Fix Trac #16517 by bumping the TcLevel for method sigs
There were actually two bugs fixed here:
- 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.
- 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 Trac #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
This patch also cleans up some Notes around error generation that
came up in conversation.
Test case: typecheck/should_fail/T16517, ghci/scripts/T16767