[gcc r13-6676] c++: constrained template friend class matching [PR96830]

[gcc r13-6676] c++: constrained template friend class matching [PR96830]

[Patrick Palka]

https://gcc.gnu.org/g:ec62dc95c4f8776c9f4eff2a9a06f9aef6a2d98a

commit r13-6676-gec62dc95c4f8776c9f4eff2a9a06f9aef6a2d98a
Author: Patrick Palka <ppalka@redhat.com>
Date:   Tue Mar 14 19:12:08 2023 -0400

    c++: constrained template friend class matching [PR96830]
    
    When instantiating a constrained template friend naming an already
    declared class template, tsubst_friend_class erroneously passes to
    redeclare_class_template the existing template's constraints instead of
    those of the friend declaration, which causes the constraint comparison
    check therein to trivially succeed and we fail to diagnose legitimate
    constraint mismatches.
    
            PR c++/96830
    
    gcc/cp/ChangeLog:
    
            * pt.cc (redeclare_class_template): Add missing "of" in
            constraint mismatch diagnostic.
            (tsubst_friend_class): For an already declared class template,
            substitute and pass the friend declaration's constraints to
            redeclare_class_template instead of passing the existing
            template's constraints.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/cpp2a/concepts-friend14.C: New test.

Diff:
---
 gcc/cp/pt.cc                                   |  8 ++++++--
 gcc/testsuite/g++.dg/cpp2a/concepts-friend14.C | 26 ++++++++++++++++++++++++++
 2 files changed, 32 insertions(+), 2 deletions(-)

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index c53d8e279c6..eb153adf383 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -6377,7 +6377,7 @@ redeclare_class_template (tree type, tree parms, tree cons)
   if (!cp_tree_equal (req1, req2))
     {
       auto_diagnostic_group d;
-      error_at (input_location, "redeclaration %q#D with different "
+      error_at (input_location, "redeclaration of %q#D with different "
                                 "constraints", tmpl);
       inform (DECL_SOURCE_LOCATION (tmpl),
               "original declaration appeared here");
@@ -11564,7 +11564,11 @@ tsubst_friend_class (tree friend_tmpl, tree args)
 						 tf_warning_or_error);
           location_t saved_input_location = input_location;
           input_location = DECL_SOURCE_LOCATION (friend_tmpl);
-          tree cons = get_constraints (tmpl);
+	  tree cons = get_constraints (friend_tmpl);
+	  ++processing_template_decl;
+	  cons = tsubst_constraint_info (cons, args, tf_warning_or_error,
+					 DECL_FRIEND_CONTEXT (friend_tmpl));
+	  --processing_template_decl;
           redeclare_class_template (TREE_TYPE (tmpl), parms, cons);
           input_location = saved_input_location;
 	}
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-friend14.C b/gcc/testsuite/g++.dg/cpp2a/concepts-friend14.C
new file mode 100644
index 00000000000..f46c7e4bdfb
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-friend14.C
@@ -0,0 +1,26 @@
+// PR c++/96830
+// { dg-do compile { target c++20 } }
+
+template<class T> requires true
+struct A {
+  template<class U> friend struct A;                   // { dg-error "different constraints" }
+  template<class U> requires (!!true) friend struct A; // { dg-error "different constraints" }
+};
+
+template<class T>
+struct B {
+  template<class U> requires true friend struct A;
+  template<class U> requires true friend struct B; // { dg-error "different constraints" }
+};
+
+template<class T> concept C = true;
+
+template<C T>
+struct D {
+  template<class U> friend struct D; // { dg-error "different constraints" }
+  template<C U> friend struct D;
+};
+
+template struct A<int>;
+template struct B<int>;
+template struct D<int>;