markused: fix `-skip-unused` on programs with generic methods (fix #12306)

vlib/v/markused/markused.v

@@ -203,6 +203,12 @@ pub fn mark_used(mut table ast.Table, pref &pref.Preferences, ast_files []&ast.F
 			all_fn_root_names << k
 			continue
 		}
+		if mfn.receiver.typ != ast.void_type && mfn.receiver.typ.has_flag(.generic) {
+			// generic methods may be used in cgen after specialisation :-|
+			// TODO: move generic method specialisation from cgen to before markused
+			all_fn_root_names << k
+			continue
+		}
 		// testing framework:
 		if pref.is_test {
 			if k.starts_with('test_') || k.contains('.test_') {

vlib/v/tests/skip_unused/generics_method.run.out

@@ -0,0 +1 @@
+0

vlib/v/tests/skip_unused/generics_method.skip_unused.run.out

@@ -0,0 +1 @@
+0

vlib/v/tests/skip_unused/generics_method.vv

@@ -0,0 +1,31 @@
+interface Something {
+	i int
+}
+
+struct Some {
+	i int
+}
+
+struct App<M> {
+	f M
+}
+
+fn (mut self App<M>) next<M, T>(input T) f64 {
+	$if M is Something {
+		return 0
+	} $else {
+		panic('${typeof(M.typ).name} is not supported')
+		return 1
+	}
+	return 1
+}
+
+fn main() {
+	mut app := App<Some>{
+		f: Some{
+			i: 10
+		}
+	}
+	assert app.next(1) == 0
+	println(app.next(1))
+}