Wrong type inferred by returning a tuple

TypeScript Version: 3.0.0-dev.20180707

Code

  const a: Array<[string, boolean]> = ["foo", "baz"].map(item => [item, !!item]);
  const b: Array<[string, boolean]> = ["foo", "baz"].map(item => { const value: [string, boolean] = [item, !!item]; return value;} );

Expected behavior: Both expressions should be allowed Actual behavior: assignment for b works, assignment for a doesn’t, as the compiler starts infering that the type for [item, !!item] is Array<string | boolean>.

[jscheiny]

Another example where a tuple inference would be desirable is in f0 below:

declare function f0<T extends any[]>(args: T): T;
declare function f1<T extends any[]>(...args: T): T;

f0(["a", 1]); // (string | number)[]
f1("a", 1); // [string, number]

This is currently blocking a typesafe implementation of Promise.all without overloads.

[austonpramodh]

+1 for different return types for different promises in promise.all