'void' parameters treated as optional only for calls, but not for assignability. #40227

TypeScript Version: 4.0.2

Search Terms: signature void parameter optional assignability

Code

declare function f(a: number, b?: void): void;
declare function g(a: number, b: void): void;
f(1); // ok
g(1); // ok

let ff: (a: number) => void = f; // ok
let gg: (a: number) => void = g; // error

Expected behavior:

g should be assignable to gg since we treat void parameters as optional.

Actual behavior:

g is not assignable to gg.

Playground Link: link

Related Issues: #39817, #29131