[gcc r13-6694] c++: injected class name as default ttp arg [PR58538]

[Jason Merrill]

https://gcc.gnu.org/g:40c1352c5a4530350012d6a922435cf491663daa

commit r13-6694-g40c1352c5a4530350012d6a922435cf491663daa
Author: Jason Merrill <jason@redhat.com>
Date:   Tue Mar 14 23:16:21 2023 -0400

    c++: injected class name as default ttp arg [PR58538]
    
    This function needs to handle this case like convert_template_argument.
    
            PR c++/58538
    
    gcc/cp/ChangeLog:
    
            * semantics.cc (check_template_template_default_arg): Check
            maybe_get_template_decl_from_type_decl.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/template/ttp7.C: Remove expected error.

Diff:
---
 gcc/cp/semantics.cc                  | 9 +++++++--
 gcc/testsuite/g++.dg/template/ttp7.C | 2 +-
 2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/gcc/cp/semantics.cc b/gcc/cp/semantics.cc
index d67a9b26719..57dd7b66da8 100644
--- a/gcc/cp/semantics.cc
+++ b/gcc/cp/semantics.cc
@@ -3468,8 +3468,13 @@ check_template_template_default_arg (tree argument)
       && TREE_CODE (argument) != UNBOUND_CLASS_TEMPLATE)
     {
       if (TREE_CODE (argument) == TYPE_DECL)
-	error ("invalid use of type %qT as a default value for a template "
-	       "template-parameter", TREE_TYPE (argument));
+	{
+	  if (tree t = maybe_get_template_decl_from_type_decl (argument))
+	    if (TREE_CODE (t) == TEMPLATE_DECL)
+	      return t;
+	  error ("invalid use of type %qT as a default value for a template "
+		 "template-parameter", TREE_TYPE (argument));
+	}
       else
 	error ("invalid default argument for a template template parameter");
       return error_mark_node;
diff --git a/gcc/testsuite/g++.dg/template/ttp7.C b/gcc/testsuite/g++.dg/template/ttp7.C
index 0bcaa8f7292..672077e7e7d 100644
--- a/gcc/testsuite/g++.dg/template/ttp7.C
+++ b/gcc/testsuite/g++.dg/template/ttp7.C
@@ -11,6 +11,6 @@ template<template<int> class = A<0> > struct B2 {};  // { dg-error "as a default
 
 template <typename T>
 struct S {
-  template <template <typename> class = S>   struct I1 {};  // { dg-error "as a default value" }
+  template <template <typename> class = S>   struct I1 {}; // PR c++/58538
   template <template <typename> class = ::S> struct I2 {};
 };