[GHC] #4877: Template Haskell panic when splicing an infix expression with a non-variable middle bit

[GHC]

#4877: Template Haskell panic when splicing an infix expression with a non-
variable middle bit
---------------------------------+------------------------------------------
    Reporter:  benmachine        |       Owner:                    
        Type:  bug               |      Status:  new               
    Priority:  normal            |   Component:  Template Haskell  
     Version:  7.0.1             |    Keywords:                    
    Testcase:                    |   Blockedby:                    
          Os:  Unknown/Multiple  |    Blocking:                    
Architecture:  Unknown/Multiple  |     Failure:  Compile-time crash
---------------------------------+------------------------------------------
 Simple enough:

 {{{
 import Language.Haskell.TH
 import Language.Haskell.TH.Syntax
 panic = $(let var = varE . mkName in infixApp (var "x") (appE (var "f")
 (var "y")) (var "z"))
 }}}

 {{{
 [1 of 1] Compiling Panic            ( Panic.hs, interpreted )
 ghc: panic! (the 'impossible' happened)
   (GHC version 7.0.1 for i386-unknown-linux):
         rnExpr: unexpected expression
     {6:11-92}
     f{v} y{v}
       x{v} z{v}

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

 Of course the expression being spliced doesn't make any sense. In fact,
 I'd think that the only things that make sense in the centre of an infix
 expression were a single variable or constructor, so that field being of
 type Exp is arguably way too permissiveP. haskell-src seems to have a data
 type especially for this purpose - `HsQOp` with constructors `HsQVarOp
 HsQName` and `HsQConOp HsQName`.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4877>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler