Union types don't match function signatures

[ghost]

TypeScript Version:

nightly (1.9.0-dev.20160303)

Code

interface Foo {
    foo( key: string ): string;
    foo( keys: string[] ): string[];
}

declare var foo: Foo;
declare const bar: string|string[];

const baz = foo.foo( bar );

Expected behavior:
The code is valid and compiles without warning, because interface Foo contains a signature for a method foo corresponding to each type in the union string|string[]. The foo method can accept a parameter list of string, and it can also accept a parameter list of string[], therefore an argument list of string|string[] is satisfiable.

I would expect the return type for foo.foo invoked in this faction to be the type union of the return types of all matching signatures (so string|string[]). This code produces the same error even if all matching signatures have the same return type.

Actual behavior:
Argument of type 'string | string[]' is not assignable to parameter of type 'string[]' Type 'string' is not assignable to type 'string[]'.

[RyanCavanaugh]

Duplicate #1805

[mhegazy]

you can also find a long discussion on why { foo(string|string[]):string|string[] } is not the same as { foo (string): string; foo (string[]): string[]; } at #7230. This should help shedding some light on how the API should be authored.

[ghost]

Maybe this is a degenerate case, but I feel like this instance should be satisfiable.
I feel like defining the interface this way is preferrable, because we can preserve the mapping for string->string and string[]->string[]. Passing in a parameter that matches both should simply result in string|string[]->string|string[]. Being less specific in what is passed in results in a result that is less specific, and vice versa.
In my actual code, both methods return void, so there really is no ambiguity; still, this is unsupported.

My actual code I'm using chrome.storage.StorageArea.remove, which can accept a string or string[]. I have keys which can be string or string[], so it's perfectly valid to pass it in; just the typings disagree with me.