[PATCH] c++: ICE with noexcept and canonical types [PR101715]

[PATCH] c++: ICE with noexcept and canonical types [PR101715]

[Marek Polacek]

This is a "canonical types differ for identical types" ICE, which started
with r11-4682.  It's a bit tricky to explain.  Consider:

  template <typename T> struct S {
    S<T> bar() noexcept(T::value);  // #1
    S<T> foo() noexcept(T::value);  // #2
  };

  template <typename T> S<T> S<T>::foo() noexcept(T::value) {}  // #3

We ICE because #3 and #2 have the same type, but their canonical types
differ: TYPE_CANONICAL (#3) == #2 but TYPE_CANONICAL (#2) == #1.

The member functions #1 and #2 have the same type.  However, since their
noexcept-specifier is deferred, when parsing them, we create a variant for
both of them, because DEFERRED_PARSE cannot be compared.  In other words,
build_cp_fntype_variant's

  tree v = TYPE_MAIN_VARIANT (type);
  for (; v; v = TYPE_NEXT_VARIANT (v))
    if (cp_check_qualified_type (v, type, type_quals, rqual, raises, late))
      return v;

will *not* find an existing variant when creating a method_type for #2, so we
have to create a new one.

But then we perform delayed parsing and call fixup_deferred_exception_variants
for #1 and #2.  f_d_e_v will replace TYPE_RAISES_EXCEPTIONS with the newly
parsed noexcept-specifier.  It also sets TYPE_CANONICAL (#2) to #1.  Both
noexcepts turned out to be the same, so now we have two equivalent variants in
the list!  I.e.,

+-----------------+      +-----------------+      +-----------------+
|      main       |      |      #2         |      |      #1         |
| S S::<T379>(S*) |----->| S S::<T37c>(S*) |----->| S S::<T37a>(S*) |----->NULL
|    -            |      |  noex(T::value) |      |  noex(T::value) |
+-----------------+      +-----------------+      +-----------------+

Then we get to #3.  As for #1 and #2, grokdeclarator calls build_memfn_type,
which ends up calling build_cp_fntype_variant, which will use the loop
above to look for an existing variant.  The first one that matches
cp_check_qualified_type will be used, so we use #2 rather than #1, and the
TYPE_CANONICAL mismatch follows.  Hopefully that makes sense.

As for the fix, I didn't think I could rewrite the method_type #2 with #1
because the type may have escaped via decltype.  So my approach is to
elide #2 from the list, so when looking for a matching variant, we always
find #1 (#2 remains live though, which admittedly sounds sort of dodgy).

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk/11?

	PR c++/101715

gcc/cp/ChangeLog:

	* tree.c (fixup_deferred_exception_variants): Remove duplicate
	variants after parsing the exception specifications.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/noexcept72.C: New test.
	* g++.dg/cpp0x/noexcept73.C: New test.
---
 gcc/cp/tree.c                           | 16 +++++++++++++++-
 gcc/testsuite/g++.dg/cpp0x/noexcept72.C | 21 +++++++++++++++++++++
 gcc/testsuite/g++.dg/cpp0x/noexcept73.C | 13 +++++++++++++
 3 files changed, 49 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/noexcept72.C
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/noexcept73.C

diff --git a/gcc/cp/tree.c b/gcc/cp/tree.c
index 7f7de86b4e8..2efad49e7c1 100644
--- a/gcc/cp/tree.c
+++ b/gcc/cp/tree.c
@@ -2804,8 +2804,9 @@ fixup_deferred_exception_variants (tree type, tree raises)
 
   /* Though sucky, this walk will process the canonical variants
      first.  */
+  tree prev = NULL_TREE;
   for (tree variant = TYPE_MAIN_VARIANT (type);
-       variant; variant = TYPE_NEXT_VARIANT (variant))
+       variant; prev = variant, variant = TYPE_NEXT_VARIANT (variant))
     if (TYPE_RAISES_EXCEPTIONS (variant) == original)
       {
 	gcc_checking_assert (variant != TYPE_MAIN_VARIANT (type));
@@ -2827,6 +2828,19 @@ fixup_deferred_exception_variants (tree type, tree raises)
 	      v = build_cp_fntype_variant (TYPE_CANONICAL (variant),
 					   rqual, cr, false);
 	    TYPE_CANONICAL (variant) = v;
+
+	    /* If VARIANT became a duplicate (cp_check_qualified_type-wise)
+	       of an existing variant in the variant list of TYPE after we
+	       have parsed its exception specification, elide it.  Otherwise,
+	       build_cp_fntype_variant would use it, leading to "canonical
+	       types differ for identical types."  */
+	    for (v = TYPE_MAIN_VARIANT (type); v; v = TYPE_NEXT_VARIANT (v))
+	      if (v != variant
+		  /* The main variant will not have TYPE_RAISES_EXCEPTIONS
+		     so PREV should never be null.  */
+		  && cp_check_qualified_type (v, variant, var_quals,
+					      rqual, cr, false))
+		TYPE_NEXT_VARIANT (prev) = TYPE_NEXT_VARIANT (variant);
 	  }
 	else
 	  TYPE_RAISES_EXCEPTIONS (variant) = raises;
diff --git a/gcc/testsuite/g++.dg/cpp0x/noexcept72.C b/gcc/testsuite/g++.dg/cpp0x/noexcept72.C
new file mode 100644
index 00000000000..f1455b3b46b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/noexcept72.C
@@ -0,0 +1,21 @@
+// PR c++/101715
+// { dg-do compile { target c++11 } }
+
+template <typename T> struct S {
+  S<T> bar() noexcept(T::value);  // #1
+  S<T> foo() noexcept(T::value);  // #2
+};
+
+template <typename T> S<T> S<T>::foo() noexcept(T::value) {}  // #3
+
+template <typename T> struct S2 {
+  S2<T> bar1() noexcept(T::value);
+  S2<T> bar2() noexcept(T::value);
+  S2<T> bar3() noexcept(T::value);
+  S2<T> bar4() noexcept(T::value);
+  S2<T> bar5() noexcept(T::value);
+  S2<T> baz() noexcept(T::value2);
+  S2<T> foo() noexcept(T::value);
+};
+
+template <typename T> S2<T> S2<T>::foo() noexcept(T::value) {}
diff --git a/gcc/testsuite/g++.dg/cpp0x/noexcept73.C b/gcc/testsuite/g++.dg/cpp0x/noexcept73.C
new file mode 100644
index 00000000000..24524f3592a
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/noexcept73.C
@@ -0,0 +1,13 @@
+// PR c++/101715
+// { dg-do compile { target c++11 } }
+
+template <typename T> struct S { };
+
+template<typename T>
+struct A
+{
+    A& foo(A&&) noexcept((S<T>::value));
+    A& assign(A&&) noexcept((S<T>::value));
+};
+template<typename T>
+A<T>& A<T>::foo(A&&) noexcept((S<T>::value)) {}

base-commit: 952b7dbb418198f86d7829aaf9d7f9fc7714a8b3
-- 
2.34.1

Re: [PATCH] c++: ICE with noexcept and canonical types [PR101715]

[Patrick Palka]

On Fri, 14 Jan 2022, Marek Polacek via Gcc-patches wrote:

> This is a "canonical types differ for identical types" ICE, which started
> with r11-4682.  It's a bit tricky to explain.  Consider:
> 
>   template <typename T> struct S {
>     S<T> bar() noexcept(T::value);  // #1
>     S<T> foo() noexcept(T::value);  // #2
>   };
> 
>   template <typename T> S<T> S<T>::foo() noexcept(T::value) {}  // #3
> 
> We ICE because #3 and #2 have the same type, but their canonical types
> differ: TYPE_CANONICAL (#3) == #2 but TYPE_CANONICAL (#2) == #1.
> 
> The member functions #1 and #2 have the same type.  However, since their
> noexcept-specifier is deferred, when parsing them, we create a variant for
> both of them, because DEFERRED_PARSE cannot be compared.  In other words,
> build_cp_fntype_variant's
> 
>   tree v = TYPE_MAIN_VARIANT (type);
>   for (; v; v = TYPE_NEXT_VARIANT (v))
>     if (cp_check_qualified_type (v, type, type_quals, rqual, raises, late))
>       return v;
> 
> will *not* find an existing variant when creating a method_type for #2, so we
> have to create a new one.
> 
> But then we perform delayed parsing and call fixup_deferred_exception_variants
> for #1 and #2.  f_d_e_v will replace TYPE_RAISES_EXCEPTIONS with the newly
> parsed noexcept-specifier.  It also sets TYPE_CANONICAL (#2) to #1.  Both
> noexcepts turned out to be the same, so now we have two equivalent variants in
> the list!  I.e.,
> 
> +-----------------+      +-----------------+      +-----------------+
> |      main       |      |      #2         |      |      #1         |
> | S S::<T379>(S*) |----->| S S::<T37c>(S*) |----->| S S::<T37a>(S*) |----->NULL
> |    -            |      |  noex(T::value) |      |  noex(T::value) |
> +-----------------+      +-----------------+      +-----------------+
> 
> Then we get to #3.  As for #1 and #2, grokdeclarator calls build_memfn_type,
> which ends up calling build_cp_fntype_variant, which will use the loop
> above to look for an existing variant.  The first one that matches
> cp_check_qualified_type will be used, so we use #2 rather than #1, and the
> TYPE_CANONICAL mismatch follows.  Hopefully that makes sense.
> 
> As for the fix, I didn't think I could rewrite the method_type #2 with #1
> because the type may have escaped via decltype.  So my approach is to
> elide #2 from the list, so when looking for a matching variant, we always
> find #1 (#2 remains live though, which admittedly sounds sort of dodgy).

I wonder about instead making build_cp_fntype_variant set the TYPE_CANONICAL for
#3 to TYPE_CANONICAL(#2) (i.e. #1) instead of to #2?  Something like:

-- >8 --

 gcc/cp/tree.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/gcc/cp/tree.c b/gcc/cp/tree.c
index 7f7de86b4e8..b89135fa121 100644
--- a/gcc/cp/tree.c
+++ b/gcc/cp/tree.c
@@ -2779,8 +2779,9 @@ build_cp_fntype_variant (tree type, cp_ref_qualifier rqual,
   else if (TYPE_CANONICAL (type) != type || cr != raises || late)
     /* Build the underlying canonical type, since it is different
        from TYPE. */
-    TYPE_CANONICAL (v) = build_cp_fntype_variant (TYPE_CANONICAL (type),
-						  rqual, cr, false);
+    TYPE_CANONICAL (v)
+      = TYPE_CANONICAL (build_cp_fntype_variant (TYPE_CANONICAL (type),
+						 rqual, cr, false));
   else
     /* T is its own canonical type. */
     TYPE_CANONICAL (v) = v;

Re: [PATCH] c++: ICE with noexcept and canonical types [PR101715]

[Marek Polacek]

