From: Jason Merrill <jason@redhat.com>
Date: Thu, 25 Feb 2021 21:47:53 +0000 (-0500)
Subject: c++: Fix class NTTP constness handling [PR98810]
X-Git-Tag: releases/gcc-10.3.0~252
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=bf49d83570ddb4df7893c3d605f7fc89db13792d;p=thirdparty%2Fgcc.git

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.
---

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index fe45b7944cce..73bc06972ddf 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -16651,13 +16651,26 @@ tsubst_copy (tree t, tree args, tsubst_flags_t complain, tree in_decl)
 		  tree type = tsubst (TREE_TYPE (t), args, complain, in_decl);
 		  return build1 (code, type, op);
 		}
-	      else
+	      else if (!CP_TYPE_CONST_P (TREE_TYPE (op)))
 		{
-		  gcc_assert (CP_TYPE_CONST_P (TREE_TYPE (op))
-			      || (TREE_CODE (op) == IMPLICIT_CONV_EXPR
-				  && IMPLICIT_CONV_EXPR_NONTYPE_ARG (op)));
-		  return 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);
+		  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.  */
+		    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 000000000000..ddd64d6165ba
--- /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; }