Desugar `?x :: t` into `IP "x" t`.

Note that the `IP` constraint requires a "flexible" context/instance,
which is why when -XImplictParams is on, it also turns on
-XFlexibleInstances and -XFlexibleContexts

2 files changed, 10 insertions, 1 deletions

diff --git a/compiler/main/DynFlags.hs b/compiler/main/DynFlags.hs
index 31eb368b3f..2d6abea4ec 100644
--- a/compiler/main/DynFlags.hs
+++ b/compiler/main/DynFlags.hs
@@ -2127,6 +2127,11 @@ impliedFlags
     , (Opt_RecordWildCards,     turnOn, Opt_DisambiguateRecordFields)
 
     , (Opt_ParallelArrays, turnOn, Opt_ParallelListComp)
+
+    -- An implicit parameter constraint, `?x::Int`, is desugared into
+    -- `IP "x" Int`, which requires a flexible context/instance.
+    , (Opt_ImplicitParams, turnOn, Opt_FlexibleContexts)
+    , (Opt_ImplicitParams, turnOn, Opt_FlexibleInstances)
   ]
 
 optLevelFlags :: [([Int], DynFlag)]
diff --git a/compiler/typecheck/TcHsType.lhs b/compiler/typecheck/TcHsType.lhs
index b780c3b2e0..b5adc925c5 100644
--- a/compiler/typecheck/TcHsType.lhs
+++ b/compiler/typecheck/TcHsType.lhs
@@ -74,6 +74,7 @@ import UniqSupply
 import Outputable
 import FastString
 import Control.Monad ( unless, when, zipWithM )
+import PrelNames(ipClassName)
 \end{code}
 
 
@@ -420,7 +421,10 @@ tc_hs_type ipTy@(HsIParamTy n ty) exp_kind
   = do { ty' <- tc_lhs_type ty 
             (EK liftedTypeKind (ptext (sLit "The type argument of the implicit parameter had")))
        ; checkExpectedKind ipTy constraintKind exp_kind
-       ; return (mkIPPred n ty') }
+       ; ipClass <- tcLookupClass ipClassName
+       ; let n' = mkStrLitTy $ occNameFS $ nameOccName $ ipNameName n
+       ; return (mkClassPred ipClass [n',ty'])
+       }
 
 tc_hs_type ty@(HsEqTy ty1 ty2) exp_kind 
   = do { (ty1', kind1) <- tc_infer_lhs_type ty1