c++: maybe_constant_init and unevaluated operands [PR109030]

This testcase in this PR (already fixed by r13-6526-ge4692319fd5fc7)
demonstrates that maybe_constant_init can be called on an unevaluated
operand (e.g. from massage_init_elt) so this entry point should also
limit constant evaluation in that case, like maybe_constant_value does.

	PR c++/109030

gcc/cp/ChangeLog:

	* constexpr.cc (maybe_constant_init_1): For an unevaluated
	non-manifestly-constant operand, don't constant evaluate
	and instead call fold_to_constant as in maybe_constant_value.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/constexpr-inst2.C: New test.

gcc/cp/constexpr.cc

@@ -8796,6 +8796,10 @@ maybe_constant_init_1 (tree t, tree decl, bool allow_non_constant,
 			&& (TREE_STATIC (decl) || DECL_EXTERNAL (decl)));
       if (is_static)
 	manifestly_const_eval = true;
+
+      if (cp_unevaluated_operand && !manifestly_const_eval)
+	return fold_to_constant (t);
+
       t = cxx_eval_outermost_constant_expr (t, allow_non_constant, !is_static,
 					    mce_value (manifestly_const_eval),
 					    false, decl);

gcc/testsuite/g++.dg/cpp2a/constexpr-inst2.C

@@ -0,0 +1,12 @@
+// { dg-do compile { target c++11 } }
+
+struct __as_receiver {
+  int empty_env;
+};
+
+template<class T>
+constexpr int f(T t) {
+  return t.fail;
+};
+
+using type = decltype(__as_receiver{f(0)}); // OK, f<int> not instantiated