Improve error messages for skolems

In error messages like this
    • Couldn't match type ‘c’ with ‘f0 (a -> b)’
      ‘c’ is a rigid type variable bound by
        the type signature for:
          f :: ((a -> b) -> b) -> forall c. c -> a

we need to take case both to actually show that 'forall c',
and to make sure that its name lines with the 'c' in the
error message.

This has been shaky for some time, and this commit puts it on solid
ground.  See TcRnTypes: Note [SigSkol SkolemInfo]

The main changes are

* SigSkol gets an extra field that records the way in which the
  type signature was skolemised.

* The type in SigSkol is now the /un/-skolemised version

* pprSkolemInfo uses the info to make the tidy type line up
  nicely

Lots of error message wibbles!

2 files changed, 4 insertions, 4 deletions

diff --git a/testsuite/tests/monadfail/MonadFailErrors.stderr b/testsuite/tests/monadfail/MonadFailErrors.stderr
index 1507984d14..81ad56c910 100644
--- a/testsuite/tests/monadfail/MonadFailErrors.stderr
+++ b/testsuite/tests/monadfail/MonadFailErrors.stderr
@@ -5,12 +5,12 @@ MonadFailErrors.hs:16:5: error:
         with the failable pattern ‘Just x’
       from the context: Monad m
         bound by the type signature for:
-                   general :: Monad m => m a
+                   general :: forall (m :: * -> *) a. Monad m => m a
         at MonadFailErrors.hs:14:1-25
       Possible fix:
         add (MonadFail m) to the context of
           the type signature for:
-            general :: Monad m => m a
+            general :: forall (m :: * -> *) a. Monad m => m a
     • In a stmt of a 'do' block: Just x <- undefined
       In the expression:
         do Just x <- undefined
diff --git a/testsuite/tests/monadfail/MonadFailWarnings.stderr b/testsuite/tests/monadfail/MonadFailWarnings.stderr
index ac16d6cd9e..7471bd9373 100644
--- a/testsuite/tests/monadfail/MonadFailWarnings.stderr
+++ b/testsuite/tests/monadfail/MonadFailWarnings.stderr
@@ -5,12 +5,12 @@ MonadFailWarnings.hs:19:5: warning: [-Wmissing-monadfail-instances (in -Wcompat)
         (this will become an error in a future GHC release)
       from the context: Monad m
         bound by the type signature for:
-                   general :: Monad m => m a
+                   general :: forall (m :: * -> *) a. Monad m => m a
         at MonadFailWarnings.hs:17:1-25
       Possible fix:
         add (MonadFail m) to the context of
           the type signature for:
-            general :: Monad m => m a
+            general :: forall (m :: * -> *) a. Monad m => m a
     • In a stmt of a 'do' block: Just x <- undefined
       In the expression:
         do Just x <- undefined