[GHC] #12229: Narrow the scope of record wildcards notation slightly

[GHC]

#12229: Narrow the scope of record wildcards notation slightly
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        Reporter:  simonpj           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 Interestingly, at least some of the things that are now rejected were ALSO
 rejected before, in a different and more confusing way.  Consider
 {{{
 module T12229 where

 data T = MkT { x, pi :: Float }

 f x = MkT { .. }  -- 'pi' is not initialised, because
                   -- there is no local binding
 }}}
 Previously the record-wildcard expansion said "oh, there is one `pi` in
 scope that isn't a record selector, so fine, let's expand it"; so we got
 {{{
 f x = MkT { x = x, pi = pi }
 }}}
 But THEN we got (from 7.10)
 {{{
 T12229 .hs:7:7:
     Ambiguous occurrence ‘pi’
     It could refer to either ‘T12229.pi’,
                              defined at T12229.hs:5:19
                           or ‘Prelude.pi’,
                              imported from ‘Prelude’ at T12229.hs:3:8-19
                              (and originally defined in ‘GHC.Float’)
 }}}
 which is stupid and confusing.  With our new change we get
 {{{
 T12229.hs:7:7: warning: [-Wmissing-fields (in -Wdefault)]
      Fields of ‘MkT’ not initialised: pi
      In the expression: MkT {..}
      In an equation for ‘f’: f x = MkT {..}
 }}}
 which is better.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12229#comment:1>
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