c++: Fix paren init of aggregates in unevaluated context [PR92947]

When I implemented C++20 parenthesized initialization of aggregates
I introduced this bogus cp_unevaluated_operand check, thus disabling
this feature in unevaluated context.  Oop.

Removing the check turned up another bug: I wasn't checking the
return value of digest_init.  So when constructible_expr called
build_new_method_call_1 to see if we can construct one type from
another, it got back a bogus INIT_EXPR that looked something like
*(struct T &) 1 = <<< error >>>.  But that isn't the error_mark_node,
so constructible_expr thought we had been successful in creating the
ctor call, and it gave the wrong answer.  Covered by paren-init17.C.

	PR c++/92947 - Paren init of aggregates in unevaluated context.
	* call.c (build_new_method_call_1): Don't check
	cp_unevaluated_operand.  Check the return value of digest_init.

	* g++.dg/cpp2a/paren-init21.C: New test.

gcc/cp/ChangeLog

+2020-02-07  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/92947 - Paren init of aggregates in unevaluated context.
+	* call.c (build_new_method_call_1): Don't check
+	cp_unevaluated_operand.  Check the return value of digest_init.
+
 2020-02-06  Jason Merrill  <jason@redhat.com>
 
 	PR c++/92654

gcc/cp/call.c

@@ -10179,7 +10179,6 @@ build_new_method_call_1 (tree instance, tree fns, vec<tree, va_gc> **args,
 	 the two.  */
       if (DECL_CONSTRUCTOR_P (fn)
 	  && !(flags & LOOKUP_ONLYCONVERTING)
-	  && !cp_unevaluated_operand
 	  && cxx_dialect >= cxx2a
 	  && CP_AGGREGATE_TYPE_P (basetype)
 	  && !user_args->is_empty ())
@@ -10194,6 +10193,8 @@ build_new_method_call_1 (tree instance, tree fns, vec<tree, va_gc> **args,
 	  else
 	    {
 	      ctor = digest_init (basetype, ctor, complain);
+	      if (ctor == error_mark_node)
+		return error_mark_node;
 	      ctor = build2 (INIT_EXPR, TREE_TYPE (instance), instance, ctor);
 	      TREE_SIDE_EFFECTS (ctor) = true;
 	      return ctor;

gcc/testsuite/ChangeLog

+2020-02-07  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/92947 - Paren init of aggregates in unevaluated context.
+	* g++.dg/cpp2a/paren-init21.C: New test.
+
 2020-02-07  Will Schmidt  <will_schmidt@vnet.ibm.com>
 
 	* testsuite/gcc.target/powerpc/pr92923-1.c: Add -mvsx.

gcc/testsuite/g++.dg/cpp2a/paren-init21.C

+// PR c++/92947 - Paren init of aggregates in unevaluated context.
+// { dg-do compile { target c++2a } }
+
+struct A {
+  int a;
+  int b;
+};
+
+int main()
+{
+  static_assert(__is_constructible(A, int, int));
+  decltype(A(1,2)) foo;
+  bool b = noexcept(A(1,2));
+}