Don't assume that coercion variables have (~) types

The constraint solver doesn't zonk the types of coercion variables
so we can't assume that a coercion variable will have a (~) type.

Fixes Trac #7090.

MERGED from commit d6918e9

compiler/typecheck/TcEvidence.lhs

@@ -36,7 +36,7 @@ import Var
 import PprCore ()   -- Instance OutputableBndr TyVar
 import TypeRep  -- Knows type representation
 import TcType
-import Type( tyConAppArgN, getEqPredTys_maybe, tyConAppTyCon_maybe, getEqPredTys )
+import Type( tyConAppArgN, tyConAppTyCon_maybe, getEqPredTys )
 import TysPrim( funTyCon )
 import TyCon
 import PrelNames
@@ -114,7 +114,7 @@ isEqVar v = case tyConAppTyCon_maybe (varType v) of
 
 isTcReflCo_maybe :: TcCoercion -> Maybe TcType
 isTcReflCo_maybe (TcRefl ty) = Just ty
-isTcReflCo_maybe _             = Nothing
+isTcReflCo_maybe _           = Nothing
 
 isTcReflCo :: TcCoercion -> Bool
 isTcReflCo (TcRefl {}) = True
@@ -185,13 +185,12 @@ mkTcInstCos co tys          = foldl TcInstCo co tys
 
 mkTcCoVarCo :: EqVar -> TcCoercion
 -- ipv :: s ~ t  (the boxed equality type)
-mkTcCoVarCo ipv
-  | ty1 `eqType` ty2 = TcRefl ty1
-  | otherwise        = TcCoVarCo ipv
-  where
-    (ty1, ty2) = case getEqPredTys_maybe (varType ipv) of
-        Nothing  -> pprPanic "mkCoVarLCo" (ppr ipv)
-        Just tys -> tys
+mkTcCoVarCo ipv = TcCoVarCo ipv
+  -- Previously I checked for (ty ~ ty) and generated Refl,
+  -- but in fact ipv may not even (visibly) have a (t1 ~ t2) type, because
+  -- the constraint solver does not substitute in the types of
+  -- evidence variables as it goes.  In any case, the optimisation
+  -- will be done in the later zonking phase
 \end{code}
 
 \begin{code}