Warning for parameter hiding on ignored parameters

[DartBot]

This issue was originally filed by [email protected]

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Here's the setup: I have nested asynchronous methods that each take a callback, and I don't care about the event being passed to the callback. For (potentially contrived) example:

var button = document.query('#some-button');
button.on.click.add((_) { // <-- I don't care about the Event, I'll call it \_
  // Button is clicked
  // ...set up some more UI...
  var anotherButton = document.query('#another-button');
  anotherButton.on.click.add((_) => window.alert('Success!')); // <-- I don't care about this Event either
});

This may be a contrived example, and in retrospect I'm not sure where I got the idea to name things _ when I don't care about them, this might be a personal convention or I may have cargo culted it from Go. If it isn't a style recommendation something like it should be perhaps.

My point is, I currently see a warning when the inner _ hides the outer _. But I don't really care, I don't care about either of them. I can rename the second to __ or something, but this arms race quickly becomes annoying.

It might be nice if Dart considered variables named _ as totally ignored placeholders, not caring if they hide other parameters, and potentially showing warnings/errors if they're ever actually used (e.g., _.size or _.stopPropagation()).

[iposva-google]

Set owner to @gbracha.
Added Area-Language, Accepted labels.

[gbracha]

Currently, _ is treated just like any other identifier. We could relax shadowing requirements for it. Or we could relax shadowing requirements altogether.

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Removed Type-Defect label.
Added Type-Enhancement label.

[DartBot]

This comment was originally written by [email protected]

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Shadowing warnings are often useful and catch bugs. But if there is a standard ignored variable (and I think that's a nice language feature) then it should be exempt from the warning.

As an alternative other languages use unused_foo or similar to mark unused variables, mostly for linting purposes.

[DartBot]

This comment was originally written by [email protected]

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(Bah. Wrong account. jasonhall == imjasonh for the record)

Knowing when a variable is unused can also help when minifying JavaScript and Dart since a method can return nothing or return a short constant variable name if it knows the result won't be used.

[gbracha]

Actually, we've decided to allow these warnings to be managed by the tools in a less "in your face fashion" so hopefully this will stop being a problem. For example, the IDE could just highlight shadowed identifiers in a special way.

[gbracha]

Added WontFix label.