Explicit Specificity

Implementation for Ticket #16393.
Explicit specificity allows users to manually create inferred type variables,
by marking them with braces.
This way, the user determines which variables can be instantiated through
visible type application.

The additional syntax is included in the parser, allowing users to write
braces in type variable binders (type signatures, data constructors etc).
This information is passed along through the renamer and verified in the
type checker.
The AST for type variable binders, data constructors, pattern synonyms,
partial signatures and Template Haskell has been updated to include the
specificity of type variables.

Minor notes:
- Bumps haddock submodule
- Disables pattern match checking in GHC.Iface.Type with GHC 8.8

1 files changed, 14 insertions, 5 deletions

diff --git a/compiler/GHC/Hs/Utils.hs b/compiler/GHC/Hs/Utils.hs
index 6e89b6844a..6301927b26 100644
--- a/compiler/GHC/Hs/Utils.hs
+++ b/compiler/GHC/Hs/Utils.hs
@@ -736,14 +736,23 @@ typeToLHsType ty
       foldl' (\f (arg, flag) ->
                let arg' = go arg in
                case flag of
-                 Inferred  -> f
-                 Specified -> f `nlHsAppKindTy` arg'
+                 -- See Note [Explicit Case Statement for Specificity]
+                 Invisible spec -> case spec of
+                   InferredSpec  -> f
+                   SpecifiedSpec -> f `nlHsAppKindTy` arg'
                  Required  -> f `nlHsAppTy`     arg')
              head (zip args arg_flags)
 
-    go_tv :: TyVar -> LHsTyVarBndr GhcPs
-    go_tv tv = noLoc $ KindedTyVar noExtField (noLoc (getRdrName tv))
-                                   (go (tyVarKind tv))
+    argf_to_spec :: ArgFlag -> Specificity
+    argf_to_spec Required      = SpecifiedSpec
+    -- see Note [Specificity in HsForAllTy] in GHC.Hs.Types
+    argf_to_spec (Invisible s) = s
+
+    go_tv :: TyVarBinder -> LHsTyVarBndr Specificity GhcPs
+    go_tv (Bndr tv argf) = noLoc $ KindedTyVar noExtField
+                                               (argf_to_spec argf)
+                                               (noLoc (getRdrName tv))
+                                               (go (tyVarKind tv))
 
 {-
 Note [Kind signatures in typeToLHsType]