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keyof "a" behaves differently when using a type parameter #23345
Description
TypeScript Version: 2.9.0-dev.20180411
Code
type A = { [P in keyof "a"]?: boolean }; // { padStart?: boolean | undefined, ... }
type Indirect<T> = { [P in keyof T]?: boolean };
type B = Indirect<"a">; // "a"
declare const a: A;
const b: B = a; // ErrorExpected behavior:
No error. B is the same as A.
Actual behavior:
Error: B is the string literal type "a".
Activity
ahejlsberg commentedon Jun 3, 2018
This is working as intended. When instantiating a homomorphic mapped type such as
{ [P in keyof T]?: boolean }we distribute over union types and leave non-object types unchanged. For example,Indirect<{ x: string } | undefined>is resolved asIndirect<{ x: string }> | undefined. For this reason,Indirect<"a">is simply"a".typescript-bot commentedon Jun 18, 2018
Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.
aleclarson commentedon Sep 13, 2018
What does it mean to "distribute over union types"?
Also, it seems that as long as I avoid using
keyofinside the generic "mapped type" declaration, I can avoid this footgun. See here: https://goo.gl/DakrYmWhat's the practical benefit of making
keyofbehave differently when inside a generic "mapped type"? Or is it simply a consequence of the current implementation?