On Sat, Jan 15, 2022 at 09:24:05AM -0500, Patrick Palka wrote:
> On Fri, 14 Jan 2022, Marek Polacek via Gcc-patches wrote:
> 
> > This is a "canonical types differ for identical types" ICE, which started
> > with r11-4682.  It's a bit tricky to explain.  Consider:
> > 
> >   template <typename T> struct S {
> >     S<T> bar() noexcept(T::value);  // #1
> >     S<T> foo() noexcept(T::value);  // #2
> >   };
> > 
> >   template <typename T> S<T> S<T>::foo() noexcept(T::value) {}  // #3
> > 
> > We ICE because #3 and #2 have the same type, but their canonical types
> > differ: TYPE_CANONICAL (#3) == #2 but TYPE_CANONICAL (#2) == #1.
> > 
> > The member functions #1 and #2 have the same type.  However, since their
> > noexcept-specifier is deferred, when parsing them, we create a variant for
> > both of them, because DEFERRED_PARSE cannot be compared.  In other words,
> > build_cp_fntype_variant's
> > 
> >   tree v = TYPE_MAIN_VARIANT (type);
> >   for (; v; v = TYPE_NEXT_VARIANT (v))
> >     if (cp_check_qualified_type (v, type, type_quals, rqual, raises, late))
> >       return v;
> > 
> > will *not* find an existing variant when creating a method_type for #2, so we
> > have to create a new one.
> > 
> > But then we perform delayed parsing and call fixup_deferred_exception_variants
> > for #1 and #2.  f_d_e_v will replace TYPE_RAISES_EXCEPTIONS with the newly
> > parsed noexcept-specifier.  It also sets TYPE_CANONICAL (#2) to #1.  Both
> > noexcepts turned out to be the same, so now we have two equivalent variants in
> > the list!  I.e.,
> > 
> > +-----------------+      +-----------------+      +-----------------+
> > |      main       |      |      #2         |      |      #1         |
> > | S S::<T379>(S*) |----->| S S::<T37c>(S*) |----->| S S::<T37a>(S*) |----->NULL
> > |    -            |      |  noex(T::value) |      |  noex(T::value) |
> > +-----------------+      +-----------------+      +-----------------+
> > 
> > Then we get to #3.  As for #1 and #2, grokdeclarator calls build_memfn_type,
> > which ends up calling build_cp_fntype_variant, which will use the loop
> > above to look for an existing variant.  The first one that matches
> > cp_check_qualified_type will be used, so we use #2 rather than #1, and the
> > TYPE_CANONICAL mismatch follows.  Hopefully that makes sense.
> > 
> > As for the fix, I didn't think I could rewrite the method_type #2 with #1
> > because the type may have escaped via decltype.  So my approach is to
> > elide #2 from the list, so when looking for a matching variant, we always
> > find #1 (#2 remains live though, which admittedly sounds sort of dodgy).
> 
> I wonder about instead making build_cp_fntype_variant set the TYPE_CANONICAL for
> #3 to TYPE_CANONICAL(#2) (i.e. #1) instead of to #2?  Something like:
> 
> -- >8 --
> 
>  gcc/cp/tree.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/gcc/cp/tree.c b/gcc/cp/tree.c
> index 7f7de86b4e8..b89135fa121 100644
> --- a/gcc/cp/tree.c
> +++ b/gcc/cp/tree.c
> @@ -2779,8 +2779,9 @@ build_cp_fntype_variant (tree type, cp_ref_qualifier rqual,
>    else if (TYPE_CANONICAL (type) != type || cr != raises || late)
>      /* Build the underlying canonical type, since it is different
>         from TYPE. */
> -    TYPE_CANONICAL (v) = build_cp_fntype_variant (TYPE_CANONICAL (type),
> -						  rqual, cr, false);
> +    TYPE_CANONICAL (v)
> +      = TYPE_CANONICAL (build_cp_fntype_variant (TYPE_CANONICAL (type),
> +						 rqual, cr, false));
>    else
>      /* T is its own canonical type. */
>      TYPE_CANONICAL (v) = v;

Thanks for looking.  I can dig that (and verified it works), but it strikes
me more as a workaround for the duplicity problem.  I also don't see
TYPE_CANONICAL (...) = TYPE_CANONICAL (build_cp_fntype_variant (...))
anywhere in the codebase, if that means anything.

Marek

Re: [PATCH] c++: ICE with noexcept and canonical types [PR101715]

[Jason Merrill]

On 1/14/22 19:22, Marek Polacek wrote:
> This is a "canonical types differ for identical types" ICE, which started
> with r11-4682.  It's a bit tricky to explain.  Consider:
> 
>    template <typename T> struct S {
>      S<T> bar() noexcept(T::value);  // #1
>      S<T> foo() noexcept(T::value);  // #2
>    };
> 
>    template <typename T> S<T> S<T>::foo() noexcept(T::value) {}  // #3
> 
> We ICE because #3 and #2 have the same type, but their canonical types
> differ: TYPE_CANONICAL (#3) == #2 but TYPE_CANONICAL (#2) == #1.
> 
> The member functions #1 and #2 have the same type.  However, since their
> noexcept-specifier is deferred, when parsing them, we create a variant for
> both of them, because DEFERRED_PARSE cannot be compared.  In other words,
> build_cp_fntype_variant's
> 
>    tree v = TYPE_MAIN_VARIANT (type);
>    for (; v; v = TYPE_NEXT_VARIANT (v))
>      if (cp_check_qualified_type (v, type, type_quals, rqual, raises, late))
>        return v;
> 
> will *not* find an existing variant when creating a method_type for #2, so we
> have to create a new one.
> 
> But then we perform delayed parsing and call fixup_deferred_exception_variants
> for #1 and #2.  f_d_e_v will replace TYPE_RAISES_EXCEPTIONS with the newly
> parsed noexcept-specifier.  It also sets TYPE_CANONICAL (#2) to #1.  Both
> noexcepts turned out to be the same, so now we have two equivalent variants in
> the list!  I.e.,
> 
> +-----------------+      +-----------------+      +-----------------+
> |      main       |      |      #2         |      |      #1         |
> | S S::<T379>(S*) |----->| S S::<T37c>(S*) |----->| S S::<T37a>(S*) |----->NULL
> |    -            |      |  noex(T::value) |      |  noex(T::value) |
> +-----------------+      +-----------------+      +-----------------+
> 
> Then we get to #3.  As for #1 and #2, grokdeclarator calls build_memfn_type,
> which ends up calling build_cp_fntype_variant, which will use the loop
> above to look for an existing variant.  The first one that matches
> cp_check_qualified_type will be used, so we use #2 rather than #1, and the
> TYPE_CANONICAL mismatch follows.  Hopefully that makes sense.

Why doesn't the TYPE_CANONICAL (v) == v check prevent this?

> As for the fix, I didn't think I could rewrite the method_type #2 with #1
> because the type may have escaped via decltype.  So my approach is to
> elide #2 from the list, so when looking for a matching variant, we always
> find #1 (#2 remains live though, which admittedly sounds sort of dodgy).
> 
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk/11?
> 
> 	PR c++/101715
> 
> gcc/cp/ChangeLog:
> 
> 	* tree.c (fixup_deferred_exception_variants): Remove duplicate
> 	variants after parsing the exception specifications.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp0x/noexcept72.C: New test.
> 	* g++.dg/cpp0x/noexcept73.C: New test.
> ---
>   gcc/cp/tree.c                           | 16 +++++++++++++++-
>   gcc/testsuite/g++.dg/cpp0x/noexcept72.C | 21 +++++++++++++++++++++
>   gcc/testsuite/g++.dg/cpp0x/noexcept73.C | 13 +++++++++++++
>   3 files changed, 49 insertions(+), 1 deletion(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp0x/noexcept72.C
>   create mode 100644 gcc/testsuite/g++.dg/cpp0x/noexcept73.C
> 
> diff --git a/gcc/cp/tree.c b/gcc/cp/tree.c
> index 7f7de86b4e8..2efad49e7c1 100644
> --- a/gcc/cp/tree.c
> +++ b/gcc/cp/tree.c
> @@ -2804,8 +2804,9 @@ fixup_deferred_exception_variants (tree type, tree raises)
>   
>     /* Though sucky, this walk will process the canonical variants
>        first.  */
> +  tree prev = NULL_TREE;
>     for (tree variant = TYPE_MAIN_VARIANT (type);
> -       variant; variant = TYPE_NEXT_VARIANT (variant))
> +       variant; prev = variant, variant = TYPE_NEXT_VARIANT (variant))
>       if (TYPE_RAISES_EXCEPTIONS (variant) == original)
>         {
>   	gcc_checking_assert (variant != TYPE_MAIN_VARIANT (type));
> @@ -2827,6 +2828,19 @@ fixup_deferred_exception_variants (tree type, tree raises)
>   	      v = build_cp_fntype_variant (TYPE_CANONICAL (variant),
>   					   rqual, cr, false);
>   	    TYPE_CANONICAL (variant) = v;
> +
> +	    /* If VARIANT became a duplicate (cp_check_qualified_type-wise)
> +	       of an existing variant in the variant list of TYPE after we
> +	       have parsed its exception specification, elide it.  Otherwise,
> +	       build_cp_fntype_variant would use it, leading to "canonical
> +	       types differ for identical types."  */
> +	    for (v = TYPE_MAIN_VARIANT (type); v; v = TYPE_NEXT_VARIANT (v))
> +	      if (v != variant
> +		  /* The main variant will not have TYPE_RAISES_EXCEPTIONS
> +		     so PREV should never be null.  */
> +		  && cp_check_qualified_type (v, variant, var_quals,
> +					      rqual, cr, false))
> +		TYPE_NEXT_VARIANT (prev) = TYPE_NEXT_VARIANT (variant);
>   	  }
>   	else
>   	  TYPE_RAISES_EXCEPTIONS (variant) = raises;
> diff --git a/gcc/testsuite/g++.dg/cpp0x/noexcept72.C b/gcc/testsuite/g++.dg/cpp0x/noexcept72.C
> new file mode 100644
> index 00000000000..f1455b3b46b
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/noexcept72.C
> @@ -0,0 +1,21 @@
> +// PR c++/101715
> +// { dg-do compile { target c++11 } }
> +
> +template <typename T> struct S {
> +  S<T> bar() noexcept(T::value);  // #1
> +  S<T> foo() noexcept(T::value);  // #2
> +};
> +
> +template <typename T> S<T> S<T>::foo() noexcept(T::value) {}  // #3
> +
> +template <typename T> struct S2 {
> +  S2<T> bar1() noexcept(T::value);
> +  S2<T> bar2() noexcept(T::value);
> +  S2<T> bar3() noexcept(T::value);
> +  S2<T> bar4() noexcept(T::value);
> +  S2<T> bar5() noexcept(T::value);
> +  S2<T> baz() noexcept(T::value2);
> +  S2<T> foo() noexcept(T::value);
> +};
> +
> +template <typename T> S2<T> S2<T>::foo() noexcept(T::value) {}
> diff --git a/gcc/testsuite/g++.dg/cpp0x/noexcept73.C b/gcc/testsuite/g++.dg/cpp0x/noexcept73.C
> new file mode 100644
> index 00000000000..24524f3592a
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/noexcept73.C
> @@ -0,0 +1,13 @@
> +// PR c++/101715
> +// { dg-do compile { target c++11 } }
> +
> +template <typename T> struct S { };
> +
> +template<typename T>
> +struct A
> +{
> +    A& foo(A&&) noexcept((S<T>::value));
> +    A& assign(A&&) noexcept((S<T>::value));
> +};
> +template<typename T>
> +A<T>& A<T>::foo(A&&) noexcept((S<T>::value)) {}
> 
> base-commit: 952b7dbb418198f86d7829aaf9d7f9fc7714a8b3

Re: [PATCH] c++: ICE with noexcept and canonical types [PR101715]

[Marek Polacek]

