vala: Search in all interfaces for an implementation match and don't bail early

The first imcompatible method match doesn't mean there is no match for another
interface. This might mean an implementation is missing which will be covered
by the semantic check of the parent class.

Fixes https://gitlab.gnome.org/GNOME/vala/issues/548

1 files changed, 21 insertions, 8 deletions

diff --git a/vala/valamethod.vala b/vala/valamethod.vala
index 294f7b397..bd203ab36 100644
--- a/vala/valamethod.vala
+++ b/vala/valamethod.vala
@@ -631,6 +631,11 @@ public class Vala.Method : Subroutine, Callable {
 	}
 
 	private void find_base_interface_method (Class cl) {
+		Method? base_match = null;
+
+		string? invalid_error = null;
+		Method? invalid_base_match = null;
+
 		foreach (DataType type in cl.get_base_types ()) {
 			if (type.data_type is Interface) {
 				if (base_interface_type != null && base_interface_type.data_type != type.data_type) {
@@ -661,20 +666,28 @@ public class Vala.Method : Subroutine, Callable {
 
 						string invalid_match = null;
 						if (!compatible (base_method, out invalid_match)) {
-							error = true;
-							var base_method_type = new MethodType (base_method);
-							Report.error (source_reference, "overriding method `%s' is incompatible with base method `%s': %s.".printf (get_full_name (), base_method_type.to_prototype_string (), invalid_match));
-							return;
+							invalid_error = invalid_match;
+							invalid_base_match = base_method;
+						} else {
+							base_match = base_method;
+							break;
 						}
-
-						_base_interface_method = base_method;
-						copy_attribute_double (base_method, "CCode", "instance_pos");
-						return;
 					}
 				}
 			}
 		}
 
+		if (base_match != null) {
+			_base_interface_method = base_match;
+			copy_attribute_double (base_match, "CCode", "instance_pos");
+			return;
+		} else if (invalid_base_match != null) {
+			error = true;
+			var base_method_type = new MethodType (invalid_base_match);
+			Report.error (source_reference, "overriding method `%s' is incompatible with base method `%s': %s.".printf (get_full_name (), base_method_type.to_prototype_string (), invalid_error));
+			return;
+		}
+
 		if (base_interface_type != null) {
 			Report.error (source_reference, "`%s': no suitable interface method found to implement".printf (get_full_name ()));
 		}