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 51abc86..6a2c4f3 100644 (file)
--- 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 (file)
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; };