Fix analyzing tuple of non-single elements as subtype of empty interf…

…ace.

src/libponyc/type/subtype.c

@@ -713,25 +713,19 @@ static bool is_tuple_sub_nominal(ast_t* sub, ast_t* super,
     if(ast_childcount(super_members) == 0)
     {
       // This is an empty interface, so we let the tuple be a subtype if the
-      // caps of the elements of the tuple are subcaps of the supertype cap.
+      // caps of the elements of the tuple are subcaps of the interface cap.
       for(ast_t* child = ast_child(sub);
         child != NULL;
         child = ast_sibling(child))
       {
-        if(!is_sub_cap_and_eph(child, super, check_cap, errorf, opt))
+        if(!is_x_sub_x(child, super, check_cap, errorf, opt))
         {
           if(errorf != NULL)
           {
-            ast_t* child_cap = cap_fetch(child);
-            ast_t* child_eph = ast_sibling(child_cap);
-            ast_t* super_cap = cap_fetch(super);
-            ast_t* super_eph = ast_sibling(super_cap);
-
             ast_error_frame(errorf, child,
-              "%s is not a subtype of %s: %s%s is not a subcap of %s%s",
+              "%s is not a subtype of %s: %s is not a subtype of %s",
               ast_print_type(sub), ast_print_type(super),
-              ast_print_type(child_cap), ast_print_type(child_eph),
-              ast_print_type(super_cap), ast_print_type(super_eph));
+              ast_print_type(child), ast_print_type(super));
           }
           return false;
         }

test/libponyc/type_check_subtype.cc

@@ -1097,6 +1097,20 @@ TEST_F(SubTypeTest, TupleTagTagNotSubAnyVal)
 }
 
 
+TEST_F(SubTypeTest, TupleRefValOrValSubAnyBox)
+{
+  const char* src =
+    "class C\n"
+    "class D\n"
+
+    "primitive P\n"
+    "  fun apply(x: (C ref, (C val | D val))) =>\n"
+    "    let x': Any box = consume x";
+
+  TEST_COMPILE(src);
+}
+
+
 TEST_F(SubTypeTest, TupleValTagSubAnyShare)
 {
   const char* src =