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.

[jakebailey]

Just looking at old issues; given #17904 (comment) and the fact that the behavior was changed, the implication is that an implicit any is no longer the desired behavior and therefore no error should appear (so this issue can be closed).