Fix #16517 by bumping the TcLevel for method sigs

There were actually two bugs fixed here:

1. candidateQTyVarsOfType needs to be careful that it does not
   try to zap metavariables from an outer scope as "naughty"
   quantification candidates. This commit adds a simple check
   to avoid doing so.

2. We weren't bumping the TcLevel in kcHsKindSig, which was used
   only for class method sigs. This mistake led to the acceptance
   of

     class C a where
       meth :: forall k. Proxy (a :: k) -> ()

   Note that k is *locally* quantified. This patch fixes the
   problem by using tcClassSigType, which correctly bumps the
   level. It's a bit inefficient because tcClassSigType does other
   work, too, but it would be tedious to repeat much of the code
   there with only a few changes. This version works well and is
   simple.

And, while updating comments, etc., I noticed that tcRnType was
missing a pushTcLevel, leading to #16767, which this patch also
fixes, by bumping the level. In the refactoring here, I also
use solveEqualities. This initially failed ghci/scripts/T15415,
but that was fixed by teaching solveEqualities to respect
-XPartialTypeSignatures.

This patch also cleans up some Notes around error generation that
came up in conversation.

Test case: typecheck/should_fail/T16517, ghci/scripts/T16767

1 files changed, 1 insertions, 0 deletions

diff --git a/testsuite/tests/ghci/scripts/all.T b/testsuite/tests/ghci/scripts/all.T
index 5dd56ca46c..db619c142b 100755
--- a/testsuite/tests/ghci/scripts/all.T
+++ b/testsuite/tests/ghci/scripts/all.T
@@ -297,3 +297,4 @@ test('T14828', normal, ghci_script, ['T14828.script'])
 test('T16376', normal, ghci_script, ['T16376.script'])
 test('T16527', normal, ghci_script, ['T16527.script'])
 test('T16569', normal, ghci_script, ['T16569.script'])
+test('T16767', normal, ghci_script, ['T16767.script'])