Weird interaction between default arguments and strict-null

[evmar]

TypeScript Version: Version 2.0.5

Code

(with --strictNullChecks on)

function x(y = 'a', z: number) {}
function x1(y: string, z: number) {}

// Construct error messages to reveal the type of these functions:
let test1: void = x;
let test2: void = x1;
// They have the same type:
// ERROR: Type '(y: string, z: number) => void' is not assignable to type 'void'.
// ERROR: Type '(y: string, z: number) => void' is not assignable to type 'void'.

// However, they have different observable behaviors:
x(undefined, 1);  // OK
x1(undefined, 1);  // ERROR
// ERROR: Argument of type 'undefined' is not assignable to parameter of type 'string'.

Note that the second parameter (z) is necessary to trigger this behavior; if you drop z from this example, then the first param x has a ? as you'd expect.

(So it's even possible this is just a bug in how the function type is printed...?)

[RyanCavanaugh]

We should be displaying the defaulted parameter as x? or : string | undefined

[sandersn]

This is a duplicate of #11900 that points out a different symptom of the bug that needs to be fixed.