tc: Never quantify over special constraints

compiler/GHC/Core/Predicate.hs

@@ -19,7 +19,7 @@ module GHC.Core.Predicate (
   mkHeteroPrimEqPred, mkHeteroReprPrimEqPred,
 
   -- Special predicates
-  SpecialPred(..), specialPredTyCon,
+  SpecialPred(..), specialPredTyCon, isSpecialPredTyCon, isSpecialPred,
 
   -- Class predicates
   mkClassPred, isDictTy,
@@ -220,6 +220,9 @@ instance Outputable SpecialPred where
 specialPredTyCon :: SpecialPred -> TyCon
 specialPredTyCon ConcretePrimPred = concretePrimTyCon
 
+isSpecialPredTyCon :: TyCon -> Bool
+isSpecialPredTyCon tc = tc `hasKey` concretePrimTyConKey
+
 {-------------------------------------------
 Predicates on PredType
 --------------------------------------------}
@@ -251,7 +254,7 @@ isEqPredClass :: Class -> Bool
 isEqPredClass cls =  cls `hasKey` eqTyConKey
                   || cls `hasKey` heqTyConKey
 
-isClassPred, isEqPred, isEqPrimPred :: PredType -> Bool
+isClassPred, isEqPred, isEqPrimPred, isSpecialPred :: PredType -> Bool
 isClassPred ty = case tyConAppTyCon_maybe ty of
     Just tyCon | isClassTyCon tyCon -> True
     _                               -> False
@@ -267,6 +270,10 @@ isEqPred ty  -- True of (a ~ b) and (a ~~ b)
 isEqPrimPred ty = isCoVarType ty
   -- True of (a ~# b) (a ~R# b)
 
+isSpecialPred ty = case tyConAppTyCon_maybe ty of
+  Just tc -> isSpecialPredTyCon tc
+  _       -> False
+
 isCTupleClass :: Class -> Bool
 isCTupleClass cls = isTupleTyCon (classTyCon cls)
 

compiler/GHC/Tc/Solver.hs

@@ -1097,7 +1097,9 @@ simplifyInfer rhs_tclvl infer_mode sigs name_taus wanteds
              wanted_transformed = dropDerivedWC wanted_transformed_incl_derivs
              quant_pred_candidates
                | definite_error = []
-               | otherwise      = ctsPreds (approximateWC False wanted_transformed)
+               | otherwise      = filterOut isSpecialPred
+                                $ ctsPreds
+                                $ approximateWC False wanted_transformed
 
        -- Decide what type variables and constraints to quantify
        -- NB: quant_pred_candidates is already fully zonked