Fix a long standing bug in constraint solving

When combining
 Inert:     [W] C ty1 ty2
 Work item: [D] C ty1 ty2

we were simply discarding the Derived one. Not good! We should turn
the inert back into [WD] or keep both.  E.g. fundeps work only on
Derived (see isImprovable).

This little patch fixes it.  The bug is hard to tickle, but #19315 did so.

The fix is a little messy (see Note [KeepBoth] plus the change in
addDictCt), but I am disinclined to refine it further because it'll
all be swept away when we Kill Deriveds.

2 files changed, 43 insertions, 0 deletions

diff --git a/testsuite/tests/typecheck/should_compile/T19315.hs b/testsuite/tests/typecheck/should_compile/T19315.hs
new file mode 100644
index 0000000000..d93f42c4d4
--- /dev/null
+++ b/testsuite/tests/typecheck/should_compile/T19315.hs
@@ -0,0 +1,42 @@
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE FunctionalDependencies #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE TypeFamilies #-}
+module Bug where
+
+import Control.Monad.Reader
+import Data.Kind
+
+type Lens f s a = (f, s, a)
+
+view :: MonadReader s m => Lens a s a -> m a
+view = undefined
+
+data TickLabels b n = TickLabels
+
+type family N a :: Type
+type instance N (TickLabels b n) = n
+
+tickLabelTextFunction ::  Lens f a (QDiagram b (N a))
+tickLabelTextFunction = undefined
+
+class HasTickLabels f a b | a -> b where
+  tickLabelFunction ::  Lens f a (N a -> String)
+
+instance HasTickLabels f (TickLabels b n) b where
+  tickLabelFunction = undefined
+
+data QDiagram b n = QD
+
+renderColourBar :: forall n b. TickLabels b n -> n -> ()
+renderColourBar cbTickLabels bnds = ()
+  where
+  f :: a -> a
+  f x = x
+
+  tickLabelXs :: String
+  tickLabelXs = view tickLabelFunction cbTickLabels bnds
+
+  drawTickLabel :: n -> QDiagram b n
+  drawTickLabel x = view tickLabelTextFunction cbTickLabels
+     where v = f x
diff --git a/testsuite/tests/typecheck/should_compile/all.T b/testsuite/tests/typecheck/should_compile/all.T
index 3842a1984c..46f2d088d1 100644
--- a/testsuite/tests/typecheck/should_compile/all.T
+++ b/testsuite/tests/typecheck/should_compile/all.T
@@ -770,3 +770,4 @@ test('InlinePatSyn_ExplicitBidiBuilder', [], makefile_test, [])
 test('InlinePatSyn_ExplicitBidiMatcher', [], makefile_test, [])
 test('T18467', normal, compile, [''])
 
+test('T19315', normal, compile, [''])