discriminated union type matching behaviour changed starting from 5.2.x

🔎 Search Terms

"key order when matching object types" "discriminated unions key order"

🕗 Version & Regression Information

• This changed between versions 5.1 and 5.2

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.2.2#code/FAFwngDgpgBAYgewQExgXhgcgIYQgGykxgB8sEAnbAOwHMiBuUSWANSnpGwCND0sAzhACW1bAGMAFsTKZx2ChQQhMTcNBgB5EJKgV+mALbD8AaxlYA7thB7VzDQGEArgJAJDB7MIoXMAhBN7B1gYABUFTnRgGFi4sgBvGLiU2OxnZGEoanEoAC4sKAUde1TUwyL8POSylLIXNw8a2tiyAAMAMwpnYRAAfQASBMQUAF825pb2gDcOKC5eKEGE9k4eQnHJ2vblXQpl7T3xphbRrcStlPTM7NyCowUBPwArZxFbXxOWmArsKpg3BRRLQvmUzsBgOIENQ3DBZmtFgVVvN1rAMHIFEoVExIdDYVwKJwChFCfN+DAklcMlkcvlCsVpAAaZq-f5teEoxbLDkLDYTUY4qEwkAwAmcABMxMiZIwFJZlQK7LmvKWQx5qPGzKpN1p9yKFBKwAFwCAA

💻 Code

type Food = 'apple' | 'orange';
type Vegetable = 'spinach' | 'carrot';
type Other = 'milk' | 'water';
type Custom = 'air' | 'soil';

type  Target =
      | {
          audience: 'earth';
          meal:
            | Custom
            | `fruit_${Food}`
            | `vegetable_${Vegetable}`
            | `other_${Other}`;
        }
      | {
          audience: 'mars' | 'jupiter';
          meal: string;
        }


const vegetable: Vegetable = 'carrot';

// ok
const target: Target =  {
    audience: 'earth',
    meal: `vegetable_${vegetable}`
};

// TS Error
// Type '{ meal: string; audience: "earth"; }' is not assignable to type 'Target'.
//  Types of property 'audience' are incompatible.
//    Type '"earth"' is not assignable to type '"mars" | "jupiter"'
const target2: Target =  {
    meal: `vegetable_${vegetable}`,
    audience: 'earth'
};

"use strict";
const vegetable = 'carrot';

const target = { // ok
    audience: 'earth',
    meal: `vegetable_${vegetable}`
};

const target2 = { // error
    meal: `vegetable_${vegetable}`,
    audience: 'earth'
};

{
  "compilerOptions": {
    "strict": true,
    "noImplicitAny": true,
    "strictNullChecks": true,
    "strictFunctionTypes": true,
    "strictPropertyInitialization": true,
    "strictBindCallApply": true,
    "noImplicitThis": true,
    "noImplicitReturns": true,
    "alwaysStrict": true,
    "esModuleInterop": true,
    "declaration": true,
    "target": "ES2017",
    "jsx": "react",
    "module": "ESNext",
    "moduleResolution": "node"
  }
}

Playground Link: Provided

🙁 Actual behavior

In the code both target and target2 have the correct structure based on the Target type.

But the order of keys is reversed in target2 which used to work but not anymore.

🙂 Expected behavior

Both target and target2 should be valid.

Additional information about the issue

Discriminated unions didn't rely on the key ordering AFAIK.

Start with the provided Playground reproduction and compare its behavior in TypeScript 5.1 and 5.2.2. Confirm whether both property orders are accepted, then trace the discriminated-union matching behavior and add a regression test if the relevant test location can be identified. Done means the reversed-key example type-checks without weakening the other diagnostics.