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, 4 insertions, 1 deletions

diff --git a/testsuite/tests/typecheck/should_fail/T3950.stderr b/testsuite/tests/typecheck/should_fail/T3950.stderr
index 0fc428e183..7b4837c8b9 100644
--- a/testsuite/tests/typecheck/should_fail/T3950.stderr
+++ b/testsuite/tests/typecheck/should_fail/T3950.stderr
@@ -1,6 +1,9 @@
 
 T3950.hs:15:13:
-    Couldn't match type ‘Id p0 x0’ with ‘Id p’
+    Couldn't match kind ‘* -> *’ with ‘*’
+    When matching types
+      w :: (* -> * -> *) -> *
+      Sealed :: (* -> *) -> *
     Expected type: w (Id p)
       Actual type: Sealed (Id p0 x0)
     Relevant bindings include