Make the desugarer a tiny bit cleverer on coercions (fixes Trac #7837)
Make the desugarer a tiny bit cleverer on coercions (fixes Trac #7837)

The desugarer was generating a redundant box/unbox pair on the
LHS of a RULE, which in turn made matching fail.

See Note [Simple coercions] in DsBinds.

gcampax/ghc,sdiehl/ghc,sdiehl/ghc,ezyang/ghc,da-x/ghc,shlevy/ghc,urbanslug/ghc,TomMD/ghc,hferreiro/replay,bitemyapp/ghc,vikraman/ghc,bitemyapp/ghc,acowley/ghc,elieux/ghc,hferreiro/replay,anton-dessiatov/ghc,da-x/ghc,acowley/ghc,sgillespie/ghc,shlevy/ghc,mcschroeder/ghc,mettekou/ghc,shlevy/ghc,mfine/ghc,wxwxwwxxx/ghc,shlevy/ghc,sgillespie/ghc,oldmanmike/ghc,lukexi/ghc,elieux/ghc,olsner/ghc,anton-dessiatov/ghc,elieux/ghc,gridaphobe/ghc,gcampax/ghc,gridaphobe/ghc,lukexi/ghc-7.8-arm64,ghc-android/ghc,oldmanmike/ghc,tibbe/ghc,forked-upstream-packages-for-ghcjs/ghc,green-haskell/ghc,ml9951/ghc,sgillespie/ghc,spacekitteh/smcghc,hferreiro/replay,hferreiro/replay,mettekou/ghc,nkaretnikov/ghc,ekmett/ghc,AlexanderPankiv/ghc,wxwxwwxxx/ghc,mfine/ghc,snoyberg/ghc,nushio3/ghc,elieux/ghc,green-haskell/ghc,da-x/ghc,forked-upstream-packages-for-ghcjs/ghc,siddhanathan/ghc,lukexi/ghc,lukexi/ghc,wxwxwwxxx/ghc,fmthoma/ghc,ezyang/ghc,GaloisInc/halvm-ghc,lukexi/ghc,da-x/ghc,shlevy/ghc,tibbe/ghc,bitemyapp/ghc,ml9951/ghc,vTurbine/ghc,nathyong/microghc-ghc,vTurbine/ghc,AlexanderPankiv/ghc,gcampax/ghc,ml9951/ghc,nathyong/microghc-ghc,siddhanathan/ghc,ryantm/ghc,siddhanathan/ghc,nkaretnikov/ghc,sdiehl/ghc,vTurbine/ghc,mfine/ghc,AlexanderPankiv/ghc,ekmett/ghc,olsner/ghc,hferreiro/replay,frantisekfarka/ghc-dsi,acowley/ghc,da-x/ghc,mcschroeder/ghc,holzensp/ghc,TomMD/ghc,mfine/ghc,tibbe/ghc,GaloisInc/halvm-ghc,olsner/ghc,mcschroeder/ghc,ezyang/ghc,gridaphobe/ghc,bitemyapp/ghc,nkaretnikov/ghc,sdiehl/ghc,urbanslug/ghc,tibbe/ghc,fmthoma/ghc,gridaphobe/ghc,wxwxwwxxx/ghc,ghc-android/ghc,sgillespie/ghc,ekmett/ghc,mcschroeder/ghc,ezyang/ghc,green-haskell/ghc,ml9951/ghc,AlexanderPankiv/ghc,nushio3/ghc,gcampax/ghc,gcampax/ghc,forked-upstream-packages-for-ghcjs/ghc,siddhanathan/ghc,vikraman/ghc,GaloisInc/halvm-ghc,nushio3/ghc,christiaanb/ghc,gridaphobe/ghc,snoyberg/ghc,elieux/ghc,ghc-android/ghc,holzensp/ghc,tjakway/ghcjvm,gcampax/ghc,mfine/ghc,TomMD/ghc,TomMD/ghc,lukexi/ghc-7.8-arm64,ml9951/ghc,fmthoma/ghc,tjakway/ghcjvm,urbanslug/ghc,jstolarek/ghc,frantisekfarka/ghc-dsi,sdiehl/ghc,anton-dessiatov/ghc,GaloisInc/halvm-ghc,siddhanathan/ghc,oldmanmike/ghc,lukexi/ghc-7.8-arm64,ekmett/ghc,vTurbine/ghc,anton-dessiatov/ghc,vikraman/ghc,holzensp/ghc,sdiehl/ghc,ryantm/ghc,nkaretnikov/ghc,da-x/ghc,oldmanmike/ghc,ryantm/ghc,christiaanb/ghc,GaloisInc/halvm-ghc,elieux/ghc,christiaanb/ghc,snoyberg/ghc,urbanslug/ghc,TomMD/ghc,green-haskell/ghc,mettekou/ghc,acowley/ghc,nathyong/microghc-ghc,TomMD/ghc,christiaanb/ghc,ml9951/ghc,nkaretnikov/ghc,mettekou/ghc,ryantm/ghc,bitemyapp/ghc,wxwxwwxxx/ghc,spacekitteh/smcghc,AlexanderPankiv/ghc,snoyberg/ghc,wxwxwwxxx/ghc,ghc-android/ghc,lukexi/ghc,tjakway/ghcjvm,TomMD/ghc,tjakway/ghcjvm,frantisekfarka/ghc-dsi,gridaphobe/ghc,sgillespie/ghc,vikraman/ghc,gcampax/ghc,ghc-android/ghc,GaloisInc/halvm-ghc,anton-dessiatov/ghc,nushio3/ghc,nushio3/ghc,lukexi/ghc-7.8-arm64,fmthoma/ghc,ekmett/ghc,jstolarek/ghc,vTurbine/ghc,christiaanb/ghc,snoyberg/ghc,fmthoma/ghc,holzensp/ghc,oldmanmike/ghc,AlexanderPankiv/ghc,ghc-android/ghc,urbanslug/ghc,spacekitteh/smcghc,olsner/ghc,sdiehl/ghc,fmthoma/ghc,oldmanmike/ghc,frantisekfarka/ghc-dsi,spacekitteh/smcghc,shlevy/ghc,nushio3/ghc,christiaanb/ghc,nathyong/microghc-ghc,GaloisInc/halvm-ghc,lukexi/ghc-7.8-arm64,mcschroeder/ghc,vikraman/ghc,tjakway/ghcjvm,shlevy/ghc,nathyong/microghc-ghc,jstolarek/ghc,ghc-android/ghc,AlexanderPankiv/ghc,forked-upstream-packages-for-ghcjs/ghc,mcschroeder/ghc,acowley/ghc,snoyberg/ghc,mfine/ghc,nathyong/microghc-ghc,nkaretnikov/ghc,olsner/ghc,snoyberg/ghc,nkaretnikov/ghc,siddhanathan/ghc,anton-dessiatov/ghc,acowley/ghc,vikraman/ghc,urbanslug/ghc,fmthoma/ghc,frantisekfarka/ghc-dsi,da-x/ghc,mettekou/ghc,nathyong/microghc-ghc,holzensp/ghc,mfine/ghc,ml9951/ghc,green-haskell/ghc,wxwxwwxxx/ghc,mettekou/ghc,ryantm/ghc,jstolarek/ghc,christiaanb/ghc,sgillespie/ghc,ml9951/ghc,nushio3/ghc,oldmanmike/ghc,mettekou/ghc,ezyang/ghc,vTurbine/ghc,mcschroeder/ghc,urbanslug/ghc,siddhanathan/ghc,ezyang/ghc,elieux/ghc,forked-upstream-packages-for-ghcjs/ghc,forked-upstream-packages-for-ghcjs/ghc,ezyang/ghc,tibbe/ghc,tjakway/ghcjvm,vikraman/ghc,olsner/ghc,gridaphobe/ghc,sgillespie/ghc,acowley/ghc,forked-upstream-packages-for-ghcjs/ghc,vTurbine/ghc,spacekitteh/smcghc,hferreiro/replay,tjakway/ghcjvm,hferreiro/replay,olsner/ghc,anton-dessiatov/ghc,jstolarek/ghc