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

[Marek Polacek <polacek@redhat.com>]

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>.

class2loc will be filled with A<char>::W<U>, added while parsing
the specialization.

> > +  struct A<int>::W<int> w2;
> > +  class A<char>::W<int> w3;
> > +  class A<int>::W<int> w4; // { dg-warning "mismatched" }
> > +}
> > 
> > base-commit: 096f034a8f5df41f610e62c1592fb90a3f551cd5
> 

Marek