Extra arguments with defaults get implicit any even with noImplicitAny

[steadicat]

TypeScript Version: 2.4.2

Code

function foo(f: (x: number) => void) {}
foo((x, arg = true) => {});

Tested in Playground with noImplicitAny on.

Expected behavior:
Type of arg is inferred to be boolean, or an error is thrown ("arg implicitly has 'any' type").

Actual behavior:
Type of arg is any, and no error is thrown.

[mhegazy]

We need a contextual type before evaluating the expression, since the type of the expression can be impacted by that. we got one from the declaration of foo, it did not have a type for arg, so it was given a type any.
the fix here is either to give the callback declaration in foo a type for arg, or give arg an explicit type parameter.

[steadicat]

Giving arg an explicit type works, but I wish there were a way to catch this issue when it's introduced. With noImplicitAny set, I was not expecting to have any types silently inferred.

[mhegazy]

that is fair. an error should have been raised here. there is parallel to this case in #11905 already detected as a no-implicit-any error if the contextual type failed to be applied.

[falsandtru]

@mhegazy @sandersn Dup of #17559

[mhegazy]

it is, thought this issue is to track raising an error under noImplicitAny, rather than changing the behavior.