Missing error on generic type parametrisation of generic function

[ConnorDuvalS]

🔎 Search Terms

"generics", "functions", "unions"

🕗 Version & Regression Information

• This is the behavior in every version I tried (v3.3.3 and v5.5.2, though v3.3.3 did not error on Case 6), and I reviewed the entire FAQ

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.5.2#code/JYOwLgpgTgZghgYwgAgJLIN4ChnLgLk2WABNCQBXAWwCNoBuZANzgBsKJCBnMKUAc2QBfejmQ1CGYmWSVaDZmw6EaAe1WsIcEMNG4Ek6eWp0ousTKmlj8syLGci93DEPXkPPiEFCs9rGAAngAOKABSyAC8RADaANLEOgDWEIGqMGgAuoSo8ZnIEAAekCAkXETucqbCyAD8yAnkEEzQwjEpaRmomYwA9L3IAERwg8gAPkM0oxODCNNDJINYWDAUIAhgwKo6MACMADwAGgXFEKXlHeloAHwAFIWEhwA0Rmgxh-lFJWUVMqAwrQASjV6sCmi0oABKTBiKAQMAUKAgUS+Fa7W7DQYvADMkL6AyocBS5S4Z1Jy1W60222QMAATEcTt9ymE7g9kM9kIEcu9MtDsLg4QikSiKXSMSMXlYZNiXix2I4AOSK4R45D9ZCE4keMkQFbiwZTKWvWWKBWEeCsUmq-GaokQEm6ilrDZbHbYxlfM4-Vn3R78sS9ABUyAAwnBrbtkEGBnt7ji1RquAALVQUVgkWSqMDiFDaApQKCqKCBkPh6106Oxg6HNkJ20ptMZrM5uh4HTQIsl3DBsMRlDYqu0mvIABkyFuYUhdeQuIbqfTmZA2dz7YLXdLfetABYh3t9r7CvX1QT7Y6QKSAHTX4gpzfllAAVj3BwiMxggxniuxisTA0bi6rvmnbFssPZlv2yAAGx7gytbxr8hCmvKyjILwHA2ie67FuUCDFnCGysIE96QQA7LBB4ztKSFykojjoSgQh-h4C7Nm2wGFqBvhAA

💻 Code

interface I {
  a: { id: number; value: string };
  b: { id: number; value: boolean };
  c: { id: number };
  d: { id: number };
  e: { };
  f: { id: string }
};

type J = { [K in keyof I]: I[K] extends { id: number } ? K : never }[keyof I]; // "a" | "b" | "c" | "d"

function f1<X extends keyof I>(x: X, id: I[X] extends { id: infer R } ? R : never) {
  return;
}

f1("a", 3); // makes sense

function f2<X extends J>(x: X, y: I[X]) {
  return;
}

f2("a", { id: 3, value: '' }); // makes sense
f2("b", { id: 3, value: false }); // makes sense

function f3<X extends J>(x: X) {
  /* Case 1 */ f1(x, 3); // should not be an error
  /* Case 2 */ f1<X>(x, 3); // should not be an error
  /* Case 3 */ f1<X & (J)>(x, 3); // should not be an error
  /* Case 4 */ f1<J>(x, 3); // makes sense... ish
  /* Case 5 */ f1<J | "f">(x, '3'); // should be an error

  /* Case 6 */ f2<X>(x, { id: 3, value: true }); // errors correctly
  /* Case 7 */ f2<J>(x, { id: 3, value: true }); // should be an error
}

🙁 Actual behavior

Cases shown in f3:
Cases 1 & 2 & 3: Errors with message

Argument of type 'number' is not assignable to parameter of type 'I[X] extends { id: infer R; } ? R : never'.

Cases 4 & 5: No error
Case 6: Errors with message

Argument of type '{ id: number; value: true; }' is not assignable to parameter of type 'I[X]'.
  Type '{ id: number; value: true; }' is not assignable to type 'never'.
    The intersection '{ id: number; value: string; } & { id: number; value: boolean; } & { id: number; } & { id: number; }' was reduced to 'never' because property 'value' has conflicting types in some constituents.

Case 7: No error

🙂 Expected behavior

As shown in the comments within f3:
Case 1 & 2: Should not be an error, X is narrowed to only be values for which a number works.
Case 3: Should not be an error, whilst X & J is narrowed to only be values for which a number works.
Case 4: Blankets the type, making the function work at the cost that it makes any additional type inference impossible. Due to I["a"] and I["b"], you would not be able to add a parameter of type I[X] to this function (as seen in cases for f2) as it has required types that do not exist in all I[J].
Case 5: Should be an error. The type of f1<X> for a generic X should be a union of functions with a specific input, not a function with a union input.
Case 6: Expected behaviour is actual behaviour.
Case 7: Should be an error for the same reason as Case 5 and Case 6.

Additional information about the issue

My assumption for the first three cases is that this was done to handle the case that X is any but, be that as it may, I do not see a similar explanation for the other cases.