Implement HasField constraint solving and modify OverloadedLabels

This implements automatic constraint solving for the new HasField class
and modifies the existing OverloadedLabels extension, as described in
the GHC proposal
(https://github.com/ghc-proposals/ghc-proposals/pull/6). Per the current
form of the proposal, it does *not* currently introduce a separate
`OverloadedRecordFields` extension.

This replaces D1687.

The users guide documentation still needs to be written, but I'll do
that after the implementation is merged, in case there are further
design changes.

Test Plan: new and modified tests in overloadedrecflds

Reviewers: simonpj, goldfire, dfeuer, bgamari, austin, hvr

Reviewed By: bgamari

Subscribers: maninalift, dfeuer, ysangkok, thomie, mpickering

Differential Revision: https://phabricator.haskell.org/D2708

3 files changed, 57 insertions, 16 deletions

diff --git a/libraries/base/GHC/OverloadedLabels.hs b/libraries/base/GHC/OverloadedLabels.hs
index f4a76cf8ea..7e27cf6bf4 100644
--- a/libraries/base/GHC/OverloadedLabels.hs
+++ b/libraries/base/GHC/OverloadedLabels.hs
@@ -1,48 +1,54 @@
-{-# LANGUAGE NoImplicitPrelude
-           , MultiParamTypeClasses
-           , MagicHash
-           , KindSignatures
+{-# LANGUAGE AllowAmbiguousTypes
            , DataKinds
+           , FlexibleInstances
+           , KindSignatures
+           , MultiParamTypeClasses
+           , ScopedTypeVariables
+           , TypeApplications
   #-}
 
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  GHC.OverloadedLabels
--- Copyright   :  (c) Adam Gundry 2015
+-- Copyright   :  (c) Adam Gundry 2015-2016
 -- License     :  see libraries/base/LICENSE
 --
 -- Maintainer  :  cvs-ghc@haskell.org
 -- Stability   :  internal
 -- Portability :  non-portable (GHC extensions)
 --
--- This module defines the `IsLabel` class is used by the
--- OverloadedLabels extension.  See the
+-- This module defines the 'IsLabel' class is used by the
+-- @OverloadedLabels@ extension.  See the
 -- <https://ghc.haskell.org/trac/ghc/wiki/Records/OverloadedRecordFields/OverloadedLabels wiki page>
 -- for more details.
 --
--- The key idea is that when GHC sees an occurrence of the new
--- overloaded label syntax @#foo@, it is replaced with
+-- When @OverloadedLabels@ is enabled, if GHC sees an occurrence of
+-- the overloaded label syntax @#foo@, it is replaced with
 --
--- > fromLabel (proxy# :: Proxy# "foo") :: alpha
+-- > fromLabel @"foo" :: alpha
 --
 -- plus a wanted constraint @IsLabel "foo" alpha@.
 --
+-- Note that if @RebindableSyntax@ is enabled, the desugaring of
+-- overloaded label syntax will make use of whatever @fromLabel@ is in
+-- scope.
+--
 -----------------------------------------------------------------------------
 
 -- Note [Overloaded labels]
 -- ~~~~~~~~~~~~~~~~~~~~~~~~
 -- An overloaded label is represented by the 'HsOverLabel' constructor
--- of 'HsExpr', which stores a 'FastString'.  It is passed through
--- unchanged by the renamer, and the type-checker transforms it into a
--- call to 'fromLabel'.  See Note [Type-checking overloaded labels] in
--- TcExpr for more details in how type-checking works.
+-- of 'HsExpr', which stores the 'FastString' text of the label and an
+-- optional id for the 'fromLabel' function to use (if
+-- RebindableSyntax is enabled) .  The type-checker transforms it into
+-- a call to 'fromLabel'.  See Note [Type-checking overloaded labels]
+-- in TcExpr for more details in how type-checking works.
 
 module GHC.OverloadedLabels
        ( IsLabel(..)
        ) where
 
 import GHC.Base ( Symbol )
-import GHC.Exts ( Proxy# )
 
 class IsLabel (x :: Symbol) a where
-  fromLabel :: Proxy# x -> a
+  fromLabel :: a
diff --git a/libraries/base/GHC/Records.hs b/libraries/base/GHC/Records.hs
new file mode 100644
index 0000000000..43c3931e86
--- /dev/null
+++ b/libraries/base/GHC/Records.hs
@@ -0,0 +1,34 @@
+{-# LANGUAGE AllowAmbiguousTypes
+           , FunctionalDependencies
+           , KindSignatures
+           , MultiParamTypeClasses
+           , PolyKinds
+  #-}
+
+-----------------------------------------------------------------------------
+-- |
+-- Module      :  GHC.Records
+-- Copyright   :  (c) Adam Gundry 2015-2016
+-- License     :  see libraries/base/LICENSE
+--
+-- Maintainer  :  cvs-ghc@haskell.org
+-- Stability   :  internal
+-- Portability :  non-portable (GHC extensions)
+--
+-- This module defines the 'HasField' class used by the
+-- @OverloadedRecordFields@ extension.  See the
+-- <https://ghc.haskell.org/trac/ghc/wiki/Records/OverloadedRecordFields
+-- wiki page> for more details.
+--
+-----------------------------------------------------------------------------
+
+module GHC.Records
+       ( HasField(..)
+       ) where
+
+-- | Constraint representing the fact that the field @x@ belongs to
+-- the record type @r@ and has field type @a@.  This will be solved
+-- automatically, but manual instances may be provided as well.
+class HasField (x :: k) r a | x r -> a where
+  -- | Selector function to extract the field from the record.
+  getField :: r -> a
diff --git a/libraries/base/base.cabal b/libraries/base/base.cabal
index 691dc83909..49e23e5c97 100644
--- a/libraries/base/base.cabal
+++ b/libraries/base/base.cabal
@@ -264,6 +264,7 @@ Library
         GHC.Ptr
         GHC.Read
         GHC.Real
+        GHC.Records
         GHC.RTS.Flags
         GHC.ST
         GHC.StaticPtr