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$ cat > Blah.hs
{-# LANGUAGE GADTs #-}

module Blah where

blah :: (dd ~ (Double, Double)) => dd -> dd
blah (ax, bx)
    | ax < bx = blah (bx, ax)
    | otherwise = (0,0)

$ cat > Meh.hs
module Meh where

import Blah

meh = blah (0,0)

If we compile with -O2, in 2 separate calls to ghc, we get:

$ ghc -O2 Blah.hs
[1 of 1] Compiling Blah             ( Blah.hs, Blah.o )
match_co baling out
    <(ghc-prim:GHC.Types.Double{(w) tc 3u},
      ghc-prim:GHC.Types.Double{(w) tc 3u})>
match_co baling out
    <(ghc-prim:GHC.Types.Double{(w) tc 3u},
      ghc-prim:GHC.Types.Double{(w) tc 3u})>
match_co baling out
    <(ghc-prim:GHC.Types.Double{(w) tc 3u},
      ghc-prim:GHC.Types.Double{(w) tc 3u})>
match_co baling out
    <(ghc-prim:GHC.Types.Double{(w) tc 3u},
      ghc-prim:GHC.Types.Double{(w) tc 3u})>
match_co baling out
    <(ghc-prim:GHC.Types.Double{(w) tc 3u},
      ghc-prim:GHC.Types.Double{(w) tc 3u})>
match_co baling out
    <(ghc-prim:GHC.Types.Double{(w) tc 3u},
      ghc-prim:GHC.Types.Double{(w) tc 3u})>

$ ghc -O2 Meh.hs
[2 of 2] Compiling Meh              ( Meh.hs, Meh.o )
ghc: panic! (the 'impossible' happened)
  (GHC version 7.4.2 for x86_64-unknown-linux):
	tcIfaceType
    Refl
        (ghc-prim:GHC.Types.Double{(w) tc 3u},
         ghc-prim:GHC.Types.Double{(w) tc 3u})

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

Trac metadata

Trac field	Value
Version	7.7
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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