Bug: when using (b=1; return) if true followed by any statement

[rosenfeld]

Choose one: is this a bug report or feature request? Bug

Input Code

a = ->
  (b=1; return) if true
  #b=2

The code above compiles successfully in the last release, but if you uncomment the last line then it throws "error: cannot use a pure statement in an expression". This used to work but after upgrading to latest CoffeeScript my existing code is broken. Was this change intentional?

• CoffeeScript version: 2.2.3
• Node.js version: 8.9.1

[rosenfeld]

This bug seems to have been introduced in release 2.0.2 since it was working on 2.0.1.

[vendethiel]

The compiler in 2.0.2+ tries to compile (b=1;return) using the comma operator.
Seems like 6faa7f2 is the culprit, removing a unwrapAll.
The removal was to accomodate with Flow, but broke this particular behavior.

[zdenko]

This looks more like #3441. The PR for it (#4849) didn't cover Return (which is the instance of Return and not StatementLiteral).

[vendethiel]

@zdenko Ah, I tend to forget Coffee doesn't have the HURL type. Anyway, if you add the unwrapAll back where I linked, you'll see the Flow tests failing, but this one bug fixed. It's better fixed where you did, though :-).

[zdenko]

@vendethiel why is this not allowed -> (yield return)?

[vendethiel]

@zdenko What do you suggest it compiles to?

[zdenko]

(function* () {}); like here?

[zdenko]

@lydell I think you might be the right person to ask about this. Why can't -> (yield return) compile to (function* () {});?

[vendethiel]

Oh, that's right, we have this syntax. Well, yield return is a YieldReturn is a Return, so the fix for this issue probably fixes that as well.

[lydell]

@zdenko Sorry, I have no idea why that doesn't compile. Your suggested compilation sounds sensible, though.