Revamp the treatment of auxiliary bindings for derived instances

This started as a simple fix for #18321 that organically grew into a
much more sweeping refactor of how auxiliary bindings for derived
instances are handled. I have rewritten `Note [Auxiliary binders]`
in `GHC.Tc.Deriv.Generate` to explain all of the moving parts, but
the highlights are:

* Previously, the OccName of each auxiliary binding would be given
  a suffix containing a hash of its package name, module name, and
  parent data type to avoid name clashes. This was needlessly
  complicated, so we take the more direct approach of generating
  `Exact` `RdrName`s for each auxiliary binding with the same
  `OccName`, but using an underlying `System` `Name` with a fresh
  `Unique` for each binding. Unlike hashes, allocating new `Unique`s
  does not require any cleverness and avoid name clashes all the
  same...
* ...speaking of which, in order to convince the renamer that multiple
  auxiliary bindings with the same `OccName` (but different
  `Unique`s) are kosher, we now use `rnLocalValBindsLHS` instead of
  `rnTopBindsLHS` to rename auxiliary bindings. Again, see
  `Note [Auxiliary binders]` for the full story.
* I have removed the `DerivHsBind` constructor for
  `DerivStuff`—which was only used for `Data.Data`-related
  auxiliary bindings—and refactored `gen_Data_binds` to use
  `DerivAuxBind` instead. This brings the treatment of
  `Data.Data`-related auxiliary bindings in line with every other
  form of auxiliary binding.

Fixes #18321.

4 files changed, 72 insertions, 51 deletions

diff --git a/testsuite/tests/deriving/should_compile/T14682.stderr b/testsuite/tests/deriving/should_compile/T14682.stderr
index 2bf9552ff9..e0c8b332ed 100644
--- a/testsuite/tests/deriving/should_compile/T14682.stderr
+++ b/testsuite/tests/deriving/should_compile/T14682.stderr
@@ -23,8 +23,8 @@ Derived class instances:
     Data.Data.gfoldl k z (T14682.Foo a1 a2)
       = ((z (\ a1 a2 -> T14682.Foo a1 a2) `k` a1) `k` a2)
     Data.Data.gunfold k z _ = k (k (z (\ a1 a2 -> T14682.Foo a1 a2)))
-    Data.Data.toConstr (T14682.Foo _ _) = T14682.$cFoo
-    Data.Data.dataTypeOf _ = T14682.$tFoo
+    Data.Data.toConstr (T14682.Foo _ _) = $cFoo
+    Data.Data.dataTypeOf _ = $tFoo
   
   instance GHC.Classes.Eq T14682.Foo where
     (GHC.Classes.==) (T14682.Foo a1 a2) (T14682.Foo b1 b2)
@@ -71,14 +71,12 @@ Derived class instances:
       = (GHC.Ix.inRange (a1, b1) c1
            GHC.Classes.&& GHC.Ix.inRange (a2, b2) c2)
   
-  T14682.$con2tag_B4iUvrAY4wB3YczpMJQUOX ::
-    T14682.Foo -> GHC.Prim.Int#
-  T14682.$con2tag_B4iUvrAY4wB3YczpMJQUOX (T14682.Foo _ _) = 0#
-  T14682.$tFoo :: Data.Data.DataType
-  T14682.$cFoo :: Data.Data.Constr
-  T14682.$tFoo = Data.Data.mkDataType "Foo" [T14682.$cFoo]
-  T14682.$cFoo
-    = Data.Data.mkConstr T14682.$tFoo "Foo" [] Data.Data.Prefix
+  $tFoo :: Data.Data.DataType
+  $cFoo :: Data.Data.Constr
+  $con2tag_Foo :: T14682.Foo -> GHC.Prim.Int#
+  $con2tag_Foo (T14682.Foo _ _) = 0#
+  $tFoo = Data.Data.mkDataType "Foo" [$cFoo]
+  $cFoo = Data.Data.mkConstr $tFoo "Foo" [] Data.Data.Prefix
 
