Regression in types after v5.5

[Titozzz]

🔎 Search Terms

cannot be used to index type
Generics
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🕗 Version & Regression Information

• This changed in commit

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.5.2#code/CYUwxgNghgTiAEYD2A7AzgF3gMwFzwB4AVAPgAo94yAPfIgSngF4T4BXFAaxSQHcVGLKrXgNmrDtz4oAUAHo58ALZROASxQBzeBgAWatPCjw4UCPDWgUGNRgCeODmBup4ICGqUaoGEIb0IIDAwSDC48oqgkLAIyOhYlMTkImJCRDIyUdBwiKiY8Jr4xG7UvijAhpI8-Mn4AN44SEh08AC+ghJc1bIKJiBKSABuGtoBBSAoQWpgufEwUBpY7p7evv66gcGh4b1ZMbP5hYSkNPWNzaJtHexd0hlx+brMOGSa9BElAA4eYLYQDrwYLYRvAkGwsGMQNQoM4dHZPgg0LppBYUHCEajsEg3B4vCgfH4dBs3Fswh89jkHlgoEUiCUyhUblIaqd4A0sRc6e1xEzuhl7BiAGJqGCYAAKsCgShAvhgSWedKhDMMrI02CC8DF1yqKIA-Jr4PgdfwProoMMtETAhA0AgAEbzFBgJ5sW3wAAGk0GQXdOJW+LWVpJITJvQFCGFoowEvm0tl8qYlyVE0ZqpQ6pgmu1t348H1YsN8C9QX58IQAA1npHxZK40ECOGkNh4LoSABtADkHI7AF0ZEA

💻 Code

declare const f: <T>(f: (x: T) => unknown) => (x: T) => unknown
// making this a real identity function eliminates the error:
// declare const f: <T>(x: T) => T

declare const g: <T extends unknown>(x: { foo: T }) => unknown
// removing the generic constraint eliminates the error:
// declare const g: <T>(x: { foo: T }) => unknown

const h = f(g)
// explicitly writing out the exact type shown in type info eliminates the error:
// declare const a: <T extends unknown>(x: { foo: T }) => unknown

type FirstParameter<T> = T extends (x: infer P) => unknown ? P : unknown
// having the else branch use `never` eliminates the error:
// type FirstParameter<T> = T extends (x: infer P) => unknown ? P : never

type X = FirstParameter<typeof h>['foo']

🙁 Actual behavior

Starting after said commit, this errors with: Type '"foo"' cannot be used to index type 'FirstParameter<(<T extends unknown>(x: { foo: T; }) => unknown)>'.

🙂 Expected behavior

No error, and types inferred as before

Additional information about the issue

This was initially detected with a more complex example, you can find it here on TS discord https://discord.com/channels/508357248330760243/1255537732730491012/1255537732730491012 but was narrowed down thanks to @mkantor.
I already pinged the initial author @Andarist and he started looking at it.