implement generic structs

vlib/compiler/tests/generic_test.v

@@ -98,3 +98,34 @@ fn test_generic_fn_in_for_in_expression() {
         assert value == 'a'
     }
 }
+
+// test generic struct
+struct DB {
+    driver string
+}
+
+struct User {
+	db DB
+mut:
+	name string
+}
+
+struct Repo<T> {
+	db DB
+mut:
+	model  T
+}
+
+fn new_repo<U>(db DB) Repo<U> {
+	return Repo<U>{db: db}
+}
+
+fn test_generic_struct() {
+	mut a :=  new_repo<User>(DB{})
+	a.model.name = 'joe'
+	mut b := Repo<User>{db: DB{}}
+	b.model.name = 'joe'
+	assert a.model.name == 'joe'
+	assert b.model.name == 'joe'
+}
+