c++: desig init in presence of list ctor [PR109871]

add_list_candidates has logic to reject designated initialization of a
non-aggregate type, but this is inadvertently being suppressed if the type
has a list constructor due to the order of case analysis, which in the
below testcase leads to us incorrectly treating the initializer list as if
it's non-designated.  This patch fixes this by making us check for invalid
designated initialization sooner.

	PR c++/109871

gcc/cp/ChangeLog:

	* call.cc (add_list_candidates): Check for invalid designated
	initialization sooner and even for types that have a list
	constructor.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/desig27.C: New test.

(cherry picked from commit d5e5007c4b534391c0a97be56f6024fde1a88682)

gcc/cp/call.cc

@@ -4128,6 +4128,14 @@ add_list_candidates (tree fns, tree first_arg,
   if (CONSTRUCTOR_NELTS (init_list) == 0
       && TYPE_HAS_DEFAULT_CONSTRUCTOR (totype))
     ;
+  else if (CONSTRUCTOR_IS_DESIGNATED_INIT (init_list)
+	   && !CP_AGGREGATE_TYPE_P (totype))
+    {
+      if (complain & tf_error)
+	error ("designated initializers cannot be used with a "
+	       "non-aggregate type %qT", totype);
+      return;
+    }
   /* If the class has a list ctor, try passing the list as a single
      argument first, but only consider list ctors.  */
   else if (TYPE_HAS_LIST_CTOR (totype))
@@ -4139,14 +4147,6 @@ add_list_candidates (tree fns, tree first_arg,
       if (any_strictly_viable (*candidates))
 	return;
     }
-  else if (CONSTRUCTOR_IS_DESIGNATED_INIT (init_list)
-	   && !CP_AGGREGATE_TYPE_P (totype))
-    {
-      if (complain & tf_error)
-	error ("designated initializers cannot be used with a "
-	       "non-aggregate type %qT", totype);
-      return;
-    }
 
   /* Expand the CONSTRUCTOR into a new argument vec.  */
   vec<tree, va_gc> *new_args;

gcc/testsuite/g++.dg/cpp2a/desig27.C

@@ -0,0 +1,16 @@
+// PR c++/109871
+// { dg-do compile { target c++11 } }
+// { dg-options "" }
+
+#include <initializer_list>
+
+struct vector {
+  vector(std::initializer_list<int>); // #1
+  vector(int); // #2
+};
+
+void f(vector);
+
+int main() {
+  f({.blah = 42}); // { dg-error "designated" } previously incorrectly selected #2
+}