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, 2 insertions, 2 deletions

diff --git a/compiler/GHC/Tc/Gen/Expr.hs b/compiler/GHC/Tc/Gen/Expr.hs
index 3a89daac0b..2d6b25df10 100644
--- a/compiler/GHC/Tc/Gen/Expr.hs
+++ b/compiler/GHC/Tc/Gen/Expr.hs
@@ -1709,7 +1709,7 @@ tcExprSig expr sig@(PartialSig { psig_name = name, sig_loc = loc })
     do { (tclvl, wanted, (expr', sig_inst))
              <- pushLevelAndCaptureConstraints  $
                 do { sig_inst <- tcInstSig sig
-                   ; expr' <- tcExtendNameTyVarEnv (sig_inst_skols sig_inst) $
+                   ; expr' <- tcExtendNameTyVarEnv (mapSnd binderVar $ sig_inst_skols sig_inst) $
                               tcExtendNameTyVarEnv (sig_inst_wcs   sig_inst) $
                               tcCheckExprNC expr (sig_inst_tau sig_inst)
                    ; return (expr', sig_inst) }
@@ -1730,7 +1730,7 @@ tcExprSig expr sig@(PartialSig { psig_name = name, sig_loc = loc })
        ; (binders, my_theta) <- chooseInferredQuantifiers inferred_theta
                                    tau_tvs qtvs (Just sig_inst)
        ; let inferred_sigma = mkInfSigmaTy qtvs inferred_theta tau
-             my_sigma       = mkForAllTys binders (mkPhiTy  my_theta tau)
+             my_sigma       = mkInvisForAllTys binders (mkPhiTy  my_theta tau)
        ; wrap <- if inferred_sigma `eqType` my_sigma -- NB: eqType ignores vis.
                  then return idHsWrapper  -- Fast path; also avoids complaint when we infer
                                           -- an ambiguous type and have AllowAmbiguousType