Array#map() is not callable on type-guarded array

[saschanaz]

TypeScript Version: Version 4.1.0-dev.20200909

Search Terms: map not callable

Code

function foo<T>(o: T) {
  if (Array.isArray(o)) {
      o.map(() => {});
  }
}

Expected behavior: It should work as it does in 4.1.0-dev.20200908.

Actual behavior:

abc.ts:3:9 - error TS2349: This expression is not callable.
  Each member of the union type '(<U>(callbackfn: (value: any, index: number, array: any[]) => U, thisArg?: any) => U[]) | (<U>(callbackfn: (value: any, index: number, array: readonly any[]) => U, thisArg?: any) => U[])' has signatures, but none of those signatures are compatible with each other.

3       o.map(() => {});
          ~~~


Found 1 error.

Playground Link: It somehow shows no error on playground. https://www.typescriptlang.org/play?ts=4.1.0-insiders.20200909#code/GYVwdgxgLglg9mABMOcA8AVAfACgA4CGATgQLYBciGAlIgN4CwAUIq4jMIjgIJEkCeAOhgBnXgPzEy1WoxZsFhEqUEArODDA4ARNuoBuZgoC+zY0A

Related Issues:

[orta]

This fails on this morning's nightly:

function foo<T>(o: T) {
  if (Array.isArray(o)) {
      o.map(() => {});
  }
}

Workbench Repro

I'm very willing to bet it's a side-effect of #39258

[weswigham]

Because it's generic, we get the apparent type of the conditional, which means getting both the read-only array branch and non-readonly-array branch of the conditional (it could be either), which then fail to unify, as they have a separate set of generics. Essentially, we can't call .map on a union of readonly any[] and any[] because they're not subtypes of one another, and don't subtype reduce to a single set of methods.

[typescript-bot]

👋 Hi, I'm the Repro bot. I can help narrow down and track compiler bugs across releases! This comment reflects the current state of the repro in this issue running against the nightly TypeScript.

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Comment by @orta

👍 Compiled

Historical Information

Comment by @saschanaz

Version	Reproduction Outputs
3.5.3, 3.6.4, 3.7.5, 3.8.3, 3.9.5, Nightly	👍 Compiled

[weswigham]

@orta this should be "fixed" by the revert to the isArray type we discussed in today's meeting - wanna include it in the list of things closed by your revert PR?

[saschanaz]

I think #41849 fixed this.