Re: [PATCH] c++: ICE with -Wmismatched-tags and member template [PR106259]

[Jason Merrill <jason@redhat.com>]

On 3/1/23 16:40, Marek Polacek wrote:
> On Wed, Mar 01, 2023 at 04:30:16PM -0500, Jason Merrill wrote:
>> On 3/1/23 15:33, Marek Polacek wrote:
>>> -Wmismatched-tags warns about the (harmless) struct/class mismatch.
>>> For, e.g.,
>>>
>>>     template<typename T> struct A { };
>>>     class A<int> a;
>>>
>>> it works by adding A<T> to the class2loc hash table while parsing the
>>> class-head and then, while parsing the elaborate type-specifier, we
>>> add A<int>.  At the end of c_parse_file we go through the table and
>>> warn about the class-key mismatches.  In this PR we crash though; we
>>> have
>>>
>>>     template<typename T> struct A {
>>>       template<typename U> struct W { };
>>>     };
>>>     struct A<int>::W<int> w; // #1
>>>
>>> where while parsing A and #1 we've stashed
>>>      A<T>
>>>      A<T>::W<U>
>>>      A<int>::W<int>
>>> into class2loc.  Then in class_decl_loc_t::diag_mismatched_tags TYPE
>>> is A<int>::W<int>, and specialization_of gets us A<int>::W<U>, which
>>> is not in class2loc, so we crash on gcc_assert (cdlguide).  But it's
>>> OK not to have found A<int>::W<U>, we should just look one "level" up,
>>> that is, A<T>::W<U>.
>>>
>>> It's important to handle class specializations, so e.g.
>>>
>>>     template<>
>>>     struct A<char> {
>>>       template<typename U>
>>>       class W { };
>>>     };
>>>
>>> where W's class-key is different than in the primary template above,
>>> so we should warn depending on whether we're looking into A<char>
>>> or into a different instantiation.
>>>
>>> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
>>>
>>> 	PR c++/106259
>>>
>>> gcc/cp/ChangeLog:
>>>
>>> 	* parser.cc (class_decl_loc_t::diag_mismatched_tags): If the first
>>> 	lookup of SPEC didn't find anything, try to look for
>>> 	most_general_template.
>>>
>>> gcc/testsuite/ChangeLog:
>>>
>>> 	* g++.dg/warn/Wmismatched-tags-11.C: New test.
>>> ---
>>>    gcc/cp/parser.cc                              | 30 +++++++++++++++----
>>>    .../g++.dg/warn/Wmismatched-tags-11.C         | 23 ++++++++++++++
>>>    2 files changed, 47 insertions(+), 6 deletions(-)
>>>    create mode 100644 gcc/testsuite/g++.dg/warn/Wmismatched-tags-11.C
>>>
>>> diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
>>> index 1a124f5395e..b528ee7b1d9 100644
>>> --- a/gcc/cp/parser.cc
>>> +++ b/gcc/cp/parser.cc
>>> @@ -34473,14 +34473,32 @@ class_decl_loc_t::diag_mismatched_tags (tree type_decl)
>>>    	 be (and inevitably is) at index zero.  */
>>>          tree spec = specialization_of (type);
>>>          cdlguide = class2loc.get (spec);
>>> +      /* It's possible that we didn't find SPEC.  Consider:
>>> +
>>> +	   template<typename T> struct A {
>>> +	     template<typename U> struct W { };
>>> +	   };
>>> +	   struct A<int>::W<int> w; // #1
>>> +
>>> +	 where while parsing A and #1 we've stashed
>>> +	   A<T>
>>> +	   A<T>::W<U>
>>> +	   A<int>::W<int>
>>> +	 into CLASS2LOC.  If TYPE is A<int>::W<int>, specialization_of
>>> +	 will yield A<int>::W<U> which may be in CLASS2LOC if we had
>>> +	 an A<int> class specialization, but otherwise won't be in it.
>>> +	 So try to look up A<T>::W<U>.  */
>>> +      if (!cdlguide)
>>> +	{
>>> +	  spec = DECL_TEMPLATE_RESULT (most_general_template (spec));
>>
>> Would it make sense to only look at most_general_template, not A<int>::W<U>
>> at all?
> 
> I think that would break with class specialization, as in...
> 
>>> +template<typename T> struct A {
>>> +  template<typename U>
>>> +  struct W { };
>>> +};
>>> +
>>> +template<>
>>> +struct A<char> {
>>> +  template<typename U>
>>> +  class W { };
>>> +};
>>> +
>>> +void
>>> +g ()
>>> +{
>>> +  struct A<char>::W<int> w1; // { dg-warning "mismatched" }
> 
> ...this, where we should first look into A<char>, and only if not
> found, go to A<T>.

I'd expect the

>       /* Stop if we run into an explicitly specialized class template.  */

code in most_general_template to avoid that problem.

Jason