From 52c5933f5838adb7bf6035e648b770b4958d137f Mon Sep 17 00:00:00 2001
From: Jason Merrill <jason@redhat.com>
Date: Mon, 11 May 2020 15:46:59 -0400
Subject: [PATCH] c++: Fix specialization of constrained member template.

The resolution of comment CA104 clarifies that we need to do direct
substitution of constraints in order to determine which member template
corresponds to an explicit specialization.

gcc/cp/ChangeLog
2020-05-11  Jason Merrill  <jason@redhat.com>

	Resolve C++20 NB comment CA104
	* pt.c (determine_specialization): Compare constraints for
	specialization of member template of class instantiation.
---
 gcc/cp/ChangeLog                            |  6 +++++
 gcc/cp/pt.c                                 | 28 ++++++++++++++++++---
 gcc/testsuite/g++.dg/cpp2a/concepts-spec1.C | 10 ++++++++
 gcc/testsuite/g++.dg/template/nontype18.C   |  2 +-
 4 files changed, 42 insertions(+), 4 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-spec1.C

diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index 5195a0a043f..5cf9dda42e2 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,3 +1,9 @@
+2020-05-11  Jason Merrill  <jason@redhat.com>
+
+	Resolve C++20 NB comment CA104
+	* pt.c (determine_specialization): Compare constraints for
+	specialization of member template of class instantiation.
+
 2020-05-11  Jason Merrill  <jason@redhat.com>
 
 	PR c++/92583
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 86f1bb7470d..84864561c25 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -2282,8 +2282,29 @@ determine_specialization (tree template_id,
              below. */
 	  if (tsk == tsk_template)
 	    {
-	      if (compparms (fn_arg_types, decl_arg_types))
-		candidates = tree_cons (NULL_TREE, fn, candidates);
+	      if (!comp_template_parms (DECL_TEMPLATE_PARMS (fn),
+					current_template_parms))
+		continue;
+	      if (!same_type_p (TREE_TYPE (TREE_TYPE (decl)),
+				TREE_TYPE (TREE_TYPE (fn))))
+		continue;
+	      if (!compparms (fn_arg_types, decl_arg_types))
+		continue;
+
+	      tree freq = get_trailing_function_requirements (fn);
+	      tree dreq = get_trailing_function_requirements (decl);
+	      if (!freq != !dreq)
+		continue;
+	      if (freq)
+		{
+		  tree fargs = DECL_TI_ARGS (fn);
+		  tsubst_flags_t complain = tf_none;
+		  freq = tsubst_constraint (freq, fargs, complain, fn);
+		  if (!cp_tree_equal (freq, dreq))
+		    continue;
+		}
+
+	      candidates = tree_cons (NULL_TREE, fn, candidates);
 	      continue;
 	    }
 
@@ -2472,7 +2493,8 @@ determine_specialization (tree template_id,
       *targs_out = copy_node (DECL_TI_ARGS (fn));
 
       /* Propagate the candidate's constraints to the declaration.  */
-      set_constraints (decl, get_constraints (fn));
+      if (tsk != tsk_template)
+	set_constraints (decl, get_constraints (fn));
 
       /* DECL is a re-declaration or partial instantiation of a template
 	 function.  */
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-spec1.C b/gcc/testsuite/g++.dg/cpp2a/concepts-spec1.C
new file mode 100644
index 00000000000..5001813d7b7
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-spec1.C
@@ -0,0 +1,10 @@
+// Example from CA 104 proposal.
+// { dg-do compile { target concepts } }
+
+template <class T> concept C = sizeof(T) == 8;
+template <class T> struct A {
+  template <class U> U f(U) requires C<typename T::type>; // #1
+  template <class U> U f(U) requires C<T>; // #2
+};
+
+template <> template <class U> U A<int>::f(U) requires C<int> { } // OK, specializes #2
diff --git a/gcc/testsuite/g++.dg/template/nontype18.C b/gcc/testsuite/g++.dg/template/nontype18.C
index cbe0a1b5a0d..b68416dca61 100644
--- a/gcc/testsuite/g++.dg/template/nontype18.C
+++ b/gcc/testsuite/g++.dg/template/nontype18.C
@@ -5,4 +5,4 @@ template<int I> struct A
     template<typename T> void foo();
 };
 
-template<int I> template<typename T> void A<0>::foo() {} // { dg-error "template parameter" }
+template<int I> template<typename T> void A<0>::foo() {} // { dg-error "" }
-- 
2.30.2