json: allow decode/encode of alias to primitive type (#18003)

vlib/json/json_encode_primite_test.v

@@ -0,0 +1,40 @@
+import json
+
+struct Test {
+	field MySumType
+}
+
+type MyInt = int
+type MyString = string
+type MySumType = MyString | int | string
+
+fn test_alias_to_primitive() {
+	mut test := Test{
+		field: MyString('foo')
+	}
+	mut encoded := json.encode(test)
+	assert dump(encoded) == '{"field":"foo"}'
+	assert json.decode(Test, '{"field":	"foo"}')!.field == MySumType('foo')
+
+	test = Test{
+		field: 'foo'
+	}
+	encoded = json.encode(test)
+	assert dump(encoded) == '{"field":"foo"}'
+	assert json.decode(Test, '{"field":"foo"}')! == test
+
+	test = Test{
+		field: 1
+	}
+	encoded = json.encode(test)
+	assert dump(encoded) == '{"field":1}'
+	assert json.decode(Test, '{"field":1}')! == test
+
+	mut test2 := MyString('foo')
+	encoded = json.encode(test2)
+	assert dump(encoded) == '"foo"'
+
+	mut test3 := MyInt(1000)
+	encoded = json.encode(test3)
+	assert dump(encoded) == '1000'
+}

vlib/v/gen/c/json.v

@@ -143,9 +143,8 @@ ${enc_fn_dec} {
 			psym := g.table.sym(parent_typ)
 			if is_js_prim(g.typ(parent_typ)) {
 				g.gen_json_for_type(parent_typ)
-				continue
-			}
-			if psym.info is ast.Struct {
+				g.gen_prim_enc_dec(parent_typ, mut enc, mut dec)
+			} else if psym.info is ast.Struct {
 				enc.writeln('\to = cJSON_CreateObject();')
 				g.gen_struct_enc_dec(utyp, psym.info, ret_styp, mut enc, mut dec)
 			} else if psym.kind == .enum_ {
@@ -261,6 +260,16 @@ fn (mut g Gen) gen_enum_enc_dec(utyp ast.Type, sym ast.TypeSymbol, mut enc strin
 	}
 }
 
+[inline]
+fn (mut g Gen) gen_prim_enc_dec(typ ast.Type, mut enc strings.Builder, mut dec strings.Builder) {
+	type_str := g.typ(typ.clear_flag(.option))
+	encode_name := js_enc_name(type_str)
+	enc.writeln('\to = ${encode_name}(val);')
+
+	dec_name := js_dec_name(type_str)
+	dec.writeln('\tres = ${dec_name}(root);')
+}
+
 [inline]
 fn (mut g Gen) gen_option_enc_dec(typ ast.Type, mut enc strings.Builder, mut dec strings.Builder) {
 	enc.writeln('\tif (val.state == 2) {')