c++: friend implicit template instantiation [PR91618]

This rule that for a friend with a qualified name we try to find a
matching template was already in C++98, but it seems we never implemented
it, and nobody reported it until 2019.

This patch sets DECL_IMPLICIT_INSTANTIATION to signal to
check_explicit_specialization that we want to find a template, like
grokfndecl already did for explicit template args.  check_classfn also needs
to call it, as check_classfn is called after the call to
check_explicit_specialization in grokfndecl, whereas the call to
set_decl_namespace comes sooner.  This inconsistency is inelegant, but safer
at this point in the release cycle; I'll unify them in stage 1.

	PR c++/91618
	PR c++/96604

gcc/cp/ChangeLog:

	* name-lookup.cc (set_decl_namespace): Set
	DECL_IMPLICIT_INSTANTIATION if no non-template match.
	* pt.cc (check_explicit_specialization): Check it.
	* decl2.cc (check_classfn): Call it.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/friend7.C: Remove xfail.
	* g++.dg/template/friend72.C: New test.
	* g++.dg/template/friend72a.C: New test.
	* g++.dg/template/friend73.C: New test.

1 files changed, 15 insertions, 0 deletions

diff --git a/gcc/cp/name-lookup.cc b/gcc/cp/name-lookup.cc
index b7fc278..118da09 100644
--- a/gcc/cp/name-lookup.cc
+++ b/gcc/cp/name-lookup.cc
@@ -5898,6 +5898,7 @@ set_decl_namespace (tree decl, tree scope, bool friendp)
 
   tree found = NULL_TREE;
   bool hidden_p = false;
+  bool saw_template = false;
 
   for (lkp_iterator iter (old); iter; ++iter)
     {
@@ -5922,6 +5923,20 @@ set_decl_namespace (tree decl, tree scope, bool friendp)
 	  found = ofn;
 	  hidden_p = iter.hidden_p ();
 	}
+      else if (TREE_CODE (decl) == FUNCTION_DECL
+	       && TREE_CODE (ofn) == TEMPLATE_DECL)
+	saw_template = true;
+    }
+
+  if (!found && friendp && saw_template)
+    {
+      /* "[if no non-template match is found,] each remaining function template
+	 is replaced with the specialization chosen by deduction from the
+	 friend declaration or discarded if deduction fails."
+
+	 So tell check_explicit_specialization to look for a match.  */
+      SET_DECL_IMPLICIT_INSTANTIATION (decl);
+      return;
     }
 
   if (found)