 Derived type family instances:
 
diff --git a/testsuite/tests/deriving/should_compile/T18321.hs b/testsuite/tests/deriving/should_compile/T18321.hs
new file mode 100644
index 0000000000..5391cf602b
--- /dev/null
+++ b/testsuite/tests/deriving/should_compile/T18321.hs
@@ -0,0 +1,15 @@
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE StandaloneDeriving #-}
+{-# LANGUAGE TemplateHaskell #-}
+module T18321 where
+
+import Data.Ix
+
+data T = MkT deriving (Eq, Ord, Ix)
+$(return [])
+deriving instance Enum T
+
+data S a = MkS
+deriving instance Enum (S Int)
+$(return [])
+deriving instance Enum (S Bool)
diff --git a/testsuite/tests/deriving/should_compile/all.T b/testsuite/tests/deriving/should_compile/all.T
index 8a363e72f9..f6e9d43b06 100644
--- a/testsuite/tests/deriving/should_compile/all.T
+++ b/testsuite/tests/deriving/should_compile/all.T
@@ -124,3 +124,4 @@ test('T17339', normal, compile,
      ['-ddump-simpl -dsuppress-idinfo -dno-typeable-binds'])
 test('T17880', normal, compile, [''])
 test('T18055', normal, compile, [''])
+test('T18321', normal, compile, [''])
diff --git a/testsuite/tests/deriving/should_compile/drv-empty-data.stderr b/testsuite/tests/deriving/should_compile/drv-empty-data.stderr
index d6e4eee4b0..cb6a89b226 100644
--- a/testsuite/tests/deriving/should_compile/drv-empty-data.stderr
+++ b/testsuite/tests/deriving/should_compile/drv-empty-data.stderr
@@ -20,7 +20,7 @@ Derived class instances:
     Data.Data.gfoldl _ _ z = case z of
     Data.Data.gunfold k z c = case Data.Data.constrIndex c of
     Data.Data.toConstr z = case z of
-    Data.Data.dataTypeOf _ = DrvEmptyData.$tVoid
+    Data.Data.dataTypeOf _ = $tVoid
     Data.Data.dataCast1 f = Data.Typeable.gcast1 f
   
   instance GHC.Base.Functor DrvEmptyData.Void where
@@ -48,8 +48,8 @@ Derived class instances:
     Language.Haskell.TH.Syntax.lift z = GHC.Base.pure (case z of)
     Language.Haskell.TH.Syntax.liftTyped z = GHC.Base.pure (case z of)
   
-  DrvEmptyData.$tVoid :: Data.Data.DataType
-  DrvEmptyData.$tVoid = Data.Data.mkDataType "Void" []
+  $tVoid :: Data.Data.DataType
+  $tVoid = Data.Data.mkDataType "Void" []
 
 Derived type family instances:
   type GHC.Generics.Rep (DrvEmptyData.Void a) = GHC.Generics.D1
@@ -64,124 +64,124 @@ Derived type family instances:
 
 
 ==================== Filling in method body ====================
-GHC.Read.Read [DrvEmptyData.Void a[ssk:2]]
+GHC.Read.Read [DrvEmptyData.Void a[ssk:1]]
   GHC.Read.readsPrec = GHC.Read.$dmreadsPrec
-                         @(DrvEmptyData.Void a[ssk:2])
+                         @(DrvEmptyData.Void a[ssk:1])
 
 
 
 ==================== Filling in method body ====================
-GHC.Show.Show [DrvEmptyData.Void a[ssk:2]]
-  GHC.Show.show = GHC.Show.$dmshow @(DrvEmptyData.Void a[ssk:2])
+GHC.Show.Show [DrvEmptyData.Void a[ssk:1]]
+  GHC.Show.show = GHC.Show.$dmshow @(DrvEmptyData.Void a[ssk:1])
 
 
 
