Contextual typing doesn't work when mixing up discriminated unions with function variant

🔎 Search Terms

contextual typing, discriminated unions

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries
• Tested with TS 5.4.5 and TS 5.9.2

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/C4TwDgpgBAcg9sA6nATgawJYDsDmBVLDOLKAXgCgoqoAfKACnoDcBDAGwFcIAuKLDgLYAjCCgCUZAHxQmcDABMxlanQDey6tQwBnAMrAU2HLwNcA3Bs1QAZll6NWnHlG0GjE0tNkKxFzQF9LNUsdfUNcAH5ea3ZtCD9qW3tmdi5efmFRDy85RQt-C3IAY2JXKGAIVwBGXngkVExcAiISUgYWbKhVAuLS4HLK4AAmWoRkdCNm4jIukL03XBMULgAaSyT2zu7yHpKsMorXAGZR+ommwmm29U1QheMbWIg1zQ36DqkuwN2+gdcAFlO40a+EurVmrzsm0+2x65FAkCgwMmYLIQQhViod3CD1M8UskOSjjSLnunW8eUsgU0wVu8xxUUebDiCSobxSTnSghE4k+FN8O0KewOgwArLxkRcWjMblp6UYlqt1lD3ltvhZhf1DsAAGwShoo6XXOZhBVMuIvRIqj6eL6C3r7LWDADs+vOoKNGLZ1rVgqAA

💻 Code

type NotWorkingUnion =
    | ((value: number) => void)
    | {
        isString: true;
        fn: ((value: string) => void);
    }
    | {
    isString?: false;
    fn: ((value: number) => void);
};

const test1: NotWorkingUnion = (a) => {}; // Works
const test2: NotWorkingUnion = { // Works
    isString: true,
    fn: (a) => {}
};
const test3: NotWorkingUnion = { // Works
    isString: false,
    fn: (a) => {}
};
const test4: NotWorkingUnion = { // Doesn't work!
    fn: (a) => {}
};

type WorkingUnion =
    | {
        isString: true;
        fn: ((value: string) => void);
    }
    | {
    isString?: false;
    fn: ((value: number) => void);
};

const test5: WorkingUnion = { // Works
    isString: true,
    fn: (a) => {}
};
const test6: WorkingUnion = { // Works
    isString: false,
    fn: (a) => {}
};
const test7: WorkingUnion = { // Works
    fn: (a) => {}
};

🙁 Actual behavior

Contextual typing breaks when an union is both discriminated by a field and one of the union variants is just a function

🙂 Expected behavior

Contextual typing should correctly work, as the type system should have enough information to discriminate between a function and an object with or without the field

Additional information about the issue

strict mode is enabled.

Tái hiện các ví dụ trong TypeScript Playground được liên kết, so sánh test4 với test7 và các trường hợp được phân biệt rõ ràng. Theo dõi cách contextual typing xử lý union chứa một biến thể hàm, sau đó thêm một regression test bao quát trường hợp thiếu discriminant và xác minh rằng tham số callback được suy luận chính xác.