Do not check synonym RHS for ambiguity
Do not check synonym RHS for ambiguity

With this patch we no longer check the RHS of a type synonym
declaration for ambiguity.  It only affects type synonyms with foralls
on the RHS (which are rare in the first place), and it's arguably
over-aggressive to check them for ambiguity.  See TcValidity
Note [When we don't check for ambiguity]

This fixes the ASSERT failures in
   th                        T3100
   typecheck/should_compile  T3692
   typecheck/should_fail     T3592

mcschroeder/ghc,snoyberg/ghc,olsner/ghc,tjakway/ghcjvm,sdiehl/ghc,olsner/ghc,sgillespie/ghc,tjakway/ghcjvm,shlevy/ghc,vikraman/ghc,tjakway/ghcjvm,sdiehl/ghc,mcschroeder/ghc,sgillespie/ghc,vikraman/ghc,snoyberg/ghc,sgillespie/ghc,shlevy/ghc,mettekou/ghc,tjakway/ghcjvm,mettekou/ghc,sdiehl/ghc,tjakway/ghcjvm,ezyang/ghc,olsner/ghc,mettekou/ghc,ezyang/ghc,ezyang/ghc,tjakway/ghcjvm,vTurbine/ghc,snoyberg/ghc,olsner/ghc,sdiehl/ghc,mettekou/ghc,snoyberg/ghc,olsner/ghc,snoyberg/ghc,sgillespie/ghc,vikraman/ghc,mcschroeder/ghc,mettekou/ghc,mcschroeder/ghc,sgillespie/ghc,ezyang/ghc,sgillespie/ghc,shlevy/ghc,sgillespie/ghc,mettekou/ghc,vikraman/ghc,vTurbine/ghc,tjakway/ghcjvm,vTurbine/ghc,vikraman/ghc,vikraman/ghc,mcschroeder/ghc,ezyang/ghc,sdiehl/ghc,olsner/ghc,ezyang/ghc,shlevy/ghc,vikraman/ghc,shlevy/ghc,mcschroeder/ghc,snoyberg/ghc,ezyang/ghc,olsner/ghc,sdiehl/ghc,snoyberg/ghc,vTurbine/ghc,vTurbine/ghc,shlevy/ghc,mettekou/ghc,shlevy/ghc,vTurbine/ghc,sdiehl/ghc,vTurbine/ghc,mcschroeder/ghc