c++: constness of decltype of NTTP object [PR99631]

This corrects resolving decltype of a (class) NTTP object as per
[dcl.type.decltype]/1.2 and [temp.param]/6 in the type-dependent case.

Note that in the non-dependent case we resolve the decltype ahead of
time, in which case finish_decltype_type drops the const VIEW_CONVERT_EXPR
wrapper around the TEMPLATE_PARM_INDEX, and the latter has the desired
non-const type.

In the type-dependent case, at instantiation time tsubst drops the
VIEW_CONVERT_EXPR since the substituted NTTP is the already-const object
created by get_template_parm_object.  So in this case finish_decltype_type
sees the const object, which this patch now adds special handling for.

	PR c++/99631

gcc/cp/ChangeLog:

	* semantics.cc (finish_decltype_type): For an NTTP object,
	return its type modulo cv-quals.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/nontype-class60.C: New test.

2 files changed, 26 insertions, 0 deletions

diff --git a/gcc/cp/semantics.cc b/gcc/cp/semantics.cc
index 4109ac3..80ef136 100644
--- a/gcc/cp/semantics.cc
+++ b/gcc/cp/semantics.cc
@@ -11606,6 +11606,14 @@ finish_decltype_type (tree expr, bool id_expression_or_member_access_p,
         case TEMPLATE_PARM_INDEX:
 	  expr = mark_type_use (expr);
           type = TREE_TYPE (expr);
+	  if (VAR_P (expr) && DECL_NTTP_OBJECT_P (expr))
+	    {
+	      /* decltype of an NTTP object is the type of the template
+		 parameter, which is the object type modulo cv-quals.  */
+	      int quals = cp_type_quals (type);
+	      gcc_checking_assert (quals & TYPE_QUAL_CONST);
+	      type = cv_unqualified (type);
+	    }
           break;
 
         case ERROR_MARK:
diff --git a/gcc/testsuite/g++.dg/cpp2a/nontype-class60.C b/gcc/testsuite/g++.dg/cpp2a/nontype-class60.C
new file mode 100644
index 0000000..9e8030b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/nontype-class60.C
@@ -0,0 +1,18 @@
+// PR c++/99631
+// { dg-do compile { target c++20 } }
+
+struct A { };
+
+template<auto V>
+void f() {
+  static_assert(__is_same(decltype(V), A));
+}
+
+template<class T, T V>
+void g() {
+  static_assert(__is_same(decltype(V), A));
+}
+
+constexpr A a;
+template void f<a>();
+template void g<A, A{}>();