Can't call array methods on type (A[] | B[])

[testerez]

TypeScript Version: 2.5.2

Code

function test(items: string[] | number[]) {
  items.forEach(item => console.log(item));
}

Expected behavior:
no error

Actual behavior:

Cannot invoke an expression whose type lacks a call signature. Type '((callbackfn: (value: string, index: number, array: string[]) => void, thisArg?: any) => void) | ...' has no compatible call signatures.

[RyanCavanaugh]

Duplicate #7294

[RyanCavanaugh]

Also #10620

[rdeneau]

It's not exactly the same but you could use a type alias to circumvent this issue:

type stringOrNumber = string | number;
function test(items: stringOrNumber[]) {
  items.forEach(item => console.log(item));
}

[kitsonk]

circumvent this issue

It isn't really an issue, in that the compiler is doing what it is instructed to do, albeit it can be slightly confusing. Also a type alias is not required:

function test(items: (string | number)[]) {
  items.forEach(item => console.log(item));
}

[testerez]

@rdeneau actually is't what I did :)