[GHC] #15720: Assign to literals is allowed in ghci

[GHC]

#15720: Assign to literals is allowed in ghci
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        Reporter:  sighingnow        |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  GHCi              |              Version:  8.6.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC accepts       |  Unknown/Multiple
  invalid program                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 This should perhaps be an on-by-default warning, but it's valid Haskell
 code. `1 = 2` is a lazy pattern match. Because it's lazy but binds no
 variables, we never see an error reported. This is maybe silly, but it
 doesn't quite seem worth putting another rule in Haskell to avoid this
 possibility.

 That said, a pattern-match that binds no variables is warned by `-Wunused-
 pattern-binds`; maybe this should really be its own warning.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15720#comment:1>
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