 ==================== Filling in method body ====================
-GHC.Show.Show [DrvEmptyData.Void a[ssk:2]]
+GHC.Show.Show [DrvEmptyData.Void a[ssk:1]]
   GHC.Show.showList = GHC.Show.$dmshowList
-                        @(DrvEmptyData.Void a[ssk:2])
+                        @(DrvEmptyData.Void a[ssk:1])
 
 
 
 ==================== Filling in method body ====================
-GHC.Classes.Ord [DrvEmptyData.Void a[ssk:2]]
-  GHC.Classes.< = GHC.Classes.$dm< @(DrvEmptyData.Void a[ssk:2])
+GHC.Classes.Ord [DrvEmptyData.Void a[ssk:1]]
+  GHC.Classes.< = GHC.Classes.$dm< @(DrvEmptyData.Void a[ssk:1])
 
 
 
 ==================== Filling in method body ====================
-GHC.Classes.Ord [DrvEmptyData.Void a[ssk:2]]
-  GHC.Classes.<= = GHC.Classes.$dm<= @(DrvEmptyData.Void a[ssk:2])
+GHC.Classes.Ord [DrvEmptyData.Void a[ssk:1]]
+  GHC.Classes.<= = GHC.Classes.$dm<= @(DrvEmptyData.Void a[ssk:1])
 
 
 
 ==================== Filling in method body ====================
-GHC.Classes.Ord [DrvEmptyData.Void a[ssk:2]]
-  GHC.Classes.> = GHC.Classes.$dm> @(DrvEmptyData.Void a[ssk:2])
+GHC.Classes.Ord [DrvEmptyData.Void a[ssk:1]]
+  GHC.Classes.> = GHC.Classes.$dm> @(DrvEmptyData.Void a[ssk:1])
 
 
 
 ==================== Filling in method body ====================
-GHC.Classes.Ord [DrvEmptyData.Void a[ssk:2]]
-  GHC.Classes.>= = GHC.Classes.$dm>= @(DrvEmptyData.Void a[ssk:2])
+GHC.Classes.Ord [DrvEmptyData.Void a[ssk:1]]
+  GHC.Classes.>= = GHC.Classes.$dm>= @(DrvEmptyData.Void a[ssk:1])
 
 
 
 ==================== Filling in method body ====================
-GHC.Classes.Ord [DrvEmptyData.Void a[ssk:2]]
-  GHC.Classes.max = GHC.Classes.$dmmax @(DrvEmptyData.Void a[ssk:2])
+GHC.Classes.Ord [DrvEmptyData.Void a[ssk:1]]
+  GHC.Classes.max = GHC.Classes.$dmmax @(DrvEmptyData.Void a[ssk:1])
 
 
 
 ==================== Filling in method body ====================
-GHC.Classes.Ord [DrvEmptyData.Void a[ssk:2]]
-  GHC.Classes.min = GHC.Classes.$dmmin @(DrvEmptyData.Void a[ssk:2])
+GHC.Classes.Ord [DrvEmptyData.Void a[ssk:1]]
+  GHC.Classes.min = GHC.Classes.$dmmin @(DrvEmptyData.Void a[ssk:1])
 
 
 
 ==================== Filling in method body ====================
-GHC.Classes.Eq [DrvEmptyData.Void a[ssk:2]]
-  GHC.Classes./= = GHC.Classes.$dm/= @(DrvEmptyData.Void a[ssk:2])
+GHC.Classes.Eq [DrvEmptyData.Void a[ssk:1]]
+  GHC.Classes./= = GHC.Classes.$dm/= @(DrvEmptyData.Void a[ssk:1])
 
 
 
 ==================== Filling in method body ====================
-Data.Data.Data [DrvEmptyData.Void a[ssk:2]]
+Data.Data.Data [DrvEmptyData.Void a[ssk:1]]
   Data.Data.dataCast2 = Data.Data.$dmdataCast2
-                          @(DrvEmptyData.Void a[ssk:2])
+                          @(DrvEmptyData.Void a[ssk:1])
 
 
 
