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Inconsistent Behavior with Equality Check Using Pick<Readonly<T>, K> in Mapped Type #58163
Description
🔎 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
💻 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
Activity
RyanCavanaugh commentedon Apr 12, 2024
This definition of
Equalintentionally 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 iswhich gives consistent results
readonlydoesn't affect assignability so can't really be used in conditional types like this. Attempts to manifest other behavior will ultimately fail.Snowflyt commentedon Apr 13, 2024
Thanks for the detailed response—it really cleared things up for me! I appreciate your help and the alternative you provided.