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, 21 insertions, 0 deletions

diff --git a/testsuite/tests/typecheck/should_compile/T13651.hs b/testsuite/tests/typecheck/should_compile/T13651.hs
index 43ae633534..63bd88eb5c 100644
--- a/testsuite/tests/typecheck/should_compile/T13651.hs
+++ b/testsuite/tests/typecheck/should_compile/T13651.hs
@@ -12,3 +12,24 @@ foo :: (F cr cu ~ Bar h (Bar r u),
         F cu cs ~ Bar (Foo h) (Bar u s))
     => Bar h (Bar r u) -> Bar (Foo h) (Bar u s) -> Foo (cr -> cs)
 foo = undefined
+
+{-  Typechecking this program used to /just/ succeed in GHC 8.2,
+    (see Trac #14745 for why), but doesn't in 8.4.
+
+[G]  F cr cu ~ Bar h (Bar r u),
+     F cu cs ~ Bar (Foo h) (Bar u s))
+
+
+[W] F cr cu0 ~ Bar h (Bar r u)
+    --> (top-level fundeps)  cr ~ Bar h (Foo r)
+                             cu0 ~ Bar h (Foo u)
+        (local fundeps)      cu ~ cu0
+
+[W] F cu0 cs ~ Bar (Foo h) (Bar u s)
+    -->  (top-level fundeps)  cu0 ~ Bar (Foo h) (Foo u)
+                              cs  ~ Bar (Foo h) (Foo s)
+         (local fundeps)      cu0 ~ cu
+
+[W] F cr (Bar (Foo h) (Fo u)) ~ Bar h (Bar r u)
+
+-}