 ==================== Filling in method body ====================
-Data.Data.Data [DrvEmptyData.Void a[ssk:2]]
-  Data.Data.gmapT = Data.Data.$dmgmapT @(DrvEmptyData.Void a[ssk:2])
+Data.Data.Data [DrvEmptyData.Void a[ssk:1]]
+  Data.Data.gmapT = Data.Data.$dmgmapT @(DrvEmptyData.Void a[ssk:1])
 
 
 
 ==================== Filling in method body ====================
-Data.Data.Data [DrvEmptyData.Void a[ssk:2]]
+Data.Data.Data [DrvEmptyData.Void a[ssk:1]]
   Data.Data.gmapQl = Data.Data.$dmgmapQl
-                       @(DrvEmptyData.Void a[ssk:2])
+                       @(DrvEmptyData.Void a[ssk:1])
 
 
 
 ==================== Filling in method body ====================
-Data.Data.Data [DrvEmptyData.Void a[ssk:2]]
+Data.Data.Data [DrvEmptyData.Void a[ssk:1]]
   Data.Data.gmapQr = Data.Data.$dmgmapQr
-                       @(DrvEmptyData.Void a[ssk:2])
+                       @(DrvEmptyData.Void a[ssk:1])
 
 
 
 ==================== Filling in method body ====================
-Data.Data.Data [DrvEmptyData.Void a[ssk:2]]
-  Data.Data.gmapQ = Data.Data.$dmgmapQ @(DrvEmptyData.Void a[ssk:2])
+Data.Data.Data [DrvEmptyData.Void a[ssk:1]]
+  Data.Data.gmapQ = Data.Data.$dmgmapQ @(DrvEmptyData.Void a[ssk:1])
 
 
 
 ==================== Filling in method body ====================
-Data.Data.Data [DrvEmptyData.Void a[ssk:2]]
+Data.Data.Data [DrvEmptyData.Void a[ssk:1]]
   Data.Data.gmapQi = Data.Data.$dmgmapQi
-                       @(DrvEmptyData.Void a[ssk:2])
+                       @(DrvEmptyData.Void a[ssk:1])
 
 
 
 ==================== Filling in method body ====================
-Data.Data.Data [DrvEmptyData.Void a[ssk:2]]
-  Data.Data.gmapM = Data.Data.$dmgmapM @(DrvEmptyData.Void a[ssk:2])
+Data.Data.Data [DrvEmptyData.Void a[ssk:1]]
+  Data.Data.gmapM = Data.Data.$dmgmapM @(DrvEmptyData.Void a[ssk:1])
 
 
 
 ==================== Filling in method body ====================
-Data.Data.Data [DrvEmptyData.Void a[ssk:2]]
+Data.Data.Data [DrvEmptyData.Void a[ssk:1]]
   Data.Data.gmapMp = Data.Data.$dmgmapMp
-                       @(DrvEmptyData.Void a[ssk:2])
+                       @(DrvEmptyData.Void a[ssk:1])
 
 
 
 ==================== Filling in method body ====================
-Data.Data.Data [DrvEmptyData.Void a[ssk:2]]
+Data.Data.Data [DrvEmptyData.Void a[ssk:1]]
   Data.Data.gmapMo = Data.Data.$dmgmapMo
-                       @(DrvEmptyData.Void a[ssk:2])
+                       @(DrvEmptyData.Void a[ssk:1])
 
 
 
@@ -193,6 +193,13 @@ Data.Foldable.Foldable [DrvEmptyData.Void]
 
 ==================== Filling in method body ====================
 Data.Foldable.Foldable [DrvEmptyData.Void]
+  Data.Foldable.foldMap' = Data.Foldable.$dmfoldMap'
+                             @(DrvEmptyData.Void)
+
+
+
+==================== Filling in method body ====================
+Data.Foldable.Foldable [DrvEmptyData.Void]
   Data.Foldable.foldr = Data.Foldable.$dmfoldr @(DrvEmptyData.Void)