Do not discard insoluble Derived constraints

This is preparing for a fix to Trac #9612. The idea is that insoluble
constraints are nice solid errors that we should not discard before
we have a chance to report them.  So TcRnTypes.dropDerivedWC now
keeps insoluble Derived constrains, and instead TcSimplify.solve_wanteds
filters them out

We get somewhat better error message for kind-equality failures too.

A slight downside is that to avoid *duplicate* kind-equality failures
when we float a kind-incompatible equality (e.g.  alpha:* ~ Int#),
I've disabled constraint-floating when there are insolubles.  But that
in turn makes a handful of error messages a little less informative;
good examples are mc21, mc22, mc25.  But I am re-jigging the
constraint floating machinery in another branch, which will make this
go back to the way it was before.

1 files changed, 7 insertions, 7 deletions

diff --git a/testsuite/tests/typecheck/should_fail/mc25.stderr b/testsuite/tests/typecheck/should_fail/mc25.stderr
index 6af388febe..e101f4b328 100644
--- a/testsuite/tests/typecheck/should_fail/mc25.stderr
+++ b/testsuite/tests/typecheck/should_fail/mc25.stderr
@@ -1,18 +1,18 @@
 
 mc25.hs:9:24:
-    No instance for (Functor t1) arising from a use of ‘fmap’
+    No instance for (Functor t2) arising from a use of ‘fmap’
     Possible fix:
-      add (Functor t1) to the context of
-        a type expected by the context: (a -> b) -> t1 a -> t1 b
-        or the inferred type of z :: [t1 t]
+      add (Functor t2) to the context of
+        a type expected by the context: (a -> b) -> t2 a -> t2 b
+        or the inferred type of z :: t (t2 t1)
     In the expression: fmap
     In a stmt of a monad comprehension: then group by x using take
     In the expression: [x | x <- [1 .. 10], then group by x using take]
 
 mc25.hs:9:46:
-    Couldn't match type ‘Int’ with ‘a -> t’
-    Expected type: (a -> t) -> [a] -> [t1 a]
+    Couldn't match type ‘Int’ with ‘a -> t1’
+    Expected type: (a -> t1) -> [a] -> t (t2 a)
       Actual type: Int -> [a] -> [a]
-    Relevant bindings include z :: [t1 t] (bound at mc25.hs:9:1)
+    Relevant bindings include z :: t (t2 t1) (bound at mc25.hs:9:1)
     In the expression: take
     In a stmt of a monad comprehension: then group by x using take