json: fix decode option string (#17812)

Author: felipensp

vlib/json/json_struct_option_test.v

@@ -0,0 +1,46 @@
+import json
+
+pub struct MyStruct[T] {
+pub mut:
+	result ?T
+	id     string
+}
+
+fn test_gn_struct_string() ! {
+	a := MyStruct[string]{
+		result: 'test'
+		id: 'some id'
+	}
+
+	encoded_string := json.encode(a)
+	dump(encoded_string)
+	test := json.decode(MyStruct[string], encoded_string)!
+	dump(test)
+	assert a == test
+}
+
+fn test_gn_struct_int() ! {
+	a := MyStruct[int]{
+		result: 1
+		id: 'some id'
+	}
+
+	encoded_string := json.encode(a)
+	dump(encoded_string)
+	test := json.decode(MyStruct[int], encoded_string)!
+	dump(test)
+	assert a == test
+}
+
+fn test_gn_struct_f64() ! {
+	a := MyStruct[f64]{
+		result: 1.2
+		id: 'some id'
+	}
+
+	encoded_string := json.encode(a)
+	dump(encoded_string)
+	test := json.decode(MyStruct[f64], encoded_string)!
+	dump(test)
+	assert a == test
+}

vlib/v/gen/c/json.v

@@ -164,7 +164,8 @@ ${enc_fn_dec} {
 			g.gen_sumtype_enc_dec(utyp, sym, mut enc, mut dec, ret_styp)
 		} else if sym.kind == .enum_ {
 			g.gen_enum_enc_dec(utyp, sym, mut enc, mut dec)
-		} else if utyp.has_flag(.option) && sym.info !is ast.Struct {
+		} else if utyp.has_flag(.option)
+			&& (is_js_prim(g.typ(utyp.clear_flag(.option))) || sym.info !is ast.Struct) {
 			g.gen_option_enc_dec(utyp, mut enc, mut dec)
 		} else {
 			enc.writeln('\to = cJSON_CreateObject();')