Inconsistent Behavior with Equality Check Using Pick<Readonly<T>, K> in Mapped Type

[Snowflyt]

🔎 Search Terms

"readonly", "pick", "equality", "equal"

🕗 Version & Regression Information

This is the behavior in every version I tried, and I reviewed the FAQ for entries about Generics.

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.4.5#code/JYOwLgpgTgZghgYwgAgPLIN4ChnJge3wC5kBnMKUAcwG4sBfLLMATwAcUBRARwFc4ANgB4AGgBpkATQB8yALw5kACiEAVaUoCU82auQQAHpBAATUshHIA-MgCMyEgCZth42eRqN2ubv1GIpuaS1nYOyI4hFLwoJPACpBB0zOwoAFK85AAKwAgA1p7ymIoA2gDSyKDIuRAs+DDIqgC6JNl5ahKl0nT0dKwcyJDkAPoADIXpWTn5qNLFAEQE+HONdAD0q7jIAHpWJBh4hCTklCBUyIzJ-a25AEoQcCb4IAIsBXJFuGUVIFU1dQ3NZDXIR3B5PF6eDpdBi9FIDCDDezva6gx7PV4zeaLZZrDa4HZ7ZBQe5ol4HYhkCjUc5MPooQZgIYRd48fjCfaLI5U07nCT7Ylg9HkrknM70aHrTYEvCCBK0uEAZQocFOEDeH2QX0q1Vq9SaJFZgiEwNUUIkwNR4Ne6ih0Nwks2jqdzu2Wzd7vd+I93p9HoaAAtgOZSP78LwBCZkAAjFD8klW4WU0XnZCIBD4KAmalgfDIAAGKPj6LzigdLvLUt9btLeM2qkDwdD4cjMcwieO1PoqYQ6cz2dzeYmYGuJftGwAdJOyAPDcITWagVMQUWITbkJ1pHmyE2I9GUHm4gkt2mM1meTn8wymSXGHT4cMAMyFJVQFVUNWYhaEHFYMvSqIoEIAC0yAAOr+iwViXPSCKMgALIUs7GkuqASF+SzSOaS6WuiQgzGh2LSBKtbSoeEBYEAA

💻 Code

interface O {
  foo: string;
}

type Equal<X, Y> =
  (<T>() => T extends X ? 1 : 2) extends <T>() => T extends Y ? 1 : 2 ? true : false;

type JustPick<T> = {
  [K in keyof T]: Pick<T, K>;
};
type test_0 = JustPick<O>["foo"];
//   ^?: { foo: string }

type PickReadonly<T> = {
  [K in keyof T]: Pick<Readonly<T>, K>;
};
type test_1 = PickReadonly<O>["foo"];
//   ^?: { readonly foo: string }

type test_2 = Equal<{ foo: string }, { readonly foo: string }>;
//   ^?: false

type Strange<T> = {
  [K in keyof T]: Equal<Pick<T, K>, Pick<Readonly<T>, K>>;
  //                    ^^^^^^^^^^  ^^^^^^^^^^^^^^^^^^^^ This should be { readonly foo: string } according to `PickReadonly`
  //                    ^^^^^^^^^^
  //     This should be { foo: string } according to `JustPick`
  // ... so `Equal<Pick<T, K>, Pick<Readonly<T>, K>>` should be `false` according to `test_2`
}
type test_3 = Strange<O>["foo"];
//   ^?: true <- Why?
type test_4 = Equal<Pick<O, "foo">, Pick<Readonly<O>, "foo">>;
//   ^?: false

🙁 Actual behavior

Type of test_3 is true instead of expected false like test_2 and test_4.

🙂 Expected behavior

Type of test_3 should be false like test_2 and test_4.

Additional information about the issue

No response

[RyanCavanaugh]

This definition of Equal intentionally does the one thing that relies on the internal representation of types in the compiler. I really do not recommend using it. A definition that isn't reverse-engineered to actively rely on implementation details is

type Equal<X, Y> = [X, Y] extends [Y, X] ? true : false;

which gives consistent results

readonly doesn't affect assignability so can't really be used in conditional types like this. Attempts to manifest other behavior will ultimately fail.

[Snowflyt]

This definition of Equal intentionally does the one thing that relies on the internal representation of types in the compiler. I really do not recommend using it. A definition that isn't reverse-engineered to actively rely on implementation details is

type Equal<X, Y> = [X, Y] extends [Y, X] ? true : false;

which gives consistent results

readonly doesn't affect assignability so can't really be used in conditional types like this. Attempts to manifest other behavior will ultimately fail.

Thanks for the detailed response—it really cleared things up for me! I appreciate your help and the alternative you provided.