On Mon, Jan 17, 2022 at 01:48:48PM -0500, Jason Merrill wrote:
> On 1/14/22 19:22, Marek Polacek wrote:
> > This is a "canonical types differ for identical types" ICE, which started
> > with r11-4682.  It's a bit tricky to explain.  Consider:
> > 
> >    template <typename T> struct S {
> >      S<T> bar() noexcept(T::value);  // #1
> >      S<T> foo() noexcept(T::value);  // #2
> >    };
> > 
> >    template <typename T> S<T> S<T>::foo() noexcept(T::value) {}  // #3
> > 
> > We ICE because #3 and #2 have the same type, but their canonical types
> > differ: TYPE_CANONICAL (#3) == #2 but TYPE_CANONICAL (#2) == #1.
> > 
> > The member functions #1 and #2 have the same type.  However, since their
> > noexcept-specifier is deferred, when parsing them, we create a variant for
> > both of them, because DEFERRED_PARSE cannot be compared.  In other words,
> > build_cp_fntype_variant's
> > 
> >    tree v = TYPE_MAIN_VARIANT (type);
> >    for (; v; v = TYPE_NEXT_VARIANT (v))
> >      if (cp_check_qualified_type (v, type, type_quals, rqual, raises, late))
> >        return v;
> > 
> > will *not* find an existing variant when creating a method_type for #2, so we
> > have to create a new one.
> > 
> > But then we perform delayed parsing and call fixup_deferred_exception_variants
> > for #1 and #2.  f_d_e_v will replace TYPE_RAISES_EXCEPTIONS with the newly
> > parsed noexcept-specifier.  It also sets TYPE_CANONICAL (#2) to #1.  Both
> > noexcepts turned out to be the same, so now we have two equivalent variants in
> > the list!  I.e.,
> > 
> > +-----------------+      +-----------------+      +-----------------+
> > |      main       |      |      #2         |      |      #1         |
> > | S S::<T379>(S*) |----->| S S::<T37c>(S*) |----->| S S::<T37a>(S*) |----->NULL
> > |    -            |      |  noex(T::value) |      |  noex(T::value) |
> > +-----------------+      +-----------------+      +-----------------+
> > 
> > Then we get to #3.  As for #1 and #2, grokdeclarator calls build_memfn_type,
> > which ends up calling build_cp_fntype_variant, which will use the loop
> > above to look for an existing variant.  The first one that matches
> > cp_check_qualified_type will be used, so we use #2 rather than #1, and the
> > TYPE_CANONICAL mismatch follows.  Hopefully that makes sense.
> 
> Why doesn't the TYPE_CANONICAL (v) == v check prevent this?

In other words, I think you're asking: why did fixup_deferred_exception_variants
set TYPE_CANONICAL (#2) to #1 (which then differs from TYPE_CANONICAL (#3),
which is #2)?

The method_type for #1 (I'll mark is as #1 here) is built with it being its own
canonical type.

The first call to fixup_deferred_exception_variants does not change it: in
there, VARIANT is #1, the loop with 'TYPE_CANONICAL (v) == v' cannot find
an existing variant that would match, so when we do

    v = build_cp_fntype_variant (TYPE_CANONICAL (variant),
                                 rqual, cr, false);
we get #1 so
    TYPE_CANONICAL (variant) = v;
is just
    TYPE_CANONICAL (#1) = #1;
so no change.

The second call to fixup_deferred_exception_variants: here we're working with
VARIANT #2.  Now we again scan the list of variants {main, #2, #1} where we
find a match for #2: #1.  #1's TYPE_CANONICAL is #1 as per above, so we set
    TYPE_CANONICAL (#2) = #1;
which I think is correct.


I think TYPE_CANONICAL (#3) should also be #1, not #2, which my patch attempts
to do.


Hope this explanation makes some sense, please ask away if it doesn't!

> > As for the fix, I didn't think I could rewrite the method_type #2 with #1
> > because the type may have escaped via decltype.  So my approach is to
> > elide #2 from the list, so when looking for a matching variant, we always
> > find #1 (#2 remains live though, which admittedly sounds sort of dodgy).
> > 
> > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk/11?
> > 
> > 	PR c++/101715
> > 
> > gcc/cp/ChangeLog:
> > 
> > 	* tree.c (fixup_deferred_exception_variants): Remove duplicate
> > 	variants after parsing the exception specifications.
> > 
> > gcc/testsuite/ChangeLog:
> > 
> > 	* g++.dg/cpp0x/noexcept72.C: New test.
> > 	* g++.dg/cpp0x/noexcept73.C: New test.
> > ---
> >   gcc/cp/tree.c                           | 16 +++++++++++++++-
> >   gcc/testsuite/g++.dg/cpp0x/noexcept72.C | 21 +++++++++++++++++++++
> >   gcc/testsuite/g++.dg/cpp0x/noexcept73.C | 13 +++++++++++++
> >   3 files changed, 49 insertions(+), 1 deletion(-)
> >   create mode 100644 gcc/testsuite/g++.dg/cpp0x/noexcept72.C
> >   create mode 100644 gcc/testsuite/g++.dg/cpp0x/noexcept73.C
> > 
> > diff --git a/gcc/cp/tree.c b/gcc/cp/tree.c
> > index 7f7de86b4e8..2efad49e7c1 100644
> > --- a/gcc/cp/tree.c
> > +++ b/gcc/cp/tree.c
> > @@ -2804,8 +2804,9 @@ fixup_deferred_exception_variants (tree type, tree raises)
> >     /* Though sucky, this walk will process the canonical variants
> >        first.  */
> > +  tree prev = NULL_TREE;
> >     for (tree variant = TYPE_MAIN_VARIANT (type);
> > -       variant; variant = TYPE_NEXT_VARIANT (variant))
> > +       variant; prev = variant, variant = TYPE_NEXT_VARIANT (variant))
> >       if (TYPE_RAISES_EXCEPTIONS (variant) == original)
> >         {
> >   	gcc_checking_assert (variant != TYPE_MAIN_VARIANT (type));
> > @@ -2827,6 +2828,19 @@ fixup_deferred_exception_variants (tree type, tree raises)
> >   	      v = build_cp_fntype_variant (TYPE_CANONICAL (variant),
> >   					   rqual, cr, false);
> >   	    TYPE_CANONICAL (variant) = v;
> > +
> > +	    /* If VARIANT became a duplicate (cp_check_qualified_type-wise)
> > +	       of an existing variant in the variant list of TYPE after we
> > +	       have parsed its exception specification, elide it.  Otherwise,
> > +	       build_cp_fntype_variant would use it, leading to "canonical
> > +	       types differ for identical types."  */
> > +	    for (v = TYPE_MAIN_VARIANT (type); v; v = TYPE_NEXT_VARIANT (v))
> > +	      if (v != variant
> > +		  /* The main variant will not have TYPE_RAISES_EXCEPTIONS
> > +		     so PREV should never be null.  */
> > +		  && cp_check_qualified_type (v, variant, var_quals,
> > +					      rqual, cr, false))
> > +		TYPE_NEXT_VARIANT (prev) = TYPE_NEXT_VARIANT (variant);
> >   	  }
> >   	else
> >   	  TYPE_RAISES_EXCEPTIONS (variant) = raises;
> > diff --git a/gcc/testsuite/g++.dg/cpp0x/noexcept72.C b/gcc/testsuite/g++.dg/cpp0x/noexcept72.C
> > new file mode 100644
> > index 00000000000..f1455b3b46b
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/cpp0x/noexcept72.C
> > @@ -0,0 +1,21 @@
> > +// PR c++/101715
> > +// { dg-do compile { target c++11 } }
> > +
> > +template <typename T> struct S {
> > +  S<T> bar() noexcept(T::value);  // #1
> > +  S<T> foo() noexcept(T::value);  // #2
> > +};
> > +
> > +template <typename T> S<T> S<T>::foo() noexcept(T::value) {}  // #3
> > +
> > +template <typename T> struct S2 {
> > +  S2<T> bar1() noexcept(T::value);
> > +  S2<T> bar2() noexcept(T::value);
> > +  S2<T> bar3() noexcept(T::value);
> > +  S2<T> bar4() noexcept(T::value);
> > +  S2<T> bar5() noexcept(T::value);
> > +  S2<T> baz() noexcept(T::value2);
> > +  S2<T> foo() noexcept(T::value);
> > +};
> > +
> > +template <typename T> S2<T> S2<T>::foo() noexcept(T::value) {}
> > diff --git a/gcc/testsuite/g++.dg/cpp0x/noexcept73.C b/gcc/testsuite/g++.dg/cpp0x/noexcept73.C
> > new file mode 100644
> > index 00000000000..24524f3592a
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/cpp0x/noexcept73.C
> > @@ -0,0 +1,13 @@
> > +// PR c++/101715
> > +// { dg-do compile { target c++11 } }
> > +
> > +template <typename T> struct S { };
> > +
> > +template<typename T>
> > +struct A
> > +{
> > +    A& foo(A&&) noexcept((S<T>::value));
> > +    A& assign(A&&) noexcept((S<T>::value));
> > +};
> > +template<typename T>
> > +A<T>& A<T>::foo(A&&) noexcept((S<T>::value)) {}
> > 
> > base-commit: 952b7dbb418198f86d7829aaf9d7f9fc7714a8b3
> 

Marek

Re: [PATCH] c++: ICE with noexcept and canonical types [PR101715]

[Jason Merrill]

On 1/18/22 11:05, Marek Polacek wrote:
> On Mon, Jan 17, 2022 at 01:48:48PM -0500, Jason Merrill wrote:
>> On 1/14/22 19:22, Marek Polacek wrote:
>>> This is a "canonical types differ for identical types" ICE, which started
>>> with r11-4682.  It's a bit tricky to explain.  Consider:
>>>
>>>     template <typename T> struct S {
>>>       S<T> bar() noexcept(T::value);  // #1
>>>       S<T> foo() noexcept(T::value);  // #2
>>>     };
>>>
>>>     template <typename T> S<T> S<T>::foo() noexcept(T::value) {}  // #3
>>>
>>> We ICE because #3 and #2 have the same type, but their canonical types
>>> differ: TYPE_CANONICAL (#3) == #2 but TYPE_CANONICAL (#2) == #1.
>>>
>>> The member functions #1 and #2 have the same type.  However, since their
>>> noexcept-specifier is deferred, when parsing them, we create a variant for
>>> both of them, because DEFERRED_PARSE cannot be compared.  In other words,
>>> build_cp_fntype_variant's
>>>
>>>     tree v = TYPE_MAIN_VARIANT (type);
>>>     for (; v; v = TYPE_NEXT_VARIANT (v))
>>>       if (cp_check_qualified_type (v, type, type_quals, rqual, raises, late))
>>>         return v;
>>>
>>> will *not* find an existing variant when creating a method_type for #2, so we
>>> have to create a new one.
>>>
>>> But then we perform delayed parsing and call fixup_deferred_exception_variants
>>> for #1 and #2.  f_d_e_v will replace TYPE_RAISES_EXCEPTIONS with the newly
>>> parsed noexcept-specifier.  It also sets TYPE_CANONICAL (#2) to #1.  Both
>>> noexcepts turned out to be the same, so now we have two equivalent variants in
>>> the list!  I.e.,
>>>
>>> +-----------------+      +-----------------+      +-----------------+
>>> |      main       |      |      #2         |      |      #1         |
>>> | S S::<T379>(S*) |----->| S S::<T37c>(S*) |----->| S S::<T37a>(S*) |----->NULL
>>> |    -            |      |  noex(T::value) |      |  noex(T::value) |
>>> +-----------------+      +-----------------+      +-----------------+
>>>
>>> Then we get to #3.  As for #1 and #2, grokdeclarator calls build_memfn_type,
>>> which ends up calling build_cp_fntype_variant, which will use the loop
>>> above to look for an existing variant.  The first one that matches
>>> cp_check_qualified_type will be used, so we use #2 rather than #1, and the
>>> TYPE_CANONICAL mismatch follows.  Hopefully that makes sense.
>>
>> Why doesn't the TYPE_CANONICAL (v) == v check prevent this?
> 
> In other words, I think you're asking: why did fixup_deferred_exception_variants
> set TYPE_CANONICAL (#2) to #1 (which then differs from TYPE_CANONICAL (#3),
> which is #2)?

