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Parsing bug for foo<typeof a[0].b>() #44857
Description
Bug Report
🔎 Search Terms
Parsing, typeof, generic, index access
🕗 Version & Regression Information
Tested with 4.3.4
⏯ Playground Link
💻 Code
let a = [{ b: 1 }] as const
let foo = <T extends 1>() => ""
foo<typeof a[0].b>();
// Unexpected error:
// Operator '<' cannot be applied to types '<T extends 1>() => string' and 'string'. (2365)
let z = a[0].b
foo<typeof z>()🙁 Actual behavior
There is an error at line 4 - Operator '<' cannot be applied to types '<T extends 1>() => string' and 'string'. (2365)
🙂 Expected behavior
There should be no errors.
Activity
whzx5byb commentedon Jul 1, 2021
In your example, the
typeofoperator appears in a type context (not expression context!), and you should use indexed access typefoo<typeof a[0]['b']>()to get the type of a property.devanshj commentedon Jul 1, 2021
Umm I don't think so, if it were in a type context
typeof a.b.cwould also not work but it does.DanielRosenwasser commentedon Jul 1, 2021
Seems possibly related to #19707.
My understanding is that we try parsing type arguments, we see
typeof a[0].bwhich is initially parsed as(typeof a)[0]. The.can't follow the type though because the left side is not valid for a qualified name though, so we determine that we can't parse the type arguments and throw the work away, back-tracking and trying to parse it as a<comparison.sandersn commentedon Jul 2, 2021
I agree, this is a duplicate of #19707