From 7c657339d6a4a671b4cd8bc62ba4e0df6bfc7c72 Mon Sep 17 00:00:00 2001
From: Jason Merrill <jason@redhat.com>
Date: Thu, 25 Feb 2021 16:47:53 -0500
Subject: [PATCH] c++: Fix class NTTP constness handling [PR98810]

Here, when substituting still-dependent args into an alias template, we see
a non-const type because the default argument is non-const, and is not a
template parm object because it's still dependent.

gcc/cp/ChangeLog:

	PR c++/98810
	* pt.c (tsubst_copy) [VIEW_CONVERT_EXPR]: Add const
	to a class non-type template argument that needs it.

gcc/testsuite/ChangeLog:

	PR c++/98810
	* g++.dg/cpp2a/nontype-class-defarg1.C: New test.
---
 gcc/cp/pt.c                                        | 27 ++++++++++++++--------
 gcc/testsuite/g++.dg/cpp2a/nontype-class-defarg1.C |  6 +++++
 2 files changed, 24 insertions(+), 9 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/nontype-class-defarg1.C

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 51abc86..6a2c4f3 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -16826,18 +16826,27 @@ tsubst_copy (tree t, tree args, tsubst_flags_t complain, tree in_decl)
 	    {
 	      /* Wrapper to make a C++20 template parameter object const.  */
 	      op = tsubst_copy (op, args, complain, in_decl);
-	      if (TREE_CODE (op) == TEMPLATE_PARM_INDEX)
+	      if (!CP_TYPE_CONST_P (TREE_TYPE (op)))
 		{
+		  /* The template argument is not const, presumably because
+		     it is still dependent, and so not the const template parm
+		     object.  */
 		  tree type = tsubst (TREE_TYPE (t), args, complain, in_decl);
-		  return build1 (code, type, op);
-		}
-	      else
-		{
-		  gcc_assert (CP_TYPE_CONST_P (TREE_TYPE (op))
-			      || (TREE_CODE (op) == IMPLICIT_CONV_EXPR
-				  && IMPLICIT_CONV_EXPR_NONTYPE_ARG (op)));
-		  return op;
+		  gcc_checking_assert (same_type_ignoring_top_level_qualifiers_p
+				       (type, TREE_TYPE (op)));
+		  if (TREE_CODE (op) == CONSTRUCTOR
+		      || TREE_CODE (op) == IMPLICIT_CONV_EXPR)
+		    {
+		      /* Don't add a wrapper to these.  */
+		      op = copy_node (op);
+		      TREE_TYPE (op) = type;
+		    }
+		  else
+		    /* Do add a wrapper otherwise (in particular, if op is
+		       another TEMPLATE_PARM_INDEX).  */
+		    op = build1 (code, type, op);
 		}
+	      return op;
 	    }
 	  /* force_paren_expr can also create a VIEW_CONVERT_EXPR.  */
 	  else if (code == VIEW_CONVERT_EXPR && REF_PARENTHESIZED_P (t))
diff --git a/gcc/testsuite/g++.dg/cpp2a/nontype-class-defarg1.C b/gcc/testsuite/g++.dg/cpp2a/nontype-class-defarg1.C
new file mode 100644
index 0000000..85e50ff
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/nontype-class-defarg1.C
@@ -0,0 +1,6 @@
+// PR c++/98810
+// { dg-do compile { target c++20 } }
+
+template <auto> struct a {};
+template <int i, a <i> s = a <i> {}> using b = a <s>;
+template <int i> constexpr auto g (const b <i> &) { return true; };
-- 
2.7.4