I meant to ask why TYPE_CANONICAL (#3) got set to #2 instead of #1?

And to answer my own question, it's because the check I mention is in 
fixup_deferred_exception_variants, and #3 doesn't go through there at 
all; the loop in build_cp_fntype_variant assumes no duplicate variants, 
which your patch fixes.

> The method_type for #1 (I'll mark is as #1 here) is built with it being its own
> canonical type.
> 
> The first call to fixup_deferred_exception_variants does not change it: in
> there, VARIANT is #1, the loop with 'TYPE_CANONICAL (v) == v' cannot find
> an existing variant that would match, so when we do
> 
>      v = build_cp_fntype_variant (TYPE_CANONICAL (variant),
>                                   rqual, cr, false);
> we get #1 so
>      TYPE_CANONICAL (variant) = v;
> is just
>      TYPE_CANONICAL (#1) = #1;
> so no change.
> 
> The second call to fixup_deferred_exception_variants: here we're working with
> VARIANT #2.  Now we again scan the list of variants {main, #2, #1} where we
> find a match for #2: #1.  #1's TYPE_CANONICAL is #1 as per above, so we set
>      TYPE_CANONICAL (#2) = #1;
> which I think is correct.
> 
> 
> I think TYPE_CANONICAL (#3) should also be #1, not #2, which my patch attempts
> to do.
> 
> 
> Hope this explanation makes some sense, please ask away if it doesn't!
> 
>>> As for the fix, I didn't think I could rewrite the method_type #2 with #1
>>> because the type may have escaped via decltype.  So my approach is to
>>> elide #2 from the list, so when looking for a matching variant, we always
>>> find #1 (#2 remains live though, which admittedly sounds sort of dodgy).
>>>
>>> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk/11?
>>>
>>> 	PR c++/101715
>>>
>>> gcc/cp/ChangeLog:
>>>
>>> 	* tree.c (fixup_deferred_exception_variants): Remove duplicate
>>> 	variants after parsing the exception specifications.
>>>
>>> gcc/testsuite/ChangeLog:
>>>
>>> 	* g++.dg/cpp0x/noexcept72.C: New test.
>>> 	* g++.dg/cpp0x/noexcept73.C: New test.
>>> ---
>>>    gcc/cp/tree.c                           | 16 +++++++++++++++-
>>>    gcc/testsuite/g++.dg/cpp0x/noexcept72.C | 21 +++++++++++++++++++++
>>>    gcc/testsuite/g++.dg/cpp0x/noexcept73.C | 13 +++++++++++++
>>>    3 files changed, 49 insertions(+), 1 deletion(-)
>>>    create mode 100644 gcc/testsuite/g++.dg/cpp0x/noexcept72.C
>>>    create mode 100644 gcc/testsuite/g++.dg/cpp0x/noexcept73.C
>>>
>>> diff --git a/gcc/cp/tree.c b/gcc/cp/tree.c
>>> index 7f7de86b4e8..2efad49e7c1 100644
>>> --- a/gcc/cp/tree.c
>>> +++ b/gcc/cp/tree.c
>>> @@ -2804,8 +2804,9 @@ fixup_deferred_exception_variants (tree type, tree raises)
>>>      /* Though sucky, this walk will process the canonical variants
>>>         first.  */
>>> +  tree prev = NULL_TREE;
>>>      for (tree variant = TYPE_MAIN_VARIANT (type);
>>> -       variant; variant = TYPE_NEXT_VARIANT (variant))
>>> +       variant; prev = variant, variant = TYPE_NEXT_VARIANT (variant))
>>>        if (TYPE_RAISES_EXCEPTIONS (variant) == original)
>>>          {
>>>    	gcc_checking_assert (variant != TYPE_MAIN_VARIANT (type));
>>> @@ -2827,6 +2828,19 @@ fixup_deferred_exception_variants (tree type, tree raises)
>>>    	      v = build_cp_fntype_variant (TYPE_CANONICAL (variant),
>>>    					   rqual, cr, false);
>>>    	    TYPE_CANONICAL (variant) = v;
>>> +
>>> +	    /* If VARIANT became a duplicate (cp_check_qualified_type-wise)
>>> +	       of an existing variant in the variant list of TYPE after we
>>> +	       have parsed its exception specification, elide it.  Otherwise,
>>> +	       build_cp_fntype_variant would use it, leading to "canonical
>>> +	       types differ for identical types."  */
>>> +	    for (v = TYPE_MAIN_VARIANT (type); v; v = TYPE_NEXT_VARIANT (v))
>>> +	      if (v != variant
>>> +		  /* The main variant will not have TYPE_RAISES_EXCEPTIONS
>>> +		     so PREV should never be null.  */
>>> +		  && cp_check_qualified_type (v, variant, var_quals,
>>> +					      rqual, cr, false))
>>> +		TYPE_NEXT_VARIANT (prev) = TYPE_NEXT_VARIANT (variant);

I think we don't two loops through the variants.  It ought to work to 
replace the existing loop with yours; if we find v, we prune and use its 
TYPE_CANONICAL.

Jason

[PATCH v2] c++: ICE with noexcept and canonical types [PR101715]

[Marek Polacek]

On Thu, Jan 20, 2022 at 03:23:24PM -0500, Jason Merrill wrote:
> On 1/18/22 11:05, Marek Polacek wrote:
> > On Mon, Jan 17, 2022 at 01:48:48PM -0500, Jason Merrill wrote:
> > > On 1/14/22 19:22, Marek Polacek wrote:
> > > > This is a "canonical types differ for identical types" ICE, which started
> > > > with r11-4682.  It's a bit tricky to explain.  Consider:
> > > > 
> > > >     template <typename T> struct S {
> > > >       S<T> bar() noexcept(T::value);  // #1
> > > >       S<T> foo() noexcept(T::value);  // #2
> > > >     };
> > > > 
> > > >     template <typename T> S<T> S<T>::foo() noexcept(T::value) {}  // #3
> > > > 
> > > > We ICE because #3 and #2 have the same type, but their canonical types
> > > > differ: TYPE_CANONICAL (#3) == #2 but TYPE_CANONICAL (#2) == #1.
> > > > 
> > > > The member functions #1 and #2 have the same type.  However, since their
> > > > noexcept-specifier is deferred, when parsing them, we create a variant for
> > > > both of them, because DEFERRED_PARSE cannot be compared.  In other words,
> > > > build_cp_fntype_variant's
> > > > 
> > > >     tree v = TYPE_MAIN_VARIANT (type);
> > > >     for (; v; v = TYPE_NEXT_VARIANT (v))
> > > >       if (cp_check_qualified_type (v, type, type_quals, rqual, raises, late))
> > > >         return v;
> > > > 
> > > > will *not* find an existing variant when creating a method_type for #2, so we
> > > > have to create a new one.
> > > > 
> > > > But then we perform delayed parsing and call fixup_deferred_exception_variants
> > > > for #1 and #2.  f_d_e_v will replace TYPE_RAISES_EXCEPTIONS with the newly
> > > > parsed noexcept-specifier.  It also sets TYPE_CANONICAL (#2) to #1.  Both
> > > > noexcepts turned out to be the same, so now we have two equivalent variants in
> > > > the list!  I.e.,
> > > > 
> > > > +-----------------+      +-----------------+      +-----------------+
> > > > |      main       |      |      #2         |      |      #1         |
> > > > | S S::<T379>(S*) |----->| S S::<T37c>(S*) |----->| S S::<T37a>(S*) |----->NULL
> > > > |    -            |      |  noex(T::value) |      |  noex(T::value) |
> > > > +-----------------+      +-----------------+      +-----------------+
> > > > 
> > > > Then we get to #3.  As for #1 and #2, grokdeclarator calls build_memfn_type,
> > > > which ends up calling build_cp_fntype_variant, which will use the loop
> > > > above to look for an existing variant.  The first one that matches
> > > > cp_check_qualified_type will be used, so we use #2 rather than #1, and the
> > > > TYPE_CANONICAL mismatch follows.  Hopefully that makes sense.
> > > 
> > > Why doesn't the TYPE_CANONICAL (v) == v check prevent this?
> > 
> > In other words, I think you're asking: why did fixup_deferred_exception_variants
> > set TYPE_CANONICAL (#2) to #1 (which then differs from TYPE_CANONICAL (#3),
> > which is #2)?
> 
> I meant to ask why TYPE_CANONICAL (#3) got set to #2 instead of #1?
> 
> And to answer my own question, it's because the check I mention is in
> fixup_deferred_exception_variants, and #3 doesn't go through there at all;
> the loop in build_cp_fntype_variant assumes no duplicate variants, which
> your patch fixes.

Right, fixup_deferred_exception_variants is only called for fn decls in
unparsed_noexcepts.

