c++: Parenthesized-init of aggregates accepts invalid code [PR94885]

Here we have (conceptually *) something like

  struct B { };
  struct D : B { };
  D(0); // invalid

and in C++20 the ()-initialization has created a { 0 } constructor that
it tries to initialize an object of type D with.  We should reject
initializing an object of type B from 0, but we wrongly accept it because
process_init_constructor_record skips initializers for empty bases/fields:
       if (DECL_SIZE (field) && integer_zerop (DECL_SIZE (field))
           && !TREE_SIDE_EFFECTS (next))
         /* Don't add trivial initialization of an empty base/field to the
            constructor, as they might not be ordered the way the back-end
            expects.  */
         continue;
but here 'next' was error_mark_node, returned by massage_elt_init, so we
wound up with { } which would validly value-initialize the object.

[*] Usually digest_init in build_new_method_call_1 would detect this,
but in this case the instance is is_dummy_object and we don't call
digest just yet.

        PR c++/94885
        * typeck2.c (process_init_constructor_record): Return PICFLAG_ERRONEOUS
        if an initializer element was erroneous.

        * g++.dg/cpp2a/paren-init26.C: New test.

gcc/cp/ChangeLog

+2020-05-08  Marek Polacek  <polacek@redhat.com>
+
+	Backported from mainline
+	2020-05-01  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/94885
+	* typeck2.c (process_init_constructor_record): Return PICFLAG_ERRONEOUS
+	if an initializer element was erroneous.
+
 2020-05-07  Marek Polacek  <polacek@redhat.com>
 	
 	Backported from mainline

gcc/cp/typeck2.c

@@ -1661,7 +1661,11 @@ process_init_constructor_record (tree type, tree init, int nested, int flags,
 	      ++idx;
 	    }
 	}
-      if (next)
+      if (next == error_mark_node)
+	/* We skip initializers for empty bases/fields, so skipping an invalid
+	   one could make us accept invalid code.  */
+	return PICFLAG_ERRONEOUS;
+      else if (next)
 	/* Already handled above.  */;
       else if (DECL_INITIAL (field))
 	{

gcc/testsuite/g++.dg/cpp2a/paren-init26.C

+// PR c++/94885 - paren-init of aggregates accepts invalid code.
+// { dg-do compile { target c++2a } }
+
+template <typename T, typename = decltype(T(0))> // { dg-error "could not convert" }
+void foo();
+
+struct base {};
+struct derived : base {};
+
+void
+bar()
+{
+  foo<derived>(); // { dg-error "no matching function" }
+}