Description
TypeScript Version: 3.1.0-dev.20180807
Search Terms:
string index inference, string key inference
Code
const SYM = Symbol();
type StringProps1<T> = T extends {[x: string]: infer U} ? U : never;
type StringProps2<T> = T extends {[K in Extract<keyof T, string>]: infer U} ? U : never;
type T1 = StringProps1<{a: 'aaa', [SYM]: 'sym'}>; // T1 = 'aaa' | 'sym'
type T2 = StringProps2<{a: 'aaa', [SYM]: 'sym'}>; // T2 = 'aaa'
Expected behavior:
I expected StringProps1
and StringProps2
to mean the same thing (infer from just the string keys).
Accordingly, T1
and T2
would both inferred as 'aaa'
, both excluding the symbol key.
Actual behavior:
T2
excludes the symbol key as expected, but T1
includes it, as shown in the code comments.
Even if StringProps1
and StringProps2
did have subtly different meanings, it doesn't seem logical to include the [SYM]: 'sym'
as part of the string indexer, since it doesn't belong there.
Related Issues:
This is a simpler repro of #26183, although that issue shows its nothing specific to conditional types. It seems to be just how string indexers work with type inference.
The search terms above brought up #25065 and #24560, but they don't look like the same issue.