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, 42 insertions, 0 deletions

diff --git a/testsuite/tests/th/T10598_TH.hs b/testsuite/tests/th/T10598_TH.hs
new file mode 100644
index 0000000000..aab8bb3aa6
--- /dev/null
+++ b/testsuite/tests/th/T10598_TH.hs
@@ -0,0 +1,42 @@
+{-# LANGUAGE DeriveAnyClass #-}
+{-# LANGUAGE DerivingStrategies #-}
+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
+{-# LANGUAGE StandaloneDeriving #-}
+{-# LANGUAGE TemplateHaskell #-}
+module T10598_TH where
+
+import Language.Haskell.TH
+
+class C a
+instance C Int
+
+class C a => D a
+instance D Int
+
+{-
+newtype Foo = MkFoo Int
+  deriving stock    Eq
+  deriving anyclass C
+  deriving newtype  Read
+
+deriving stock    instance Ord  Foo
+deriving anyclass instance D    Foo
+deriving newtype  instance Show Foo
+-}
+
+$(do fooDataName  <- newName "Foo"
+     mkFooConName <- newName "MkFoo"
+     let fooType = conT fooDataName
+     sequence [ newtypeD (cxt []) fooDataName [] Nothing
+                (normalC mkFooConName
+                  [ bangType (bang noSourceUnpackedness noSourceStrictness)
+                             [t| Int |] ])
+                [ derivClause (Just Stock)    [ [t| Eq   |] ]
+                , derivClause (Just Anyclass) [ [t| C    |] ]
+                , derivClause (Just Newtype)  [ [t| Read |] ] ]
+             , standaloneDerivWithStrategyD (Just Stock)
+                 (cxt []) [t| Ord $(fooType) |]
+             , standaloneDerivWithStrategyD (Just Anyclass)
+                 (cxt []) [t| D $(fooType) |]
+             , standaloneDerivWithStrategyD (Just Newtype)
+                 (cxt []) [t| Show $(fooType) |] ])