markused: fix generic fn mark as used (fix #15387) (#15406)

vlib/v/markused/markused.v

@@ -204,7 +204,7 @@ 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) {
+		if mfn.receiver.typ != ast.void_type && mfn.generic_names.len > 0 {
 			// generic methods may be used in cgen after specialisation :-|
 			// TODO: move generic method specialisation from cgen to before markused
 			all_fn_root_names << k

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

@@ -0,0 +1 @@
+OK!!

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

@@ -0,0 +1 @@
+OK!!

vlib/v/tests/skip_unused/nested_generics_method.vv

@@ -0,0 +1,54 @@
+struct Calc<S> {
+mut:
+	typ S
+}
+
+struct TypeA {}
+
+struct TypeB {}
+
+fn (mut c Calc<S>) next<T>(input T) f64 {
+	$if S is TypeA || S is TypeB {
+		return c.typ.next(input)
+	} $else {
+		return 99.0
+	}
+}
+
+fn (mut t TypeA) next<T>(input T) f64 {
+	return 10
+}
+
+fn (mut t TypeB) next<T>(input T) f64 {
+	return 11
+}
+
+fn new<S>() Calc<S> {
+	$if S is TypeA {
+		return Calc<TypeA>{
+			typ: TypeA{}
+		}
+	} $else $if S is TypeB {
+		return Calc<TypeB>{
+			typ: TypeB{}
+		}
+	} $else {
+		panic('unknown type $S.name')
+	}
+}
+
+fn main() {
+	{
+		mut c := Calc<TypeA>{
+			typ: TypeA{}
+		}
+		assert c.next(100) == 10.0
+	}
+	{
+		mut c := Calc<TypeB>{
+			typ: TypeB{}
+		}
+		assert c.next(100) == 11.0
+	}
+	println('OK!!')
+}