> > The method_type for #1 (I'll mark is as #1 here) is built with it being its own
> > canonical type.
> > 
> > The first call to fixup_deferred_exception_variants does not change it: in
> > there, VARIANT is #1, the loop with 'TYPE_CANONICAL (v) == v' cannot find
> > an existing variant that would match, so when we do
> > 
> >      v = build_cp_fntype_variant (TYPE_CANONICAL (variant),
> >                                   rqual, cr, false);
> > we get #1 so
> >      TYPE_CANONICAL (variant) = v;
> > is just
> >      TYPE_CANONICAL (#1) = #1;
> > so no change.
> > 
> > The second call to fixup_deferred_exception_variants: here we're working with
> > VARIANT #2.  Now we again scan the list of variants {main, #2, #1} where we
> > find a match for #2: #1.  #1's TYPE_CANONICAL is #1 as per above, so we set
> >      TYPE_CANONICAL (#2) = #1;
> > which I think is correct.
> > 
> > 
> > I think TYPE_CANONICAL (#3) should also be #1, not #2, which my patch attempts
> > to do.
> > 
> > 
> > Hope this explanation makes some sense, please ask away if it doesn't!
> > 
> > > > As for the fix, I didn't think I could rewrite the method_type #2 with #1
> > > > because the type may have escaped via decltype.  So my approach is to
> > > > elide #2 from the list, so when looking for a matching variant, we always
> > > > find #1 (#2 remains live though, which admittedly sounds sort of dodgy).
> > > > 
> > > > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk/11?
> > > > 
> > > > 	PR c++/101715
> > > > 
> > > > gcc/cp/ChangeLog:
> > > > 
> > > > 	* tree.c (fixup_deferred_exception_variants): Remove duplicate
> > > > 	variants after parsing the exception specifications.
> > > > 
> > > > gcc/testsuite/ChangeLog:
> > > > 
> > > > 	* g++.dg/cpp0x/noexcept72.C: New test.
> > > > 	* g++.dg/cpp0x/noexcept73.C: New test.
> > > > ---
> > > >    gcc/cp/tree.c                           | 16 +++++++++++++++-
> > > >    gcc/testsuite/g++.dg/cpp0x/noexcept72.C | 21 +++++++++++++++++++++
> > > >    gcc/testsuite/g++.dg/cpp0x/noexcept73.C | 13 +++++++++++++
> > > >    3 files changed, 49 insertions(+), 1 deletion(-)
> > > >    create mode 100644 gcc/testsuite/g++.dg/cpp0x/noexcept72.C
> > > >    create mode 100644 gcc/testsuite/g++.dg/cpp0x/noexcept73.C
> > > > 
> > > > diff --git a/gcc/cp/tree.c b/gcc/cp/tree.c
> > > > index 7f7de86b4e8..2efad49e7c1 100644
> > > > --- a/gcc/cp/tree.c
> > > > +++ b/gcc/cp/tree.c
> > > > @@ -2804,8 +2804,9 @@ fixup_deferred_exception_variants (tree type, tree raises)
> > > >      /* Though sucky, this walk will process the canonical variants
> > > >         first.  */
> > > > +  tree prev = NULL_TREE;
> > > >      for (tree variant = TYPE_MAIN_VARIANT (type);
> > > > -       variant; variant = TYPE_NEXT_VARIANT (variant))
> > > > +       variant; prev = variant, variant = TYPE_NEXT_VARIANT (variant))
> > > >        if (TYPE_RAISES_EXCEPTIONS (variant) == original)
> > > >          {
> > > >    	gcc_checking_assert (variant != TYPE_MAIN_VARIANT (type));
> > > > @@ -2827,6 +2828,19 @@ fixup_deferred_exception_variants (tree type, tree raises)
> > > >    	      v = build_cp_fntype_variant (TYPE_CANONICAL (variant),
> > > >    					   rqual, cr, false);
> > > >    	    TYPE_CANONICAL (variant) = v;
> > > > +
> > > > +	    /* If VARIANT became a duplicate (cp_check_qualified_type-wise)
> > > > +	       of an existing variant in the variant list of TYPE after we
> > > > +	       have parsed its exception specification, elide it.  Otherwise,
> > > > +	       build_cp_fntype_variant would use it, leading to "canonical
> > > > +	       types differ for identical types."  */
> > > > +	    for (v = TYPE_MAIN_VARIANT (type); v; v = TYPE_NEXT_VARIANT (v))
> > > > +	      if (v != variant
> > > > +		  /* The main variant will not have TYPE_RAISES_EXCEPTIONS
> > > > +		     so PREV should never be null.  */
> > > > +		  && cp_check_qualified_type (v, variant, var_quals,
> > > > +					      rqual, cr, false))
> > > > +		TYPE_NEXT_VARIANT (prev) = TYPE_NEXT_VARIANT (variant);
> 
> I think we don't two loops through the variants.  It ought to work to
> replace the existing loop with yours; if we find v, we prune and use its
> TYPE_CANONICAL.

Ah yes, good idea; I don't actually need to wait till TYPE_RAISES_EXCEPTIONS
is set on variant!  The following seems to work just as well.

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

-- >8 --
This is a "canonical types differ for identical types" ICE, which started
with r11-4682.  It's a bit tricky to explain.  Consider:

  template <typename T> struct S {
    S<T> bar() noexcept(T::value);  // #1
    S<T> foo() noexcept(T::value);  // #2
  };

  template <typename T> S<T> S<T>::foo() noexcept(T::value) {}  // #3

We ICE because #3 and #2 have the same type, but their canonical types
differ: TYPE_CANONICAL (#3) == #2 but TYPE_CANONICAL (#2) == #1.

The member functions #1 and #2 have the same type.  However, since their
noexcept-specifier is deferred, when parsing them, we create a variant for
both of them, because DEFERRED_PARSE cannot be compared.  In other words,
build_cp_fntype_variant's

  tree v = TYPE_MAIN_VARIANT (type);
  for (; v; v = TYPE_NEXT_VARIANT (v))
    if (cp_check_qualified_type (v, type, type_quals, rqual, raises, late))
      return v;

will *not* find an existing variant when creating a method_type for #2, so we
have to create a new one.

But then we perform delayed parsing and call fixup_deferred_exception_variants
for #1 and #2.  f_d_e_v will replace TYPE_RAISES_EXCEPTIONS with the newly
parsed noexcept-specifier.  It also sets TYPE_CANONICAL (#2) to #1.  Both
noexcepts turned out to be the same, so now we have two equivalent variants in
the list!  I.e.,

+-----------------+      +-----------------+      +-----------------+
|      main       |      |      #2         |      |      #1         |
| S S::<T379>(S*) |----->| S S::<T37c>(S*) |----->| S S::<T37a>(S*) |----->NULL
|    -            |      |  noex(T::value) |      |  noex(T::value) |
+-----------------+      +-----------------+      +-----------------+

Then we get to #3.  As for #1 and #2, grokdeclarator calls build_memfn_type,
which ends up calling build_cp_fntype_variant, which will use the loop
above to look for an existing variant.  The first one that matches
cp_check_qualified_type will be used, so we use #2 rather than #1, and the
TYPE_CANONICAL mismatch follows.  Hopefully that makes sense.

As for the fix, I didn't think I could rewrite the method_type #2 with #1
because the type may have escaped via decltype.  So my approach is to
elide #2 from the list, so when looking for a matching variant, we always
find #1 (#2 remains live though, which admittedly sounds sort of dodgy).

	PR c++/101715

gcc/cp/ChangeLog:

	* tree.c (fixup_deferred_exception_variants): Remove duplicate
	variants after parsing the exception specifications.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/noexcept72.C: New test.
	* g++.dg/cpp0x/noexcept73.C: New test.
---
 gcc/cp/tree.cc                          | 16 ++++++++++++++--
 gcc/testsuite/g++.dg/cpp0x/noexcept72.C | 21 +++++++++++++++++++++
 gcc/testsuite/g++.dg/cpp0x/noexcept73.C | 13 +++++++++++++
 3 files changed, 48 insertions(+), 2 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/noexcept72.C
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/noexcept73.C

diff --git a/gcc/cp/tree.cc b/gcc/cp/tree.cc
index bcd44e73921..17436f0512d 100644
--- a/gcc/cp/tree.cc
+++ b/gcc/cp/tree.cc
@@ -2804,8 +2804,9 @@ fixup_deferred_exception_variants (tree type, tree raises)
 
   /* Though sucky, this walk will process the canonical variants
      first.  */
+  tree prev = NULL_TREE;
   for (tree variant = TYPE_MAIN_VARIANT (type);
-       variant; variant = TYPE_NEXT_VARIANT (variant))
+       variant; prev = variant, variant = TYPE_NEXT_VARIANT (variant))
     if (TYPE_RAISES_EXCEPTIONS (variant) == original)
       {
 	gcc_checking_assert (variant != TYPE_MAIN_VARIANT (type));
@@ -2815,12 +2816,23 @@ fixup_deferred_exception_variants (tree type, tree raises)
 	    cp_cv_quals var_quals = TYPE_QUALS (variant);
 	    cp_ref_qualifier rqual = type_memfn_rqual (variant);
 
+	    /* If VARIANT would become a dup (cp_check_qualified_type-wise)
+	       of an existing variant in the variant list of TYPE after its
+	       exception specification has been parsed, elide it.  Otherwise,
+	       build_cp_fntype_variant could use it, leading to "canonical
+	       types differ for identical types."  */
 	    tree v = TYPE_MAIN_VARIANT (type);
 	    for (; v; v = TYPE_NEXT_VARIANT (v))
 	      if (TYPE_CANONICAL (v) == v
+		  && v != variant
 		  && cp_check_qualified_type (v, variant, var_quals,
 					      rqual, cr, false))
-		break;
+		{
+		  /* The main variant will not match V, so PREV will never
+		     be null.  */
+		  TYPE_NEXT_VARIANT (prev) = TYPE_NEXT_VARIANT (variant);
+		  break;
+		}
 	    TYPE_RAISES_EXCEPTIONS (variant) = raises;
 
 	    if (!v)
diff --git a/gcc/testsuite/g++.dg/cpp0x/noexcept72.C b/gcc/testsuite/g++.dg/cpp0x/noexcept72.C
new file mode 100644
index 00000000000..f1455b3b46b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/noexcept72.C
@@ -0,0 +1,21 @@
+// PR c++/101715
+// { dg-do compile { target c++11 } }
+
+template <typename T> struct S {
+  S<T> bar() noexcept(T::value);  // #1
+  S<T> foo() noexcept(T::value);  // #2
+};
+
+template <typename T> S<T> S<T>::foo() noexcept(T::value) {}  // #3
+
+template <typename T> struct S2 {
+  S2<T> bar1() noexcept(T::value);
+  S2<T> bar2() noexcept(T::value);
+  S2<T> bar3() noexcept(T::value);
+  S2<T> bar4() noexcept(T::value);
+  S2<T> bar5() noexcept(T::value);
+  S2<T> baz() noexcept(T::value2);
+  S2<T> foo() noexcept(T::value);
+};
+
+template <typename T> S2<T> S2<T>::foo() noexcept(T::value) {}
diff --git a/gcc/testsuite/g++.dg/cpp0x/noexcept73.C b/gcc/testsuite/g++.dg/cpp0x/noexcept73.C
new file mode 100644
index 00000000000..24524f3592a
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/noexcept73.C
@@ -0,0 +1,13 @@
+// PR c++/101715
+// { dg-do compile { target c++11 } }
+
+template <typename T> struct S { };
+
+template<typename T>
+struct A
+{
+    A& foo(A&&) noexcept((S<T>::value));
+    A& assign(A&&) noexcept((S<T>::value));
+};
+template<typename T>
+A<T>& A<T>::foo(A&&) noexcept((S<T>::value)) {}

base-commit: d2ad748eeef0dd260f3993b8dcbffbded3240a0a
-- 
2.34.1

Re: [PATCH v2] c++: ICE with noexcept and canonical types [PR101715]

[Jason Merrill]

