Refactor treatment of wildcards

This patch began as a modest refactoring of HsType and friends, to
clarify and tidy up exactly where quantification takes place in types.
Although initially driven by making the implementation of wildcards more
tidy (and fixing a number of bugs), I gradually got drawn into a pretty
big process, which I've been doing on and off for quite a long time.

There is one compiler performance regression as a result of all
this, in perf/compiler/T3064.  I still need to look into that.

* The principal driving change is described in Note [HsType binders]
  in HsType.  Well worth reading!

* Those data type changes drive almost everything else.  In particular
  we now statically know where

       (a) implicit quantification only (LHsSigType),
           e.g. in instance declaratios and SPECIALISE signatures

       (b) implicit quantification and wildcards (LHsSigWcType)
           can appear, e.g. in function type signatures

* As part of this change, HsForAllTy is (a) simplified (no wildcards)
  and (b) split into HsForAllTy and HsQualTy.  The two contructors
  appear when and only when the correponding user-level construct
  appears.  Again see Note [HsType binders].

  HsExplicitFlag disappears altogether.

* Other simplifications

     - ExprWithTySig no longer needs an ExprWithTySigOut variant

     - TypeSig no longer needs a PostRn name [name] field
       for wildcards

     - PatSynSig records a LHsSigType rather than the decomposed
       pieces

     - The mysterious 'GenericSig' is now 'ClassOpSig'

* Renamed LHsTyVarBndrs to LHsQTyVars

* There are some uninteresting knock-on changes in Haddock,
  because of the HsSyn changes

I also did a bunch of loosely-related changes:

* We already had type synonyms CoercionN/CoercionR for nominal and
  representational coercions.  I've added similar treatment for

      TcCoercionN/TcCoercionR

      mkWpCastN/mkWpCastN

  All just type synonyms but jolly useful.

* I record-ised ForeignImport and ForeignExport

* I improved the (poor) fix to Trac #10896, by making
  TcTyClsDecls.checkValidTyCl recover from errors, but adding a
  harmless, abstract TyCon to the envt if so.

* I did some significant refactoring in RnEnv.lookupSubBndrOcc,
  for reasons that I have (embarrassingly) now totally forgotten.
  It had to do with something to do with import and export

Updates haddock submodule.

1 files changed, 5 insertions, 3 deletions

diff --git a/compiler/hsSyn/HsPat.hs b/compiler/hsSyn/HsPat.hs
index 320956261e..24ef065e2a 100644
--- a/compiler/hsSyn/HsPat.hs
+++ b/compiler/hsSyn/HsPat.hs
@@ -157,6 +157,8 @@ data Pat id
         pat_binds :: TcEvBinds,         -- Bindings involving those dictionaries
         pat_args  :: HsConPatDetails id,
         pat_wrap  :: HsWrapper          -- Extra wrapper to pass to the matcher
+                                        -- Only relevant for pattern-synonyms;
+                                        --   ignored for data cons
     }
 
         ------------ View patterns ---------------
@@ -199,9 +201,9 @@ data Pat id
   -- | - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnDcolon'
 
   -- For details on above see note [Api annotations] in ApiAnnotation
-  | SigPatIn        (LPat id)                  -- Pattern with a type signature
-                    (HsWithBndrs id (LHsType id)) -- Signature can bind both
-                                                  -- kind and type vars
+  | SigPatIn        (LPat id)                 -- Pattern with a type signature
+                    (LHsSigWcType id)         -- Signature can bind both
+                                              -- kind and type vars
 
   | SigPatOut       (LPat id)           -- Pattern with a type signature
                     Type