checker: fix vls test compilation

vlib/v/checker/fn.v

@@ -1552,6 +1552,15 @@ pub fn (mut c Checker) method_call(mut node ast.CallExpr) ast.Type {
 			c.table.register_fn_concrete_types(method.fkey(), concrete_types)
 		}
 
+		// resolve return generics struct to concrete type
+		if method.generic_names.len > 0 && method.return_type.has_flag(.generic)
+			&& !isnil(c.table.cur_fn) && c.table.cur_fn.generic_names.len == 0 {
+			node.return_type = c.table.unwrap_generic_type(method.return_type, method.generic_names,
+				concrete_types)
+		} else {
+			node.return_type = method.return_type
+		}
+
 		if node.concrete_types.len > 0 && node.concrete_types.all(!it.has_flag(.generic))
 			&& method.return_type.has_flag(.generic) && method.generic_names.len > 0
 			&& method.generic_names.len == node.concrete_types.len {

vlib/v/tests/generics_method_returning_optional_test.v

@@ -0,0 +1,30 @@
+import json
+
+pub struct NotificationMessage<T> {
+pub:
+	method string
+	params T
+}
+
+struct Abc {}
+
+pub fn (x &Abc) notification_at<T>() ?NotificationMessage<T> {
+	return json.decode(NotificationMessage<T>, '{}')
+}
+
+pub fn (x &Abc) generic_method<T>(method_name string) ?NotificationMessage<T> {
+	return x.notification_at<T>()
+}
+
+struct Res {}
+
+pub fn (mut x Abc) diagnostics() ?Res {
+	got := x.generic_method<Res>('xyz')?
+	return got.params
+}
+
+fn test_generic_method_returning_optional() ? {
+	mut a := Abc{}
+	a.diagnostics()?
+	assert true
+}