On 1/20/22 20:03, Marek Polacek wrote:
> On Thu, Jan 20, 2022 at 03:23:24PM -0500, Jason Merrill wrote:
>> On 1/18/22 11:05, Marek Polacek wrote:
>>> On Mon, Jan 17, 2022 at 01:48:48PM -0500, Jason Merrill wrote:
>>>> On 1/14/22 19:22, Marek Polacek wrote:
>>>>> This is a "canonical types differ for identical types" ICE, which started
>>>>> with r11-4682.  It's a bit tricky to explain.  Consider:
>>>>>
>>>>>      template <typename T> struct S {
>>>>>        S<T> bar() noexcept(T::value);  // #1
>>>>>        S<T> foo() noexcept(T::value);  // #2
>>>>>      };
>>>>>
>>>>>      template <typename T> S<T> S<T>::foo() noexcept(T::value) {}  // #3
>>>>>
>>>>> We ICE because #3 and #2 have the same type, but their canonical types
>>>>> differ: TYPE_CANONICAL (#3) == #2 but TYPE_CANONICAL (#2) == #1.
>>>>>
>>>>> The member functions #1 and #2 have the same type.  However, since their
>>>>> noexcept-specifier is deferred, when parsing them, we create a variant for
>>>>> both of them, because DEFERRED_PARSE cannot be compared.  In other words,
>>>>> build_cp_fntype_variant's
>>>>>
>>>>>      tree v = TYPE_MAIN_VARIANT (type);
>>>>>      for (; v; v = TYPE_NEXT_VARIANT (v))
>>>>>        if (cp_check_qualified_type (v, type, type_quals, rqual, raises, late))
>>>>>          return v;
>>>>>
>>>>> will *not* find an existing variant when creating a method_type for #2, so we
>>>>> have to create a new one.
>>>>>
>>>>> But then we perform delayed parsing and call fixup_deferred_exception_variants
>>>>> for #1 and #2.  f_d_e_v will replace TYPE_RAISES_EXCEPTIONS with the newly
>>>>> parsed noexcept-specifier.  It also sets TYPE_CANONICAL (#2) to #1.  Both
>>>>> noexcepts turned out to be the same, so now we have two equivalent variants in
>>>>> the list!  I.e.,
>>>>>
>>>>> +-----------------+      +-----------------+      +-----------------+
>>>>> |      main       |      |      #2         |      |      #1         |
>>>>> | S S::<T379>(S*) |----->| S S::<T37c>(S*) |----->| S S::<T37a>(S*) |----->NULL
>>>>> |    -            |      |  noex(T::value) |      |  noex(T::value) |
>>>>> +-----------------+      +-----------------+      +-----------------+
>>>>>
>>>>> Then we get to #3.  As for #1 and #2, grokdeclarator calls build_memfn_type,
>>>>> which ends up calling build_cp_fntype_variant, which will use the loop
>>>>> above to look for an existing variant.  The first one that matches
>>>>> cp_check_qualified_type will be used, so we use #2 rather than #1, and the
>>>>> TYPE_CANONICAL mismatch follows.  Hopefully that makes sense.
>>>>
>>>> Why doesn't the TYPE_CANONICAL (v) == v check prevent this?
>>>
>>> In other words, I think you're asking: why did fixup_deferred_exception_variants
>>> set TYPE_CANONICAL (#2) to #1 (which then differs from TYPE_CANONICAL (#3),
>>> which is #2)?
>>
>> I meant to ask why TYPE_CANONICAL (#3) got set to #2 instead of #1?
>>
>> And to answer my own question, it's because the check I mention is in
>> fixup_deferred_exception_variants, and #3 doesn't go through there at all;
>> the loop in build_cp_fntype_variant assumes no duplicate variants, which
>> your patch fixes.
> 
> Right, fixup_deferred_exception_variants is only called for fn decls in
> unparsed_noexcepts.
> 
>>> The method_type for #1 (I'll mark is as #1 here) is built with it being its own
>>> canonical type.
>>>
>>> The first call to fixup_deferred_exception_variants does not change it: in
>>> there, VARIANT is #1, the loop with 'TYPE_CANONICAL (v) == v' cannot find
>>> an existing variant that would match, so when we do
>>>
>>>       v = build_cp_fntype_variant (TYPE_CANONICAL (variant),
>>>                                    rqual, cr, false);
>>> we get #1 so
>>>       TYPE_CANONICAL (variant) = v;
>>> is just
>>>       TYPE_CANONICAL (#1) = #1;
>>> so no change.
>>>
>>> The second call to fixup_deferred_exception_variants: here we're working with
>>> VARIANT #2.  Now we again scan the list of variants {main, #2, #1} where we
>>> find a match for #2: #1.  #1's TYPE_CANONICAL is #1 as per above, so we set
>>>       TYPE_CANONICAL (#2) = #1;
>>> which I think is correct.
>>>
>>>
>>> I think TYPE_CANONICAL (#3) should also be #1, not #2, which my patch attempts
>>> to do.
>>>
>>>
>>> Hope this explanation makes some sense, please ask away if it doesn't!
>>>
>>>>> As for the fix, I didn't think I could rewrite the method_type #2 with #1
>>>>> because the type may have escaped via decltype.  So my approach is to
>>>>> elide #2 from the list, so when looking for a matching variant, we always
>>>>> find #1 (#2 remains live though, which admittedly sounds sort of dodgy).
>>>>>
>>>>> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk/11?
>>>>>
>>>>> 	PR c++/101715
>>>>>
>>>>> gcc/cp/ChangeLog:
>>>>>
>>>>> 	* tree.c (fixup_deferred_exception_variants): Remove duplicate
>>>>> 	variants after parsing the exception specifications.
>>>>>
>>>>> gcc/testsuite/ChangeLog:
>>>>>
>>>>> 	* g++.dg/cpp0x/noexcept72.C: New test.
>>>>> 	* g++.dg/cpp0x/noexcept73.C: New test.
>>>>> ---
>>>>>     gcc/cp/tree.c                           | 16 +++++++++++++++-
>>>>>     gcc/testsuite/g++.dg/cpp0x/noexcept72.C | 21 +++++++++++++++++++++
>>>>>     gcc/testsuite/g++.dg/cpp0x/noexcept73.C | 13 +++++++++++++
>>>>>     3 files changed, 49 insertions(+), 1 deletion(-)
>>>>>     create mode 100644 gcc/testsuite/g++.dg/cpp0x/noexcept72.C
>>>>>     create mode 100644 gcc/testsuite/g++.dg/cpp0x/noexcept73.C
>>>>>
>>>>> diff --git a/gcc/cp/tree.c b/gcc/cp/tree.c
>>>>> index 7f7de86b4e8..2efad49e7c1 100644
>>>>> --- a/gcc/cp/tree.c
>>>>> +++ b/gcc/cp/tree.c
>>>>> @@ -2804,8 +2804,9 @@ fixup_deferred_exception_variants (tree type, tree raises)
>>>>>       /* Though sucky, this walk will process the canonical variants
>>>>>          first.  */
>>>>> +  tree prev = NULL_TREE;
>>>>>       for (tree variant = TYPE_MAIN_VARIANT (type);
>>>>> -       variant; variant = TYPE_NEXT_VARIANT (variant))
>>>>> +       variant; prev = variant, variant = TYPE_NEXT_VARIANT (variant))
>>>>>         if (TYPE_RAISES_EXCEPTIONS (variant) == original)
>>>>>           {
>>>>>     	gcc_checking_assert (variant != TYPE_MAIN_VARIANT (type));
>>>>> @@ -2827,6 +2828,19 @@ fixup_deferred_exception_variants (tree type, tree raises)
>>>>>     	      v = build_cp_fntype_variant (TYPE_CANONICAL (variant),
>>>>>     					   rqual, cr, false);
>>>>>     	    TYPE_CANONICAL (variant) = v;
>>>>> +
>>>>> +	    /* If VARIANT became a duplicate (cp_check_qualified_type-wise)
>>>>> +	       of an existing variant in the variant list of TYPE after we
>>>>> +	       have parsed its exception specification, elide it.  Otherwise,
>>>>> +	       build_cp_fntype_variant would use it, leading to "canonical
>>>>> +	       types differ for identical types."  */
>>>>> +	    for (v = TYPE_MAIN_VARIANT (type); v; v = TYPE_NEXT_VARIANT (v))
>>>>> +	      if (v != variant
>>>>> +		  /* The main variant will not have TYPE_RAISES_EXCEPTIONS
>>>>> +		     so PREV should never be null.  */
>>>>> +		  && cp_check_qualified_type (v, variant, var_quals,
>>>>> +					      rqual, cr, false))
>>>>> +		TYPE_NEXT_VARIANT (prev) = TYPE_NEXT_VARIANT (variant);
>>
>> I think we don't two loops through the variants.  It ought to work to
>> replace the existing loop with yours; if we find v, we prune and use its
>> TYPE_CANONICAL.
> 
> Ah yes, good idea; I don't actually need to wait till TYPE_RAISES_EXCEPTIONS
> is set on variant!  The following seems to work just as well.
> 
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> 
> -- >8 --
> This is a "canonical types differ for identical types" ICE, which started
> with r11-4682.  It's a bit tricky to explain.  Consider:
> 
>    template <typename T> struct S {
>      S<T> bar() noexcept(T::value);  // #1
>      S<T> foo() noexcept(T::value);  // #2
>    };
> 
>    template <typename T> S<T> S<T>::foo() noexcept(T::value) {}  // #3
> 
> We ICE because #3 and #2 have the same type, but their canonical types
> differ: TYPE_CANONICAL (#3) == #2 but TYPE_CANONICAL (#2) == #1.
> 
> The member functions #1 and #2 have the same type.  However, since their
> noexcept-specifier is deferred, when parsing them, we create a variant for
> both of them, because DEFERRED_PARSE cannot be compared.  In other words,
> build_cp_fntype_variant's
> 
>    tree v = TYPE_MAIN_VARIANT (type);
>    for (; v; v = TYPE_NEXT_VARIANT (v))
>      if (cp_check_qualified_type (v, type, type_quals, rqual, raises, late))
>        return v;
> 
> will *not* find an existing variant when creating a method_type for #2, so we
> have to create a new one.
> 
> But then we perform delayed parsing and call fixup_deferred_exception_variants
> for #1 and #2.  f_d_e_v will replace TYPE_RAISES_EXCEPTIONS with the newly
> parsed noexcept-specifier.  It also sets TYPE_CANONICAL (#2) to #1.  Both
> noexcepts turned out to be the same, so now we have two equivalent variants in
> the list!  I.e.,
> 
> +-----------------+      +-----------------+      +-----------------+
> |      main       |      |      #2         |      |      #1         |
> | S S::<T379>(S*) |----->| S S::<T37c>(S*) |----->| S S::<T37a>(S*) |----->NULL
> |    -            |      |  noex(T::value) |      |  noex(T::value) |
> +-----------------+      +-----------------+      +-----------------+
> 
> Then we get to #3.  As for #1 and #2, grokdeclarator calls build_memfn_type,
> which ends up calling build_cp_fntype_variant, which will use the loop
> above to look for an existing variant.  The first one that matches
> cp_check_qualified_type will be used, so we use #2 rather than #1, and the
> TYPE_CANONICAL mismatch follows.  Hopefully that makes sense.
> 
> As for the fix, I didn't think I could rewrite the method_type #2 with #1
> because the type may have escaped via decltype.  So my approach is to
> elide #2 from the list, so when looking for a matching variant, we always
> find #1 (#2 remains live though, which admittedly sounds sort of dodgy).
> 
> 	PR c++/101715
> 
> gcc/cp/ChangeLog:
> 
> 	* tree.c (fixup_deferred_exception_variants): Remove duplicate
> 	variants after parsing the exception specifications.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp0x/noexcept72.C: New test.
> 	* g++.dg/cpp0x/noexcept73.C: New test.
> ---
>   gcc/cp/tree.cc                          | 16 ++++++++++++++--
>   gcc/testsuite/g++.dg/cpp0x/noexcept72.C | 21 +++++++++++++++++++++
>   gcc/testsuite/g++.dg/cpp0x/noexcept73.C | 13 +++++++++++++
>   3 files changed, 48 insertions(+), 2 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp0x/noexcept72.C
>   create mode 100644 gcc/testsuite/g++.dg/cpp0x/noexcept73.C
> 
> diff --git a/gcc/cp/tree.cc b/gcc/cp/tree.cc
> index bcd44e73921..17436f0512d 100644
> --- a/gcc/cp/tree.cc
> +++ b/gcc/cp/tree.cc
> @@ -2804,8 +2804,9 @@ fixup_deferred_exception_variants (tree type, tree raises)
>   
>     /* Though sucky, this walk will process the canonical variants
>        first.  */
> +  tree prev = NULL_TREE;
>     for (tree variant = TYPE_MAIN_VARIANT (type);
> -       variant; variant = TYPE_NEXT_VARIANT (variant))
> +       variant; prev = variant, variant = TYPE_NEXT_VARIANT (variant))
>       if (TYPE_RAISES_EXCEPTIONS (variant) == original)
>         {
>   	gcc_checking_assert (variant != TYPE_MAIN_VARIANT (type));
> @@ -2815,12 +2816,23 @@ fixup_deferred_exception_variants (tree type, tree raises)
>   	    cp_cv_quals var_quals = TYPE_QUALS (variant);
>   	    cp_ref_qualifier rqual = type_memfn_rqual (variant);
>   
> +	    /* If VARIANT would become a dup (cp_check_qualified_type-wise)
> +	       of an existing variant in the variant list of TYPE after its
> +	       exception specification has been parsed, elide it.  Otherwise,
> +	       build_cp_fntype_variant could use it, leading to "canonical
> +	       types differ for identical types."  */
>   	    tree v = TYPE_MAIN_VARIANT (type);
>   	    for (; v; v = TYPE_NEXT_VARIANT (v))
>   	      if (TYPE_CANONICAL (v) == v

I think we want to drop the TYPE_CANONICAL check here, and below change

TYPE_CANONICAL (variant) = v;

to

TYPE_CANONICAL (variant) = TYPE_CANONICAL (v);

so that this also works for e.g. signatures involving typedefs.

