Assignability of functions with a rest parameter of an empty tuple possibly incorrect

[bterlson]

TypeScript Version: 3.1.0-dev.20180722
Code

declare var f1: (... args: any[]) => void;

declare var f2: () => void;
declare var f3: (...args: []) => void;
declare var f4: (a: never) => void;

f1 = f2; // ok
f1 = f3; // error
f1 = f4; // error

Expected behavior:
I want to create function types that are assignable to f1 using tuple types ala f3. I expected f3 to behave more like f2 than f4.

Actual behavior:
f3 cannot be assigned to f2.

But adding any type into f3's args tuple makes the error go away.

[bterlson]

/cc @sandersn

[mhegazy]

Just to be clear, this issue is about missing error for:

declare var f3: (...args: [number]) => void;

f1 = f3;

correct?

[bterlson]

There may be two issues. To clarify a bit about my scenario:

declare var cbf: (cb: (... args: any[]) => void) => void; // a library function I don't control
declare var myCB: (... args: []) => void; // a callback generated from a library I do, whose signature in this case doesn't take any arguments
cbf(myCB); // but I can't pass myCB to cbf.

It should be possible to use tuple types to generate callback types that are assignment compatible with normal signatures.

[mhegazy]

Looks like there are two missing cases here:

declare var f1: (...args: any[]) => void;
declare var f5: (...args: []) => void;  // should be equivalent to ()=>void
declare var f6: (...arg: [number]) => void; // should be equivalent to (arg0: number) => void

f1 = f5; // Should be allowed, but errors
f1 = f6; // Should error, but does not

[ahejlsberg]

@mhegazy In your example, f1 = f6 isn't and shouldn't be an error. Any parameter list should be assignable to (...args: any[]) => xxx.

[mhegazy]

yes. you are right. we allow that though it is unsafe :)