UnusedParameter and UnusedVariable false positives in lambda default arguments

[ryangreenberg]

This sample code:

function ex(
	int $i,
	(function(int): int) $fn = ($a) ==> {
		foo($a);
		return 5;
	},
): void {
	$fn($i);
}

function foo(int $_i): void {}

produces these errors:

Parameter is unused
  Linter: Facebook\HHAST\UnusedParameterLinter
  Location: /Users/greenberg/code/repro/unused_repro.hack:3:29
@@ -1,6 +1,6 @@
 function ex(
  int $i,
- (function(int): int) $fn = ($a) ==> {
+ (function(int): int) $fn = ($_a) ==> {
    foo($a);
    return 5;
  },

Would you like to apply this fix?
  [y]es/[n]o/yes to [a]ll/n[o] to all: Variable is unused
  Linter: Facebook\HHAST\UnusedVariableLinter
  Location: /Users/greenberg/code/repro/unused_repro.hack:4:6
  Code:
  >$a

Both are very confused about what to do with the lambda as a parameter default value. I think UnusedParameter needs to look for parameters that are children of the declaration rather than descendants.