> +		  && v != variant

I think we don't need this check since we haven't changed 
TYPE_RAISES_EXCEPTIONS yet.

>   		  && cp_check_qualified_type (v, variant, var_quals,
>   					      rqual, cr, false))
> -		break;
> +		{
> +		  /* The main variant will not match V, so PREV will never
> +		     be null.  */
> +		  TYPE_NEXT_VARIANT (prev) = TYPE_NEXT_VARIANT (variant);
> +		  break;
> +		}
>   	    TYPE_RAISES_EXCEPTIONS (variant) = raises;
>   
>   	    if (!v)
> diff --git a/gcc/testsuite/g++.dg/cpp0x/noexcept72.C b/gcc/testsuite/g++.dg/cpp0x/noexcept72.C
> new file mode 100644
> index 00000000000..f1455b3b46b
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/noexcept72.C
> @@ -0,0 +1,21 @@
> +// PR c++/101715
> +// { dg-do compile { target c++11 } }
> +
> +template <typename T> struct S {
> +  S<T> bar() noexcept(T::value);  // #1
> +  S<T> foo() noexcept(T::value);  // #2
> +};
> +
> +template <typename T> S<T> S<T>::foo() noexcept(T::value) {}  // #3
> +
> +template <typename T> struct S2 {
> +  S2<T> bar1() noexcept(T::value);
> +  S2<T> bar2() noexcept(T::value);
> +  S2<T> bar3() noexcept(T::value);
> +  S2<T> bar4() noexcept(T::value);
> +  S2<T> bar5() noexcept(T::value);
> +  S2<T> baz() noexcept(T::value2);
> +  S2<T> foo() noexcept(T::value);
> +};
> +
> +template <typename T> S2<T> S2<T>::foo() noexcept(T::value) {}
> diff --git a/gcc/testsuite/g++.dg/cpp0x/noexcept73.C b/gcc/testsuite/g++.dg/cpp0x/noexcept73.C
> new file mode 100644
> index 00000000000..24524f3592a
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/noexcept73.C
> @@ -0,0 +1,13 @@
> +// PR c++/101715
> +// { dg-do compile { target c++11 } }
> +
> +template <typename T> struct S { };
> +
> +template<typename T>
> +struct A
> +{
> +    A& foo(A&&) noexcept((S<T>::value));
> +    A& assign(A&&) noexcept((S<T>::value));
> +};
> +template<typename T>
> +A<T>& A<T>::foo(A&&) noexcept((S<T>::value)) {}
> 
> base-commit: d2ad748eeef0dd260f3993b8dcbffbded3240a0a

[PATCH v3] c++: ICE with noexcept and canonical types [PR101715]

[Marek Polacek]

On Fri, Jan 21, 2022 at 09:27:17AM -0500, Jason Merrill wrote:
> On 1/20/22 20:03, Marek Polacek wrote:
> > @@ -2815,12 +2816,23 @@ fixup_deferred_exception_variants (tree type, tree raises)
> >   	    cp_cv_quals var_quals = TYPE_QUALS (variant);
> >   	    cp_ref_qualifier rqual = type_memfn_rqual (variant);
> > +	    /* If VARIANT would become a dup (cp_check_qualified_type-wise)
> > +	       of an existing variant in the variant list of TYPE after its
> > +	       exception specification has been parsed, elide it.  Otherwise,
> > +	       build_cp_fntype_variant could use it, leading to "canonical
> > +	       types differ for identical types."  */
> >   	    tree v = TYPE_MAIN_VARIANT (type);
> >   	    for (; v; v = TYPE_NEXT_VARIANT (v))
> >   	      if (TYPE_CANONICAL (v) == v
> 
> I think we want to drop the TYPE_CANONICAL check here, and below change
> 
> TYPE_CANONICAL (variant) = v;
> 
> to
> 
> TYPE_CANONICAL (variant) = TYPE_CANONICAL (v);

OK.  I couldn't really find a way to test it; clang++ rejected
my attempts with "error: exception specifications are not allowed in
typedefs" so I'm not sure if I want to add such tests even though we
happen to accept it currently.
 
> so that this also works for e.g. signatures involving typedefs.
> 
> > +		  && v != variant
> 
> I think we don't need this check since we haven't changed
> TYPE_RAISES_EXCEPTIONS yet.

And variant will never be the main variant, because of the

  if (TYPE_RAISES_EXCEPTIONS (variant) == original)

check.  Ok, so the following should be enough:

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

-- >8 --
This is a "canonical types differ for identical types" ICE, which started
with r11-4682.  It's a bit tricky to explain.  Consider:

  template <typename T> struct S {
    S<T> bar() noexcept(T::value);  // #1
    S<T> foo() noexcept(T::value);  // #2
  };

  template <typename T> S<T> S<T>::foo() noexcept(T::value) {}  // #3

We ICE because #3 and #2 have the same type, but their canonical types
differ: TYPE_CANONICAL (#3) == #2 but TYPE_CANONICAL (#2) == #1.

The member functions #1 and #2 have the same type.  However, since their
noexcept-specifier is deferred, when parsing them, we create a variant for
both of them, because DEFERRED_PARSE cannot be compared.  In other words,
build_cp_fntype_variant's

  tree v = TYPE_MAIN_VARIANT (type);
  for (; v; v = TYPE_NEXT_VARIANT (v))
    if (cp_check_qualified_type (v, type, type_quals, rqual, raises, late))
      return v;

will *not* find an existing variant when creating a method_type for #2, so we
have to create a new one.

But then we perform delayed parsing and call fixup_deferred_exception_variants
for #1 and #2.  f_d_e_v will replace TYPE_RAISES_EXCEPTIONS with the newly
parsed noexcept-specifier.  It also sets TYPE_CANONICAL (#2) to #1.  Both
noexcepts turned out to be the same, so now we have two equivalent variants in
the list!  I.e.,

+-----------------+      +-----------------+      +-----------------+
|      main       |      |      #2         |      |      #1         |
| S S::<T379>(S*) |----->| S S::<T37c>(S*) |----->| S S::<T37a>(S*) |----->NULL
|    -            |      |  noex(T::value) |      |  noex(T::value) |
+-----------------+      +-----------------+      +-----------------+

Then we get to #3.  As for #1 and #2, grokdeclarator calls build_memfn_type,
which ends up calling build_cp_fntype_variant, which will use the loop
above to look for an existing variant.  The first one that matches
cp_check_qualified_type will be used, so we use #2 rather than #1, and the
TYPE_CANONICAL mismatch follows.  Hopefully that makes sense.

As for the fix, I didn't think I could rewrite the method_type #2 with #1
because the type may have escaped via decltype.  So my approach is to
elide #2 from the list, so when looking for a matching variant, we always
find #1 (#2 remains live though, which admittedly sounds sort of dodgy).

	PR c++/101715

gcc/cp/ChangeLog:

	* tree.c (fixup_deferred_exception_variants): Remove duplicate
	variants after parsing the exception specifications.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/noexcept72.C: New test.
	* g++.dg/cpp0x/noexcept73.C: New test.
---
 gcc/cp/tree.cc                          | 22 ++++++++++++++++------
 gcc/testsuite/g++.dg/cpp0x/noexcept72.C | 21 +++++++++++++++++++++
 gcc/testsuite/g++.dg/cpp0x/noexcept73.C | 13 +++++++++++++
 3 files changed, 50 insertions(+), 6 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/noexcept72.C
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/noexcept73.C

diff --git a/gcc/cp/tree.cc b/gcc/cp/tree.cc
index bcd44e73921..f88006aec4f 100644
--- a/gcc/cp/tree.cc
+++ b/gcc/cp/tree.cc
@@ -2804,8 +2804,9 @@ fixup_deferred_exception_variants (tree type, tree raises)
 
   /* Though sucky, this walk will process the canonical variants
      first.  */
+  tree prev = NULL_TREE;
   for (tree variant = TYPE_MAIN_VARIANT (type);
-       variant; variant = TYPE_NEXT_VARIANT (variant))
+       variant; prev = variant, variant = TYPE_NEXT_VARIANT (variant))
     if (TYPE_RAISES_EXCEPTIONS (variant) == original)
       {
 	gcc_checking_assert (variant != TYPE_MAIN_VARIANT (type));
@@ -2815,18 +2816,27 @@ fixup_deferred_exception_variants (tree type, tree raises)
 	    cp_cv_quals var_quals = TYPE_QUALS (variant);
 	    cp_ref_qualifier rqual = type_memfn_rqual (variant);
 
