"no-unused-vars" doesn't work with recursive function correctly, when it is declared with function expression

[aliaksandr-yermalayeu]

Tell us about your environment

• ESLint Version: 5.7.0
• Node Version: 8.11.1
• npm Version: 5.6.0

What parser (default, Babel-ESLint, etc.) are you using?
default

Please show your full configuration:

{
    "rules": {
        "no-unused-vars": "error"
    }
}

Configuration

{
    "rules": {
        "no-unused-vars": "error"
    }
}

What did you do? Please include the actual source code causing the issue, as well as the command that you used to run ESLint.

function declaration

// considered error
function a () {
    a();
}

function expression

// error is not happened
var a = function () {
    a();
}

What did you expect to happen?
Second case should cause error the same way as the first one.

[platinumazure]

Hi @aliaksandr-yermalayeu, thanks for the issue.

It seems to me the recursive declaration case should be flagged as used (i.e., neither example should produce a warning). But I'm not 100% sure.

@not-an-aardvark @mysticatea Any thoughts?

[not-an-aardvark]

The firat example should produce a warning because the function can never be initially called, since it's only referenced from inside itself.

The second example could be made to produce a warning for the same reason, although this would require modifying the rule to be able to infer that the function can only be accessed through the a variable.

[mysticatea]

It has been introduced in #2095; the rule is addressing function declarations to detect recursive calls, but not variable declarations. We can enhance the rule. Maybe...

• the rule should ignore references in the VariableDeclarator#init and AssignmentPattern#right if duplicate variables don't exist.

[kaicataldo]

@mysticatea Just to clarify, are you saying this is a bug?

[mysticatea]

Looks like an enhancement to me.

Re-looked, #2095 was handled as a bug, so this is a bug.