Implement deriving strategies

Allows users to explicitly request which approach to `deriving` to use
via keywords, e.g.,

```
newtype Foo = Foo Bar
  deriving Eq
  deriving stock    Ord
  deriving newtype Show
```

Fixes #10598. Updates haddock submodule.

Test Plan: ./validate

Reviewers: hvr, kosmikus, goldfire, alanz, bgamari, simonpj, austin,
erikd, simonmar

Reviewed By: alanz, bgamari, simonpj

Subscribers: thomie, mpickering, oerjan

Differential Revision: https://phabricator.haskell.org/D2280

GHC Trac Issues: #10598

1 files changed, 29 insertions, 11 deletions

diff --git a/libraries/template-haskell/Language/Haskell/TH/Syntax.hs b/libraries/template-haskell/Language/Haskell/TH/Syntax.hs
index 00ac0b308b..afe961b50e 100644
--- a/libraries/template-haskell/Language/Haskell/TH/Syntax.hs
+++ b/libraries/template-haskell/Language/Haskell/TH/Syntax.hs
@@ -1549,13 +1549,15 @@ data Dec
   | ValD Pat Body [Dec]           -- ^ @{ p = b where decs }@
   | DataD Cxt Name [TyVarBndr]
           (Maybe Kind)            -- Kind signature (allowed only for GADTs)
-          [Con] Cxt
+          [Con] [DerivClause]
                                   -- ^ @{ data Cxt x => T x = A x | B (T x)
-                                  --       deriving (Z,W)}@
+                                  --       deriving (Z,W)
+                                  --       deriving stock Eq }@
   | NewtypeD Cxt Name [TyVarBndr]
              (Maybe Kind)         -- Kind signature
-             Con Cxt              -- ^ @{ newtype Cxt x => T x = A (B x)
-                                  --       deriving (Z,W Q)}@
+             Con [DerivClause]    -- ^ @{ newtype Cxt x => T x = A (B x)
+                                  --       deriving (Z,W Q)
+                                  --       deriving stock Eq }@
   | TySynD Name [TyVarBndr] Type  -- ^ @{ type T x = (x,x) }@
   | ClassD Cxt Name [TyVarBndr]
          [FunDep] [Dec]           -- ^ @{ class Eq a => Ord a where ds }@
@@ -1578,14 +1580,18 @@ data Dec
 
   | DataInstD Cxt Name [Type]
              (Maybe Kind)         -- Kind signature
-             [Con] Cxt            -- ^ @{ data instance Cxt x => T [x]
-                                  --       = A x | B (T x) deriving (Z,W)}@
+             [Con] [DerivClause]  -- ^ @{ data instance Cxt x => T [x]
+                                  --       = A x | B (T x)
+                                  --       deriving (Z,W)
+                                  --       deriving stock Eq }@
 
   | NewtypeInstD Cxt Name [Type]
-                 (Maybe Kind)     -- Kind signature
-                 Con Cxt          -- ^ @{ newtype instance Cxt x => T [x]
-                                  --        = A (B x) deriving (Z,W)}@
-  | TySynInstD Name TySynEqn      -- ^ @{ type instance ... }@
+                 (Maybe Kind)      -- Kind signature
+                 Con [DerivClause] -- ^ @{ newtype instance Cxt x => T [x]
+                                   --        = A (B x)
+                                   --        deriving (Z,W)
+                                   --        deriving stock Eq }@
+  | TySynInstD Name TySynEqn       -- ^ @{ type instance ... }@
 
   -- | open type families (may also appear in [Dec] of 'ClassD' and 'InstanceD')
   | OpenTypeFamilyD TypeFamilyHead
@@ -1595,7 +1601,8 @@ data Dec
        -- ^ @{ type family F a b = (r :: *) | r -> a where ... }@
 
   | RoleAnnotD Name [Role]     -- ^ @{ type role T nominal representational }@
-  | StandaloneDerivD Cxt Type  -- ^ @{ deriving instance Ord a => Ord (Foo a) }@
+  | StandaloneDerivD (Maybe DerivStrategy) Cxt Type
+       -- ^ @{ deriving stock instance Ord a => Ord (Foo a) }@
   | DefaultSigD Name Type      -- ^ @{ default size :: Data a => a -> Int }@
 
   -- | Pattern Synonyms
@@ -1620,6 +1627,17 @@ data Overlap = Overlappable   -- ^ May be overlapped by more specific instances
                               -- available.
   deriving( Show, Eq, Ord, Data, Generic )
 
+-- | A single @deriving@ clause at the end of a datatype.
+data DerivClause = DerivClause (Maybe DerivStrategy) Cxt
+    -- ^ @{ deriving stock (Eq, Ord) }@
+  deriving( Show, Eq, Ord, Data, Generic )
+
+-- | What the user explicitly requests when deriving an instance.
+data DerivStrategy = Stock    -- ^ A \"standard\" derived instance
+                   | Anyclass -- ^ @-XDeriveAnyClass@
+                   | Newtype  -- ^ @-XGeneralizedNewtypeDeriving@
+  deriving( Show, Eq, Ord, Data, Generic )
+
 -- | A Pattern synonym's type. Note that a pattern synonym's *fully*
 -- specified type has a peculiar shape coming with two forall
 -- quantifiers and two constraint contexts. For example, consider the