[GHC] #10072: Panic: generalised wildcards in RULES

[GHC]

#10072: Panic: generalised wildcards in RULES
-------------------------------------+-------------------------------------
              Reporter:  thomasw     |             Owner:  thomasw
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.10.1-rc2
  (Type checker)                     |  Operating System:  Unknown/Multiple
              Keywords:              |   Type of failure:  Compile-time
          Architecture:              |  crash
  Unknown/Multiple                   |        Blocked By:
             Test Case:              |   Related Tickets:
              Blocking:              |
Differential Revisions:              |
-------------------------------------+-------------------------------------
 Generalised wildcards (PartialTypeSignatures) in the binder type
 annotation of a RULE cause panics.

 Minimal example:
 {{{#!hs
 module WildcardInRuleBndrSig where
 {-# RULES
 "map/empty" forall (f :: a -> _). map f [] = []
   #-}
 }}}

 Output:
 {{{
 WildcardInRuleBndrSig.hs:3:31:ghc-stage1: panic! (the 'impossible'
 happened)
   (GHC version 7.11.20150209 for x86_64-unknown-linux):
         No skolem info: w__alY[sk]
 }}}


 When a wildcard is generalised over, the error message reporting the
 inferred type gives some extra info about the type variables occurring in
 the inferred type. This extra information is retrieved by looking up the
 skolem information (`getSkolemInfo`) for the type variables in the
 enclosing implications (`cec_encl`). The problem is that there are no
 enclosing implications in this case, hence the panic.

 Note that with the flags `-XPartialTypeSignatures` and `-fno-warn-partial-
 type-signatures` enabled, there is no panic, as no error/warning message
 is constructed.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10072>
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