Function should be assignable to (...args: any[]) => any

Obviously all functions have type Function. And likewise all functions can be given the type

type AnyFunc = (...args: any[]) => any;

AFAIK, there is nothing one can do with something of type Function that cannot be done with something of type AnyFunc and vice versa. However they are not equivalently assignable:

(f: AnyFunc): Function => f;
(f: Function): AnyFunc => f;
//                        ^
// Type 'Function' is not assignable to type 'AnyFunc'.
//   Type 'Function' provides no match for the signature '(...args: any[]): any'.

If I edit lib.d.ts to add a callable signature to Function:

interface Function {
  (...argArray: any[]): any;
  //...
}

it seems to "fix" this issue; is there any reason not to make this change for real?

Start by reviewing the Function declaration in lib.d.ts and the assignability behavior shown in the issue. Investigate existing tests for Function and callable signatures, then assess whether adding the signature preserves expected type-system behavior. Done means the proposed assignability is addressed with coverage for the demonstrated cases and any compatibility concerns are resolved.