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Spurious "A type predicate's type must be assignable to its parameter's type" #21185
Description
TypeScript Version: 2.7.0-dev.20180113
Code
This type operator from the Unionize library:
type Predicates<TaggedRecord> = {
[T in keyof TaggedRecord]: (variant: TaggedRecord[keyof TaggedRecord]) => variant is TaggedRecord[T]
}Expected behavior:
Should compile, and does in typescript@2.6.2.
Actual behavior:
Fails to compile in typescript@next with
A type predicate's type must be assignable to its parameter's type.
Type 'TaggedRecord[T]' is not assignable to type 'TaggedRecord[keyof TaggedRecord]'.
Activity
sandersn commentedon Jan 17, 2018
Caused by #17912. Before that, the assignability was not checked at all; the constraint of both types was incorrectly
any. Now, neither type has a constraint, so they are incorrectly not assignable. Turns out our assignability rules for indexed access types are incomplete.I'll put up a PR after lunch; I need to write some more tests.
sandersn commentedon Jan 17, 2018
Fix is up at #21242