<T extends unknown> does not narrow

[asmundg]

I'm not entirely sure why you would want to extend unknown. But if you end up doing it, it seems a bit surprising that the narrowing behavior is not the same as for unknown itself.

TypeScript Version: 4.0.0-dev.20200509

Search Terms: narrow unknown

Code

function works(arg: unknown): void {
  typeof arg === "object" && !!arg && Object.keys(arg);
}

function fails<T extends unknown>(arg: T): void {
  // No overload matches this call.
  // Overload 1 of 2, '(o: {}): string[]', gave the following error.
  //   Argument of type 'T' is not assignable to parameter of type '{}'.
  //     Type 'unknown' is not assignable to type '{}'.
  // Overload 2 of 2, '(o: object): string[]', gave the following error.
  //   Argument of type 'T' is not assignable to parameter of type 'object'.
  //     Type 'unknown' is not assignable to type 'object'.
  typeof arg === "object" && !!arg && Object.keys(arg);
}

Expected behavior:

Both examples compile.

Actual behavior:

Only works compiles.

Playground Link: https://www.typescriptlang.org/play?ts=4.0.0-dev.20200509#code/GYVwdgxgLglg9mABAdzgJwNYGcAUBDNAcwC5FwMw5kwBKUgNzhgBNEBvAKEUSgE8AHAKZxgiAoUQBeaYgBEcAEYArQdFmIAZBsQBCHeM3aA8stVQAdBkG9c4mgG4OAXw4dQkWAkTA8MADZYADwAKoiCAB5QgmDMWGRgFFRgAHz4RKTBdIiMLOxciAD0BYgAcnCIcPSCaH5weKwAtnhQEAAWgnFQrTBxEHh+fub5RYhGVTV1rACMFaIATAA0iADkOHCkbE5ZWFBoMGCEANoAustLhHhVPO3ecANU+xLVaOhD3CPcAIJEIA3RULMeAJBCtgstED1EJQAXgsFgYIQwHgFH4QVByvwCHg-lE0IC+EIVptlm9CsVuNxgsCVuRKNRwZDoWI4QikSi0eUCSDlsTSSMxtVavVEHNAYsVmtSIoVNBtrtHiczogLlcuiDgHdashHmE0C80HzyYhvoRfv98dTlmCIXEmbD4YjkaieBisTjqhbCctpWYScMjZTLbSkgzbXAYSzHeyXUCvT7oH7uFyRGIiFIZPJTGpDLp9GmtKMsxYrDY0oQHM4gA

Related Issues:

[RyanCavanaugh]

In general a T extends U won't behave exactly like U in any number of cases. Different things having different behavior is why we have different things.

[typescript-bot]

This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.

[benwiley4000]

@RyanCavanaugh I had a very similar issue to @asmundg . I was surprised to learn that T extends unknown can't narrow.. what do you see as the intended use case of <T extends unknown> ? Why not just use <T>? I found that this does compile when you remove the extends unknown (even in strict mode), but that said I can't figure out what the point of extends unknown is.

BTW I think this came up for me because I was working in some .tsx files where a plain <T> on an arrow function is mistaken for a JSX tag. For some reason I started using extends unknown which was working in some places, but clearly not everywhere. Anyway my workaround is to just use a regular function with <T> but I'm trying to understand the deal with extends unknown.

[sebasthuis]

@benwiley4000 Did you manage to find out what the point is of extends unknown?

[jsejcksn]

@sebasthuis My current understanding is that it's used to trigger distribution and create a distributive type as explained in the v3.0 release notes.