Record pattern synonyms

This patch implements an extension to pattern synonyms which allows user
to specify pattern synonyms using record syntax. Doing so generates
appropriate selectors and update functions.

=== Interaction with Duplicate Record Fields ===

The implementation given here isn't quite as general as it could be with
respect to the recently-introduced `DuplicateRecordFields` extension.

Consider the following module:

    {-# LANGUAGE DuplicateRecordFields #-}
    {-# LANGUAGE PatternSynonyms #-}

    module Main where

    pattern S{a, b} = (a, b)
    pattern T{a}    = Just a

    main = do
      print S{ a = "fst", b = "snd" }
      print T{ a = "a" }

In principle, this ought to work, because there is no ambiguity. But at
the moment it leads to a "multiple declarations of a" error. The problem
is that pattern synonym record selectors don't do the same name mangling
as normal datatypes when DuplicateRecordFields is enabled. They could,
but this would require some work to track the field label and selector
name separately.

In particular, we currently represent datatype selectors in the third
component of AvailTC, but pattern synonym selectors are just represented
as Avails (because they don't have a corresponding type constructor).
Moreover, the GlobalRdrElt for a selector currently requires it to have
a parent tycon.

(example due to Adam Gundry)

=== Updating Explicitly Bidirectional Pattern Synonyms ===

Consider the following

```
pattern Silly{a} <- [a] where
  Silly a = [a, a]

f1 = a [5] -- 5

f2 = [5] {a = 6} -- currently [6,6]
```

=== Fixing Polymorphic Updates ===

They were fixed by adding these two lines in `dsExpr`. This might break
record updates but will be easy to fix.

```
+ ; let req_wrap = mkWpTyApps (mkTyVarTys univ_tvs)

- , pat_wrap = idHsWrapper }
+, pat_wrap = req_wrap }
```

=== Mixed selectors error ===

Note [Mixed Record Field Updates]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Consider the following pattern synonym.

    data MyRec = MyRec { foo :: Int, qux :: String }

    pattern HisRec{f1, f2} = MyRec{foo = f1, qux=f2}

This allows updates such as the following

    updater :: MyRec -> MyRec
    updater a = a {f1 = 1 }

It would also make sense to allow the following update (which we
reject).

    updater a = a {f1 = 1, qux = "two" } ==? MyRec 1 "two"

This leads to confusing behaviour when the selectors in fact refer the
same field.

    updater a = a {f1 = 1, foo = 2} ==? ???

For this reason, we reject a mixture of pattern synonym and normal
record selectors in the same update block. Although of course we still
allow the following.

    updater a = (a {f1 = 1}) {foo = 2}

    > updater (MyRec 0 "str")
    MyRec 2 "str"

1 files changed, 26 insertions, 5 deletions

diff --git a/compiler/basicTypes/Id.hs b/compiler/basicTypes/Id.hs
index 7b54baae15..e22a77c07c 100644
--- a/compiler/basicTypes/Id.hs
+++ b/compiler/basicTypes/Id.hs
@@ -54,11 +54,13 @@ module Id (
         isStrictId,
         isExportedId, isLocalId, isGlobalId,
         isRecordSelector, isNaughtyRecordSelector,
+        isPatSynRecordSelector,
+        isDataConRecordSelector,
         isClassOpId_maybe, isDFunId,
         isPrimOpId, isPrimOpId_maybe,
         isFCallId, isFCallId_maybe,
         isDataConWorkId, isDataConWorkId_maybe, isDataConId_maybe, idDataCon,
-        isConLikeId, isBottomingId, idIsFrom,
+        idConLike, isConLikeId, isBottomingId, idIsFrom,
         hasNoBinding,
 
         -- ** Evidence variables
@@ -114,7 +116,6 @@ import Var( Id, DictId,
             isId, isLocalId, isGlobalId, isExportedId )
 import qualified Var
 
-import TyCon
 import Type
 import TysPrim
 import DataCon
@@ -132,6 +133,7 @@ import UniqSupply
 import FastString
 import Util
 import StaticFlags
+import {-# SOURCE #-} ConLike ( ConLike(..) )
 
 -- infixl so you can say (id `set` a `set` b)
 infixl  1 `setIdUnfoldingLazily`,
@@ -354,14 +356,17 @@ That is what is happening in, say tidy_insts in TidyPgm.
 -}
 
 -- | If the 'Id' is that for a record selector, extract the 'sel_tycon'. Panic otherwise.
-recordSelectorTyCon :: Id -> TyCon
+recordSelectorTyCon :: Id -> RecSelParent
 recordSelectorTyCon id
   = case Var.idDetails id of
-        RecSelId { sel_tycon = tycon } -> tycon
+        RecSelId { sel_tycon = parent } -> parent
         _ -> panic "recordSelectorTyCon"
 
+
 isRecordSelector        :: Id -> Bool
 isNaughtyRecordSelector :: Id -> Bool
+isPatSynRecordSelector  :: Id -> Bool
+isDataConRecordSelector  :: Id -> Bool
 isPrimOpId              :: Id -> Bool
 isFCallId               :: Id -> Bool
 isDataConWorkId         :: Id -> Bool
@@ -373,7 +378,15 @@ isFCallId_maybe         :: Id -> Maybe ForeignCall
 isDataConWorkId_maybe   :: Id -> Maybe DataCon
 
 isRecordSelector id = case Var.idDetails id of
-                        RecSelId {}  -> True
+                        RecSelId {}     -> True
+                        _               -> False
+
+isDataConRecordSelector id = case Var.idDetails id of
+                        RecSelId {sel_tycon = RecSelData _} -> True
+                        _               -> False
+
+isPatSynRecordSelector id = case Var.idDetails id of
+                        RecSelId {sel_tycon = RecSelPatSyn _} -> True
                         _               -> False
 
 isNaughtyRecordSelector id = case Var.idDetails id of
@@ -424,6 +437,14 @@ idDataCon :: Id -> DataCon
 -- INVARIANT: @idDataCon (dataConWrapId d) = d@: remember, 'dataConWrapId' can return either the wrapper or the worker
 idDataCon id = isDataConId_maybe id `orElse` pprPanic "idDataCon" (ppr id)
 
+idConLike :: Id -> ConLike
+idConLike id =
+  case Var.idDetails id of
+       DataConWorkId con -> RealDataCon con
+       DataConWrapId con -> RealDataCon con
+       PatSynBuilderId ps -> PatSynCon ps
+       _               -> pprPanic "idConLike" (ppr id)
+
 hasNoBinding :: Id -> Bool
 -- ^ Returns @True@ of an 'Id' which may not have a
 -- binding, even though it is defined in this module.