Prioritise equalities when solving, incl deriveds

We already prioritise equalities when solving, but
Trac #14723 showed that we were not doing so consistently
enough, and as a result the type checker could go into a loop.
Yikes.

See Note [Prioritise equalities] in TcSMonad.

Fixng this bug changed the solve order enough to demonstrate
a problem with fundeps: Trac #14745.

1 files changed, 70 insertions, 0 deletions

diff --git a/testsuite/tests/polykinds/T14723.hs b/testsuite/tests/polykinds/T14723.hs
new file mode 100644
index 0000000000..9b2f3bf75e
--- /dev/null
+++ b/testsuite/tests/polykinds/T14723.hs
@@ -0,0 +1,70 @@
+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE FunctionalDependencies #-}
+{-# LANGUAGE KindSignatures #-}
+{-# LANGUAGE PolyKinds #-}
+{-# LANGUAGE RankNTypes #-}
+{-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE UndecidableInstances #-}
+
+module T14723 () where
+
+import Data.Coerce( coerce )
+import Data.Kind (Type)
+import Data.Proxy (Proxy(..))
+import Data.String (fromString)
+import Data.Int (Int64)
+import GHC.Stack (HasCallStack)
+import GHC.TypeLits (Nat, Symbol)
+
+data JType = Iface Symbol
+
+data J (a :: JType)
+
+newIterator
+  :: IO (J ('Iface "java.util.Iterator"))
+newIterator = do
+    let tblPtr :: Int64
+        tblPtr = undefined
+    iterator <-
+         (qqMarker (Proxy :: Proxy "wuggle")
+                   (Proxy :: Proxy "waggle")
+                   (Proxy :: Proxy "tblPtr")
+                   (Proxy :: Proxy 106)
+                   (tblPtr, ())
+                   Proxy
+                   (undefined :: IO Int))
+    undefined
+
+class Coercible (a :: Type) where
+  type Ty a :: JType
+
+instance Coercible Int64 where
+  type Ty Int64 = Iface "Int64"
+instance Coercible Int where
+  type Ty Int = Iface "Int"
+
+class Coercibles xs (tys :: k) | xs -> tys
+instance Coercibles () ()
+instance (ty ~ Ty x, Coercible x, Coercibles xs tys) => Coercibles (x, xs) '(ty, tys)
+
+qqMarker
+  :: forall
+     -- k                -- the kind variable shows up in Core
+     (args_tys :: k)     -- JType's of arguments
+     tyres               -- JType of result
+     (input :: Symbol)   -- input string of the quasiquoter
+     (mname :: Symbol)   -- name of the method to generate
+     (antiqs :: Symbol)  -- antiquoted variables as a comma-separated list
+     (line :: Nat)       -- line number of the quasiquotation
+     args_tuple          -- uncoerced argument types
+     b.                  -- uncoerced result type
+     (tyres ~ Ty b, Coercibles args_tuple args_tys, Coercible b, HasCallStack)
+  => Proxy input
+  -> Proxy mname
+  -> Proxy antiqs
+  -> Proxy line
+  -> args_tuple
+  -> Proxy args_tys
+  -> IO b
+  -> IO b
+qqMarker = undefined