[gcc r12-9217] c++: ICE with -Wmismatched-tags and member template [PR106259]

[Marek Polacek <mpolacek@gcc.gnu.org>]

https://gcc.gnu.org/g:3a2310d4b229e707bbc5440150bf180e0499273a

commit r12-9217-g3a2310d4b229e707bbc5440150bf180e0499273a
Author: Marek Polacek <polacek@redhat.com>
Date:   Wed Mar 1 14:28:46 2023 -0500

    c++: ICE with -Wmismatched-tags and member template [PR106259]
    
    -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.
    
            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.
    
    (cherry picked from commit 71afd0628419c5d670701cb35bc9860380c7d9fb)

Diff:
---
 gcc/cp/parser.cc                                | 30 ++++++++++++++++++++-----
 gcc/testsuite/g++.dg/warn/Wmismatched-tags-11.C | 23 +++++++++++++++++++
 2 files changed, 47 insertions(+), 6 deletions(-)

diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index 39f7eda4b56..e5211d8075e 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -33670,14 +33670,32 @@ 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));
+	  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" }
+}