Constrained types is not assignable to constrainer union types, and failed to narrow its type

[falsandtru]

TypeScript Version:

master

Code

function f<T extends string[] | {}>(a: T) {
  if (a instanceof Array) {
    a = a;
  }
}

Expected behavior:

$ node built/local/tsc.js index.ts

Actual behavior:

$ node built/local/tsc.js index.ts
index.ts(3,5): error TS2322: Type 'any[]' is not assignable to type 'T'.

[ahejlsberg]

Hmm. This one is tricky. Currently the only type we consider assignable to a type parameter is that same type parameter (and any of course). In order for the above to work we would basically need to create a new type parameter that is a subtype of T and has a narrowed constraint string[]. It's possible, but it isn't a simple fix.

[DanielRosenwasser]

I would think that #7422 would have allowed this. Care to weigh in Nathan Shively-Sanders (@sandersn)?

[sandersn]

Yeah, it did basically what Anders Hejlsberg (@ahejlsberg) said -- but (1) only locally, when getting the narrowed type, and (2) not updated for control-flow rewrite of narrowing. So it can't be used as-is.

[ahejlsberg]

I don't think the solution in #7422 works. The thing that makes this issue complicated is that you can't just clone type parameters without also being prepared to treat the clones as aliases for the original type parameter in the type instantiation logic. Otherwise you'll "leak" type parameters.

[ahejlsberg]

This has worked correctly for a while now. The type guard produces type T & any[] which is assignable to T so there is no error. I'm closing this issue.