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

[Jason Merrill <jason@redhat.com>]

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?

> +	  cdlguide = class2loc.get (spec);
> +	}
> +      /* Now we really should have found something.  */
>         gcc_assert (cdlguide != NULL);
>       }
> -  else
> -    {
> -      /* Skip declarations that consistently use the same class-key.  */
> -      if (def_class_key != none_type)
> -	return;
> -    }
> +  /* Skip declarations that consistently use the same class-key.  */
> +  else if (def_class_key != none_type)
> +    return;
>   
>     /* Set if a definition for the class has been seen.  */
>     const bool def_p = cdlguide->def_p ();
> diff --git a/gcc/testsuite/g++.dg/warn/Wmismatched-tags-11.C b/gcc/testsuite/g++.dg/warn/Wmismatched-tags-11.C
> new file mode 100644
> index 00000000000..6c4e571726a
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/warn/Wmismatched-tags-11.C
> @@ -0,0 +1,23 @@
> +// PR c++/106259
> +// { dg-do compile }
> +// { dg-options "-Wmismatched-tags" }
> +
> +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" }
> +  struct A<int>::W<int> w2;
> +  class A<char>::W<int> w3;
> +  class A<int>::W<int> w4; // { dg-warning "mismatched" }
> +}
> 
> base-commit: 096f034a8f5df41f610e62c1592fb90a3f551cd5