+	    /* If VARIANT would become a dup (cp_check_qualified_type-wise)
+	       of an existing variant in the variant list of TYPE after its
+	       exception specification has been parsed, elide it.  Otherwise,
+	       build_cp_fntype_variant could use it, leading to "canonical
+	       types differ for identical types."  */
 	    tree v = TYPE_MAIN_VARIANT (type);
 	    for (; v; v = TYPE_NEXT_VARIANT (v))
-	      if (TYPE_CANONICAL (v) == v
-		  && cp_check_qualified_type (v, variant, var_quals,
-					      rqual, cr, false))
-		break;
+	      if (cp_check_qualified_type (v, variant, var_quals,
+					   rqual, cr, false))
+		{
+		  /* The main variant will not match V, so PREV will never
+		     be null.  */
+		  TYPE_NEXT_VARIANT (prev) = TYPE_NEXT_VARIANT (variant);
+		  break;
+		}
 	    TYPE_RAISES_EXCEPTIONS (variant) = raises;
 
 	    if (!v)
 	      v = build_cp_fntype_variant (TYPE_CANONICAL (variant),
 					   rqual, cr, false);
-	    TYPE_CANONICAL (variant) = v;
+	    TYPE_CANONICAL (variant) = TYPE_CANONICAL (v);
 	  }
 	else
 	  TYPE_RAISES_EXCEPTIONS (variant) = raises;
diff --git a/gcc/testsuite/g++.dg/cpp0x/noexcept72.C b/gcc/testsuite/g++.dg/cpp0x/noexcept72.C
new file mode 100644
index 00000000000..f1455b3b46b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/noexcept72.C
@@ -0,0 +1,21 @@
+// PR c++/101715
+// { dg-do compile { target c++11 } }
+
+template <typename T> struct S {
+  S<T> bar() noexcept(T::value);  // #1
+  S<T> foo() noexcept(T::value);  // #2
+};
+
+template <typename T> S<T> S<T>::foo() noexcept(T::value) {}  // #3
+
+template <typename T> struct S2 {
+  S2<T> bar1() noexcept(T::value);
+  S2<T> bar2() noexcept(T::value);
+  S2<T> bar3() noexcept(T::value);
+  S2<T> bar4() noexcept(T::value);
+  S2<T> bar5() noexcept(T::value);
+  S2<T> baz() noexcept(T::value2);
+  S2<T> foo() noexcept(T::value);
+};
+
+template <typename T> S2<T> S2<T>::foo() noexcept(T::value) {}
diff --git a/gcc/testsuite/g++.dg/cpp0x/noexcept73.C b/gcc/testsuite/g++.dg/cpp0x/noexcept73.C
new file mode 100644
index 00000000000..24524f3592a
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/noexcept73.C
@@ -0,0 +1,13 @@
+// PR c++/101715
+// { dg-do compile { target c++11 } }
+
+template <typename T> struct S { };
+
+template<typename T>
+struct A
+{
+    A& foo(A&&) noexcept((S<T>::value));
+    A& assign(A&&) noexcept((S<T>::value));
+};
+template<typename T>
+A<T>& A<T>::foo(A&&) noexcept((S<T>::value)) {}

base-commit: 45cae5b6392496028f35c5948f7fae0af81d135b
-- 
2.34.1

Re: [PATCH v3] c++: ICE with noexcept and canonical types [PR101715]

[Jason Merrill]

On 1/21/22 12:42, Marek Polacek wrote:
> On Fri, Jan 21, 2022 at 09:27:17AM -0500, Jason Merrill wrote:
>> On 1/20/22 20:03, Marek Polacek wrote:
>>> @@ -2815,12 +2816,23 @@ fixup_deferred_exception_variants (tree type, tree raises)
>>>    	    cp_cv_quals var_quals = TYPE_QUALS (variant);
>>>    	    cp_ref_qualifier rqual = type_memfn_rqual (variant);
>>> +	    /* If VARIANT would become a dup (cp_check_qualified_type-wise)
>>> +	       of an existing variant in the variant list of TYPE after its
>>> +	       exception specification has been parsed, elide it.  Otherwise,
>>> +	       build_cp_fntype_variant could use it, leading to "canonical
>>> +	       types differ for identical types."  */
>>>    	    tree v = TYPE_MAIN_VARIANT (type);
>>>    	    for (; v; v = TYPE_NEXT_VARIANT (v))
>>>    	      if (TYPE_CANONICAL (v) == v
>>
>> I think we want to drop the TYPE_CANONICAL check here, and below change
>>
>> TYPE_CANONICAL (variant) = v;
>>
>> to
>>
>> TYPE_CANONICAL (variant) = TYPE_CANONICAL (v);
> 
> OK.  I couldn't really find a way to test it; clang++ rejected
> my attempts with "error: exception specifications are not allowed in
> typedefs" so I'm not sure if I want to add such tests even though we
> happen to accept it currently.
>   
>> so that this also works for e.g. signatures involving typedefs.
>>
>>> +		  && v != variant
>>
>> I think we don't need this check since we haven't changed
>> TYPE_RAISES_EXCEPTIONS yet.
> 
> And variant will never be the main variant, because of the
> 
>    if (TYPE_RAISES_EXCEPTIONS (variant) == original)
> 
> check.  Ok, so the following should be enough:
> 
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

OK, thanks.

> -- >8 --
> This is a "canonical types differ for identical types" ICE, which started
> with r11-4682.  It's a bit tricky to explain.  Consider:
> 
>    template <typename T> struct S {
>      S<T> bar() noexcept(T::value);  // #1
>      S<T> foo() noexcept(T::value);  // #2
>    };
> 
>    template <typename T> S<T> S<T>::foo() noexcept(T::value) {}  // #3
> 
> We ICE because #3 and #2 have the same type, but their canonical types
> differ: TYPE_CANONICAL (#3) == #2 but TYPE_CANONICAL (#2) == #1.
> 
> The member functions #1 and #2 have the same type.  However, since their
> noexcept-specifier is deferred, when parsing them, we create a variant for
> both of them, because DEFERRED_PARSE cannot be compared.  In other words,
> build_cp_fntype_variant's
> 
>    tree v = TYPE_MAIN_VARIANT (type);
>    for (; v; v = TYPE_NEXT_VARIANT (v))
>      if (cp_check_qualified_type (v, type, type_quals, rqual, raises, late))
>        return v;
> 
> will *not* find an existing variant when creating a method_type for #2, so we
> have to create a new one.
> 
> But then we perform delayed parsing and call fixup_deferred_exception_variants
> for #1 and #2.  f_d_e_v will replace TYPE_RAISES_EXCEPTIONS with the newly
> parsed noexcept-specifier.  It also sets TYPE_CANONICAL (#2) to #1.  Both
> noexcepts turned out to be the same, so now we have two equivalent variants in
> the list!  I.e.,
> 
> +-----------------+      +-----------------+      +-----------------+
> |      main       |      |      #2         |      |      #1         |
> | S S::<T379>(S*) |----->| S S::<T37c>(S*) |----->| S S::<T37a>(S*) |----->NULL
> |    -            |      |  noex(T::value) |      |  noex(T::value) |
> +-----------------+      +-----------------+      +-----------------+
> 
> Then we get to #3.  As for #1 and #2, grokdeclarator calls build_memfn_type,
> which ends up calling build_cp_fntype_variant, which will use the loop
> above to look for an existing variant.  The first one that matches
> cp_check_qualified_type will be used, so we use #2 rather than #1, and the
> TYPE_CANONICAL mismatch follows.  Hopefully that makes sense.
> 
> As for the fix, I didn't think I could rewrite the method_type #2 with #1
> because the type may have escaped via decltype.  So my approach is to
> elide #2 from the list, so when looking for a matching variant, we always
> find #1 (#2 remains live though, which admittedly sounds sort of dodgy).
> 
> 	PR c++/101715
> 
> gcc/cp/ChangeLog:
> 
> 	* tree.c (fixup_deferred_exception_variants): Remove duplicate
> 	variants after parsing the exception specifications.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp0x/noexcept72.C: New test.
> 	* g++.dg/cpp0x/noexcept73.C: New test.
> ---
>   gcc/cp/tree.cc                          | 22 ++++++++++++++++------
>   gcc/testsuite/g++.dg/cpp0x/noexcept72.C | 21 +++++++++++++++++++++
>   gcc/testsuite/g++.dg/cpp0x/noexcept73.C | 13 +++++++++++++
>   3 files changed, 50 insertions(+), 6 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp0x/noexcept72.C
>   create mode 100644 gcc/testsuite/g++.dg/cpp0x/noexcept73.C
> 
> diff --git a/gcc/cp/tree.cc b/gcc/cp/tree.cc
> index bcd44e73921..f88006aec4f 100644
> --- a/gcc/cp/tree.cc
> +++ b/gcc/cp/tree.cc
> @@ -2804,8 +2804,9 @@ fixup_deferred_exception_variants (tree type, tree raises)
>   
>     /* Though sucky, this walk will process the canonical variants
>        first.  */
> +  tree prev = NULL_TREE;
>     for (tree variant = TYPE_MAIN_VARIANT (type);
> -       variant; variant = TYPE_NEXT_VARIANT (variant))
> +       variant; prev = variant, variant = TYPE_NEXT_VARIANT (variant))
>       if (TYPE_RAISES_EXCEPTIONS (variant) == original)
>         {
>   	gcc_checking_assert (variant != TYPE_MAIN_VARIANT (type));
> @@ -2815,18 +2816,27 @@ fixup_deferred_exception_variants (tree type, tree raises)
>   	    cp_cv_quals var_quals = TYPE_QUALS (variant);
>   	    cp_ref_qualifier rqual = type_memfn_rqual (variant);
>   
> +	    /* If VARIANT would become a dup (cp_check_qualified_type-wise)
> +	       of an existing variant in the variant list of TYPE after its
> +	       exception specification has been parsed, elide it.  Otherwise,
> +	       build_cp_fntype_variant could use it, leading to "canonical
> +	       types differ for identical types."  */
>   	    tree v = TYPE_MAIN_VARIANT (type);
>   	    for (; v; v = TYPE_NEXT_VARIANT (v))
> -	      if (TYPE_CANONICAL (v) == v
> -		  && cp_check_qualified_type (v, variant, var_quals,
> -					      rqual, cr, false))
> -		break;
> +	      if (cp_check_qualified_type (v, variant, var_quals,
> +					   rqual, cr, false))
> +		{
> +		  /* The main variant will not match V, so PREV will never
> +		     be null.  */
> +		  TYPE_NEXT_VARIANT (prev) = TYPE_NEXT_VARIANT (variant);
> +		  break;
> +		}
>   	    TYPE_RAISES_EXCEPTIONS (variant) = raises;
>   
>   	    if (!v)
>   	      v = build_cp_fntype_variant (TYPE_CANONICAL (variant),
>   					   rqual, cr, false);
> -	    TYPE_CANONICAL (variant) = v;
> +	    TYPE_CANONICAL (variant) = TYPE_CANONICAL (v);
>   	  }
>   	else
>   	  TYPE_RAISES_EXCEPTIONS (variant) = raises;
> diff --git a/gcc/testsuite/g++.dg/cpp0x/noexcept72.C b/gcc/testsuite/g++.dg/cpp0x/noexcept72.C
> new file mode 100644
> index 00000000000..f1455b3b46b
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/noexcept72.C
> @@ -0,0 +1,21 @@
> +// PR c++/101715
> +// { dg-do compile { target c++11 } }
> +
> +template <typename T> struct S {
> +  S<T> bar() noexcept(T::value);  // #1
> +  S<T> foo() noexcept(T::value);  // #2
> +};
> +
> +template <typename T> S<T> S<T>::foo() noexcept(T::value) {}  // #3
> +
> +template <typename T> struct S2 {
> +  S2<T> bar1() noexcept(T::value);
> +  S2<T> bar2() noexcept(T::value);
> +  S2<T> bar3() noexcept(T::value);
> +  S2<T> bar4() noexcept(T::value);
> +  S2<T> bar5() noexcept(T::value);
> +  S2<T> baz() noexcept(T::value2);
> +  S2<T> foo() noexcept(T::value);
> +};
> +
> +template <typename T> S2<T> S2<T>::foo() noexcept(T::value) {}
> diff --git a/gcc/testsuite/g++.dg/cpp0x/noexcept73.C b/gcc/testsuite/g++.dg/cpp0x/noexcept73.C
> new file mode 100644
> index 00000000000..24524f3592a
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/noexcept73.C
> @@ -0,0 +1,13 @@
> +// PR c++/101715
> +// { dg-do compile { target c++11 } }
> +
> +template <typename T> struct S { };
> +
> +template<typename T>
> +struct A
> +{
> +    A& foo(A&&) noexcept((S<T>::value));
> +    A& assign(A&&) noexcept((S<T>::value));
> +};
> +template<typename T>
> +A<T>& A<T>::foo(A&&) noexcept((S<T>::value)) {}
> 
> base-commit: 45cae5b6392496028f35c5948f7fae0af81d135b