vala: Improve error reporting for invalid interface prerequisites

Fixes https://gitlab.gnome.org/GNOME/vala/issues/437

tests/Makefile.am

@@ -702,6 +702,8 @@ TESTS = \
 	semantic/foreach-next-void.test \
 	semantic/foreach-wrong-types.test \
 	semantic/initializer-unknown-type.test \
+	semantic/interface-prerequisite-invalid.test \
+	semantic/interface-prerequisite-less-accessible.test \
 	semantic/localvariable-owned-to-unowned.test \
 	semantic/localvariable-var-static-access-instance-field.test \
 	semantic/localvariable-var-static-access-instance-method.test \

tests/semantic/interface-prerequisite-invalid.test

@@ -0,0 +1,11 @@
+Invalid Code
+
+struct Bar {
+	public int i;
+}
+
+interface Foo : Bar {
+}
+
+void main () {
+}

tests/semantic/interface-prerequisite-less-accessible.test

@@ -0,0 +1,10 @@
+Invalid Code
+
+class Bar {
+}
+
+public interface Foo : Bar {
+}
+
+void main () {
+}

vala/valainterface.vala

@@ -214,28 +214,21 @@ public class Vala.Interface : ObjectTypeSymbol {
 		/* check prerequisites */
 		Class prereq_class = null;
 		foreach (DataType prereq in get_prerequisites ()) {
-			TypeSymbol class_or_interface = prereq.data_type;
-			/* skip on previous errors */
-			if (class_or_interface == null) {
+			if (!(prereq is ObjectType)) {
 				error = true;
-				continue;
-			}
-
-			if (!(class_or_interface is ObjectTypeSymbol)) {
-				error = true;
-				Report.error (source_reference, "Prerequisite `%s' of interface `%s' is not a class or interface".printf (get_full_name (), class_or_interface.to_string ()));
+				Report.error (source_reference, "Prerequisite `%s' of interface `%s' is not a class or interface".printf (prereq.to_string (), get_full_name ()));
 				return false;
 			}
 
 			/* interfaces are not allowed to have multiple instantiable prerequisites */
-			if (class_or_interface is Class) {
+			if (prereq.data_type is Class) {
 				if (prereq_class != null) {
 					error = true;
-					Report.error (source_reference, "%s: Interfaces cannot have multiple instantiable prerequisites (`%s' and `%s')".printf (get_full_name (), class_or_interface.get_full_name (), prereq_class.get_full_name ()));
+					Report.error (source_reference, "%s: Interfaces cannot have multiple instantiable prerequisites (`%s' and `%s')".printf (get_full_name (), prereq.data_type.get_full_name (), prereq_class.get_full_name ()));
 					return false;
 				}
 
-				prereq_class = (Class) class_or_interface;
+				prereq_class = (Class) prereq.data_type;
 			}
 		}