[ Typechecker built-in lint ] "Loop variable `_` shadows a local variable" (5568) ignores lambda cow

[lexidor]

Describe the bug hh_client --lint believes a variable that is declared outside a lambda can be shadowed inside a lambda.

Standalone code, or other way to reproduce the problem

function false_shadow(): void {
    $false_shadow = 42;
    (vec<int> $things) ==> {
        foreach ($things as $false_shadow) {
            // do something with $false_shadow
        }
    };
}

Steps to reproduce the behavior:

1. Go to Invoke hh_client --lint this_file.hack

Expected behavior

The foreach loop inside the lambda does not write to the outer $false_shadow. The lambda has a cow scope.

Actual behavior

Lint[5568] Loop variable $false_shadow shadows a local variable defined or last assigned here:
File "path/to/this_file.hack", line 2, characters 5-17: [1]

src/false_shadow.hack:4:29
    2 |     $false_shadow = 42;
    3 |     (vec<int> $things) ==> {
[1] 4 |         foreach ($things as $false_shadow) {
    5 |             // do something with $false_shadow
    6 |         }

Environment

• Operating system

Irrelevant

• Installation method

hhvm/hhvm on dockerhub

• HHVM Version

HipHop VM 4.146.0 (rel) (non-lowptr)
Compiler: 1643142756_285035995
Repo schema: f559125a34cec281a617647f42d18bd793318580
hackc-9ddd1899e3c5cf4954e903514c8f4d89beadd202-4.146.0

Additional context The same thing also applies to async {} blocks.

This code is still smelly. The lint error message led me to believe that renaming this variable might change the behavior of the code. The enclosing function in which I encountered this lint error was rather large. It took me a while to realize that they never shadowed each other.

[Wilfred]

Should be fixed by D39003052.