json: fix encode/decode support for generic structs (#6489)

vlib/json/json_test.v

@@ -218,7 +218,7 @@ fn test_nested_type() {
 			assert data2.countries[i].cities[j].name == data.countries[i].cities[j].name
 		}
 	}
-	
+
 	for key, user in data.users {
 		assert data2.users[key].age == user.age
 		assert data2.users[key].nums == user.nums
@@ -235,6 +235,25 @@ fn test_nested_type() {
 	}
 }
 
+struct Foo<T> {
+pub:
+	name string
+	data T
+}
+
+fn test_generic_struct() {
+	foo_int := Foo<int>{'bar', 12}
+	foo_enc := json.encode(foo_int)
+	assert foo_enc == '{"name":"bar","data":12}'
+
+	foo_dec := json.decode(Foo<int>, foo_enc) or {
+		exit(1)
+	}
+
+	assert foo_dec.name == 'bar'
+	assert foo_dec.data == 12
+}
+
 fn test_errors() {
 	invalid_array := fn () {
 		data := '{"countries":[{"cities":[{"name":"London"},{"name":"Manchester"}],"name":"UK"},{"cities":{"name":"Donlon"},"name":"KU"}],"users":{"Foo":{"age":10,"nums":[1,2,3],"lastName":"Johnson","IsRegistered":true,"type":0,"pet_animals":"little foo"},"Boo":{"age":20,"nums":[5,3,1],"lastName":"Smith","IsRegistered":false,"type":4,"pet_animals":"little boo"}},"extra":{"2":{"n1":2,"n2":4,"n3":8,"n4":16},"3":{"n1":3,"n2":9,"n3":27,"n4":81}}}'