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

[Marek Polacek <polacek@redhat.com>]

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