Refactor the treatment of predicate types

Trac #15648 showed that GHC was a bit confused about the
difference between the types for

* Predicates
* Coercions
* Evidence (in the typechecker constraint solver)

This patch cleans it up. See especially Type.hs
Note [Types for coercions, predicates, and evidence]

Particular changes

* Coercion types (a ~# b) and (a ~#R b) are not predicate types
  (so isPredTy reports False for them) and are not implicitly
  instantiated by the type checker.  This is a real change, but
  it consistently reflects that fact that (~#) and (~R#) really
  are different from predicates.

* isCoercionType is renamed to isCoVarType

* During type inference, simplifyInfer, we do /not/ want to infer
  a constraint (a ~# b), because that is no longer a predicate type.
  So we 'lift' it to (a ~ b). See TcType
  Note [Lift equality constaints when quantifying]

* During type inference for pattern synonyms, we need to 'lift'
  provided constraints of type (a ~# b) to (a ~ b).  See
  Note [Equality evidence in pattern synonyms] in PatSyn

* But what about (forall a. Eq a => a ~# b)? Is that a
  predicate type?  No -- it does not have kind Constraint.
  Is it an evidence type?  Perhaps, but awkwardly so.

  In the end I decided NOT to make it an evidence type,
  and to ensure the the type inference engine never
  meets it.  This made me /simplify/ the code in
  TcCanonical.makeSuperClasses; see TcCanonical
  Note [Equality superclasses in quantified constraints]

  Instead I moved the special treatment for primitive
  equality to TcInteract.doTopReactOther.  See TcInteract
  Note [Looking up primitive equalities in quantified constraints]

  Also see Note [Evidence for quantified constraints] in Type.

  All this means I can have
     isEvVarType ty = isCoVarType ty || isPredTy ty
  which is nice.

All in all, rather a lot of work for a small refactoring,
but I think it's a real improvement.

1 files changed, 9 insertions, 9 deletions

diff --git a/compiler/basicTypes/Id.hs b/compiler/basicTypes/Id.hs
index c1d281edd6..78518ee094 100644
--- a/compiler/basicTypes/Id.hs
+++ b/compiler/basicTypes/Id.hs
@@ -267,20 +267,20 @@ mkVanillaGlobalWithInfo = mkGlobalId VanillaId
 -- | For an explanation of global vs. local 'Id's, see "Var#globalvslocal"
 mkLocalId :: Name -> Type -> Id
 mkLocalId name ty = mkLocalIdWithInfo name ty vanillaIdInfo
- -- It's tempting to ASSERT( not (isCoercionType ty) ), but don't. Sometimes,
+ -- It's tempting to ASSERT( not (isCoVarType ty) ), but don't. Sometimes,
  -- the type is a panic. (Search invented_id)
 
 -- | Make a local CoVar
 mkLocalCoVar :: Name -> Type -> CoVar
 mkLocalCoVar name ty
-  = ASSERT( isCoercionType ty )
+  = ASSERT( isCoVarType ty )
     Var.mkLocalVar CoVarId name ty vanillaIdInfo
 
 -- | Like 'mkLocalId', but checks the type to see if it should make a covar
 mkLocalIdOrCoVar :: Name -> Type -> Id
 mkLocalIdOrCoVar name ty
-  | isCoercionType ty = mkLocalCoVar name ty
-  | otherwise         = mkLocalId    name ty
+  | isCoVarType ty = mkLocalCoVar name ty
+  | otherwise      = mkLocalId    name ty
 
 -- | Make a local id, with the IdDetails set to CoVarId if the type indicates
 -- so.
@@ -288,8 +288,8 @@ mkLocalIdOrCoVarWithInfo :: Name -> Type -> IdInfo -> Id
 mkLocalIdOrCoVarWithInfo name ty info
   = Var.mkLocalVar details name ty info
   where
-    details | isCoercionType ty = CoVarId
-            | otherwise         = VanillaId
+    details | isCoVarType ty = CoVarId
+            | otherwise      = VanillaId
 
     -- proper ids only; no covars!
 mkLocalIdWithInfo :: Name -> Type -> IdInfo -> Id
@@ -311,7 +311,7 @@ mkExportedVanillaId name ty = Var.mkExportedLocalVar VanillaId name ty vanillaId
 -- | Create a system local 'Id'. These are local 'Id's (see "Var#globalvslocal")
 -- that are created by the compiler out of thin air
 mkSysLocal :: FastString -> Unique -> Type -> Id
-mkSysLocal fs uniq ty = ASSERT( not (isCoercionType ty) )
+mkSysLocal fs uniq ty = ASSERT( not (isCoVarType ty) )
                         mkLocalId (mkSystemVarName uniq fs) ty
 
 -- | Like 'mkSysLocal', but checks to see if we have a covar type
@@ -328,7 +328,7 @@ mkSysLocalOrCoVarM fs ty
 
 -- | Create a user local 'Id'. These are local 'Id's (see "Var#globalvslocal") with a name and location that the user might recognize
 mkUserLocal :: OccName -> Unique -> Type -> SrcSpan -> Id
-mkUserLocal occ uniq ty loc = ASSERT( not (isCoercionType ty) )
+mkUserLocal occ uniq ty loc = ASSERT( not (isCoVarType ty) )
                               mkLocalId (mkInternalName uniq occ loc) ty
 
 -- | Like 'mkUserLocal', but checks if we have a coercion type
@@ -585,7 +585,7 @@ isDeadBinder bndr | isId bndr = isDeadOcc (idOccInfo bndr)
 -}
 
 isEvVar :: Var -> Bool
-isEvVar var = isPredTy (varType var)
+isEvVar var = isEvVarType (varType var)
 
 isDictId :: Id -> Bool
 isDictId id = isDictTy (idType id)