[GHC] #15607: RebindableSyntax warns `return` is not in scope when trying to call `pure`, but does not desugar between the two

[GHC]

#15607: RebindableSyntax warns `return` is not in scope when trying to call `pure`,
but does not desugar between the two
-------------------------------------+-------------------------------------
        Reporter:  isovector         |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.6.1-beta1
      Resolution:                    |             Keywords:
                                     |  RebindableSyntax
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Description changed by isovector:

Old description:

> The following (incorrect) program fails to compile:
>
> {{{#!hs
> {-# LANGUAGE RebindableSyntax #-}
>
> import Prelude hiding (pure, return)
>
> t = do
>   pure 5
> }}}
>
> {{{
>     Not in scope: ‘return’
>     Perhaps you want to remove ‘return’ from the explicit hiding list
>     in the import of ‘Prelude’ (/home/sandy/Bug.hs:3:1-40).
>   |
> 6 |   pure 5
>   |
> }}}
>
> Changing the import of Prelude to no longer hide `return` now gives the
> correct error:
>
> {{{#!hs
> import Prelude hiding (pure)
> }}}
>
> {{{
>     • Variable not in scope: pure :: Integer -> t
>     • Perhaps you want to remove ‘pure’ from the explicit hiding list
>       in the import of ‘Prelude’ (/home/sandy/Bug.hs:3:1-28).
>   |
> 6 |   pure 5
>   |   ^^^^
> }}}
>

> This is particularly confusing when `pure` is in scope and would
> typecheck but `return` is not. The error suggests that there is
> desugaring going on behind the scenes to turn `pure` into `return` via
> rebindable syntax.

New description:

 The following (incorrect) program fails to compile:

 {{{#!hs
 {-# LANGUAGE RebindableSyntax #-}

 import Prelude hiding (pure, return)

 t = do
   pure 5
 }}}

 {{{
     Not in scope: ‘return’
     Perhaps you want to remove ‘return’ from the explicit hiding list
     in the import of ‘Prelude’ (/home/sandy/Bug.hs:3:1-40).
   |
 6 |   pure 5
   |
 }}}

 Notice that this error mentions `return` not being in scope, despite
 trying to call `pure`. This suggests to the user that there is some
 desugaring going on to transform pure into return (but there isn't!).

 Changing the import of Prelude to no longer hide `return` now gives the
 correct error:

 {{{#!hs
 import Prelude hiding (pure)
 }}}

 {{{
     • Variable not in scope: pure :: Integer -> t
     • Perhaps you want to remove ‘pure’ from the explicit hiding list
       in the import of ‘Prelude’ (/home/sandy/Bug.hs:3:1-28).
   |
 6 |   pure 5
   |   ^^^^
 }}}


 This is particularly confusing when `pure` is in scope and would typecheck
 but `return` is not. The error suggests that there is desugaring going on
 behind the scenes to turn `pure` into `return` via rebindable syntax.

--

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15607#comment:1>
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