checker: fix generics that return reference generics struct (fix #6218) (#9869)

Author: yuyi98

vlib/v/checker/checker.v

@@ -1457,7 +1457,7 @@ fn (mut c Checker) check_return_generics_struct(return_type ast.Type, mut call_e
 			idx := c.table.type_idxs[nrt]
 			if idx != 0 {
 				c.ensure_type_exists(idx, call_expr.pos) or {}
-				call_expr.return_type = ast.new_type(idx).derive(return_type)
+				call_expr.return_type = ast.new_type(idx).derive(return_type).clear_flag(.generic)
 			} else {
 				mut fields := rts.info.fields.clone()
 				if rts.info.generic_types.len == concrete_types.len {
@@ -1481,7 +1481,7 @@ fn (mut c Checker) check_return_generics_struct(return_type ast.Type, mut call_e
 						mod: c.mod
 						info: info
 					})
-					call_expr.return_type = ast.new_type(stru_idx)
+					call_expr.return_type = ast.new_type(stru_idx).derive(return_type).clear_flag(.generic)
 				}
 			}
 		}

vlib/v/tests/generics_return_reference_generics_struct_test.v

@@ -0,0 +1,15 @@
+struct Foo<T> {
+	data []T
+}
+
+fn new_foo<T>(len int) &Foo<T> {
+	return &Foo{
+		data: []T{len: len}
+	}
+}
+
+fn test_generics_return_reference_generics_struct() {
+	f := new_foo<int>(4)
+	println(f)
+	assert f.data == [0, 0, 0, 0]
+}