Refactor the typechecker to use ExpTypes.wip/exp-types

The idea here is described in [wiki:Typechecker]. Briefly,
this refactor keeps solid track of "synthesis" mode vs
"checking" in GHC's bidirectional type-checking algorithm.
When in synthesis mode, the expected type is just an IORef
to write to.

In addition, this patch does a significant reworking of
RebindableSyntax, allowing much more freedom in the types
of the rebindable operators. For example, we can now have
`negate :: Int -> Bool` and
`(>>=) :: m a -> (forall x. a x -> m b) -> m b`. The magic
is in tcSyntaxOp.

This addresses tickets #11397, #11452, and #11458.

Tests:
  typecheck/should_compile/{RebindHR,RebindNegate,T11397,T11458}
  th/T11452

1 files changed, 5 insertions, 3 deletions

diff --git a/compiler/rename/RnEnv.hs b/compiler/rename/RnEnv.hs
index d1ec1de6e6..868712b829 100644
--- a/compiler/rename/RnEnv.hs
+++ b/compiler/rename/RnEnv.hs
@@ -1562,21 +1562,23 @@ lookupIfThenElse
        ; if not rebindable_on
          then return (Nothing, emptyFVs)
          else do { ite <- lookupOccRn (mkVarUnqual (fsLit "ifThenElse"))
-                 ; return (Just (HsVar (noLoc ite)), unitFV ite) } }
+                 ; return ( Just (mkRnSyntaxExpr ite)
+                          , unitFV ite ) } }
 
 lookupSyntaxName :: Name                                -- The standard name
                  -> RnM (SyntaxExpr Name, FreeVars)     -- Possibly a non-standard name
 lookupSyntaxName std_name
   = do { rebindable_on <- xoptM LangExt.RebindableSyntax
        ; if not rebindable_on then
-           return (HsVar (noLoc std_name), emptyFVs)
+           return (mkRnSyntaxExpr std_name, emptyFVs)
          else
             -- Get the similarly named thing from the local environment
            do { usr_name <- lookupOccRn (mkRdrUnqual (nameOccName std_name))
-              ; return (HsVar (noLoc usr_name), unitFV usr_name) } }
+              ; return (mkRnSyntaxExpr usr_name, unitFV usr_name) } }
 
 lookupSyntaxNames :: [Name]                          -- Standard names
                   -> RnM ([HsExpr Name], FreeVars)   -- See comments with HsExpr.ReboundNames
+   -- this works with CmdTop, which wants HsExprs, not SyntaxExprs
 lookupSyntaxNames std_names
   = do { rebindable_on <- xoptM LangExt.RebindableSyntax
        ; if not rebindable_on then