cgen,table: fix generic interface used in an implements clause (fix #22161) (#22165)

vlib/v/ast/table.v

@@ -2087,7 +2087,7 @@ pub fn (mut t Table) unwrap_generic_type(typ Type, generic_names []string, concr
 				}
 			}
 			mut info := ts.info
-			info.is_generic = false
+			info.is_generic = final_concrete_types.any(it.has_flag(.generic))
 			info.concrete_types = final_concrete_types
 			info.parent_type = typ.set_flag(.generic)
 			info.fields = fields

vlib/v/tests/interface_generic_implements_test.v

@@ -0,0 +1,19 @@
+interface Foo[T] {
+	val T
+	foo() T
+}
+
+struct Bar[T] implements Foo[T] {
+	val T
+}
+
+fn (b Bar[T]) foo() T {
+	return b.val
+}
+
+fn test_main() {
+	b := Bar{
+		val: 0
+	}
+	assert b.foo() == 0
+}