extends causing string literal can't be used as index

[troy351]

Bug Report

🔎 Search Terms

cannot be used as an index type

🕗 Version & Regression Information

• This changed between versions 4.9.5 and 5.0.4

⏯ Playground Link

Playground link with relevant code

💻 Code

interface Dict {
  foo: 1
  bar: 1
}

type A = Dict['foo'] // works

type F = 'foo'
type B = Dict[F] // works

type FF = 'foo' extends ('foo' | 'bar') ? 'foo' : never // in theory, FF === F
type C = Dict[FF] // not working, Type '"foo"' cannot be used as an index type.(2538)

🙁 Actual behavior

type C throws error

🙂 Expected behavior

no error

[jcalz]

Could you please edit this to conform to the bug report template? I'd hate for this to be closed for not following the template when it looks like a real issue

[troy351]

I just did

[RyanCavanaugh]

What's the version of this code that you originally had?

[troy351]

This is a testing code when I debugging a complicated generic type, and the generic type works.
I know this kind of code nearly won't happen in real world, but it's still a bug.

[RyanCavanaugh]

Right, but I'd really like a "real" version so we can make a testcase in the PR

[troy351]

Sorry, there is no "real" version. Maybe consider the provided playground code as real