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, 9 insertions, 8 deletions

diff --git a/testsuite/tests/typecheck/should_fail/mc22.stderr b/testsuite/tests/typecheck/should_fail/mc22.stderr
index f3da3c5eeb..44a2eebdc7 100644
--- a/testsuite/tests/typecheck/should_fail/mc22.stderr
+++ b/testsuite/tests/typecheck/should_fail/mc22.stderr
@@ -1,21 +1,22 @@
 
 mc22.hs:10:9:
-    No instance for (Functor t) arising from a use of ‘fmap’
+    No instance for (Functor t1) arising from a use of ‘fmap’
     Possible fix:
-      add (Functor t) to the context of
-        a type expected by the context: (a -> b) -> t a -> t b
-        or the inferred type of foo :: [t [Char]]
+      add (Functor t1) to the context of
+        a type expected by the context: (a -> b) -> t1 a -> t1 b
+        or the inferred type of foo :: t (t1 [Char])
     In the expression: fmap
     In a stmt of a monad comprehension: then group using take 5
     In the expression:
       [x + 1 | x <- ["Hello", "World"], then group using take 5]
 
 mc22.hs:10:26:
-    Couldn't match type ‘a’ with ‘t a’
+    Couldn't match type ‘a’ with ‘t1 a’
       ‘a’ is a rigid type variable bound by
-          a type expected by the context: [a] -> [t a] at mc22.hs:10:9
-    Expected type: [a] -> [t a]
+          a type expected by the context: [a] -> t (t1 a) at mc22.hs:10:9
+    Expected type: [a] -> t (t1 a)
       Actual type: [a] -> [a]
-    Relevant bindings include foo :: [t [Char]] (bound at mc22.hs:8:1)
+    Relevant bindings include
+      foo :: t (t1 [Char]) (bound at mc22.hs:8:1)
     In the expression: take 5
     In a stmt of a monad comprehension: then group using take 5