From: Patrick Palka <ppalka@redhat.com>
Date: Tue, 14 Mar 2023 23:12:08 +0000 (-0400)
Subject: c++: constrained template friend class matching [PR96830]
X-Git-Tag: basepoints/gcc-14~531
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=ec62dc95c4f8776c9f4eff2a9a06f9aef6a2d98a;p=thirdparty%2Fgcc.git

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 --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index c53d8e279c62..eb153adf383d 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 000000000000..f46c7e4bdfb3
--- /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>;