Incorrect partial union signature matching

[zhuravlikjb]

function g1(b?: string, ...a: string[]) { }
function g2(a?: string, b?: string) { }

var s: typeof g1 | typeof g2;

s("a", "b", "c") // should be an error: 0..2 parameters are allowed

[mhegazy]

@sandersn can you take a look.

[sandersn]

Sure.

[sandersn]

Seems like it only repros with optional arguments. This correctly errors:

function g1(b: string, ...a: string[]) { }
function g2(a: string, b: string) { }

var s: typeof g1 | typeof g2;

s("a", "b", "c") // should be an error: 0..2 parameters are allowed

[sandersn]

It's not in the union code like the last bug. The two signatures now union without a problem.

[sandersn]

It looks like resolveCall just treats unioned function types as an overload set and accepts a call if any member of the union matches. It should be all.

But I may be mis-reading the code -- I don't know why it only repros with optional arguments.

[sandersn]

With required parameters, only the fixed-arg signature matches with the rest-arg signature. compareSignatures is not commutative, and it happens that

{ (...a: string[]): string } `match` { (a:string, b:string): string }

but not

{ (a: string, b: string): string } `match` { (...a: string[]): string }

So when overload resolution happens, only { (a: string, b: string): string } is considered. However, when the parameters are all optional, then the overload set includes both:

1. { (...a: string[]): string }
2. { (a?: string, b?: string): string}

And overload resolution just takes the first member of the union.