Spurious "A type predicate's type must be assignable to its parameter's type"

[pelotom]

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 [email protected].

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]'.

[sandersn]

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]

Fix is up at #21242