"cannot invoke an expression whose type lacks a call signature" on union of types both having this call signature

[fabiospampinato]

TypeScript Version: 2.0.3

Code

function foo ( arg: number[] | string[] ) {
    arg.find ( () => false ); // cannot invoke an expression whose type lacks a call signature
} 

function bar ( arg: (number | string)[] ) {
    arg.find ( () => false );
}

Expected behavior:

Inside foo, arg is always an array, either of numbers or of strings, so I expect the find method to be recognized by the compiler.

Actual behavior:

It doesn't recognize it, and it throws a cannot invoke an expression whose type lacks a call signature error.

I don't see any good reason why inside bar the compiler understands that arg is always an array, but not inside foo.

[kitsonk]

Dupe of #10620 (second one today, after #11205)

[fabiospampinato]

Sorry about that. I didn't find them on my searches.