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

[Jason Merrill <jason@redhat.com>]

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