Clean up the inferred type variable restriction

This patch primarily:

* Documents `checkInferredVars` (previously called
  `check_inferred_vars`) more carefully. This is the
  function which throws an error message if a user quantifies an
  inferred type variable in a place where specificity cannot be
  observed. See `Note [Unobservably inferred type variables]` in
  `GHC.Rename.HsType`.

  Note that I now invoke `checkInferredVars` _alongside_
  `rnHsSigType`, `rnHsWcSigType`, etc. rather than doing so _inside_
  of these functions. This results in slightly more call sites for
  `checkInferredVars`, but it makes it much easier to enumerate the
  spots where the inferred type variable restriction comes into
  effect.
* Removes the inferred type variable restriction for default method
  type signatures, per the discussion in #18432. As a result, this
  patch fixes #18432.

Along the way, I performed some various cleanup:

* I moved `no_nested_foralls_contexts_err` into `GHC.Rename.Utils`
  (under the new name `noNestedForallsContextsErr`), since it now
  needs to be invoked from multiple modules. I also added a helper
  function `addNoNestedForallsContextsErr` that throws the error
  message after producing it, as this is a common idiom.
* In order to ensure that users cannot sneak inferred type variables
  into `SPECIALISE instance` pragmas by way of nested `forall`s, I
  now invoke `addNoNestedForallsContextsErr` when renaming
  `SPECIALISE instance` pragmas, much like when we rename normal
  instance declarations. (This probably should have originally been
  done as a part of the fix for #18240, but this task was somehow
  overlooked.) As a result, this patch fixes #18455 as a side effect.

2 files changed, 11 insertions, 0 deletions

diff --git a/testsuite/tests/quantified-constraints/T18432.hs b/testsuite/tests/quantified-constraints/T18432.hs
new file mode 100644
index 0000000000..ba50727420
--- /dev/null
+++ b/testsuite/tests/quantified-constraints/T18432.hs
@@ -0,0 +1,10 @@
+{-# LANGUAGE QuantifiedConstraints #-}
+module Bug where
+
+import Data.Proxy
+
+class C a where
+  m :: Proxy a
+
+f :: (forall {a}. C a) => Proxy Int
+f = m
diff --git a/testsuite/tests/quantified-constraints/all.T b/testsuite/tests/quantified-constraints/all.T
index da3a6fecec..9bcc30b1a9 100644
--- a/testsuite/tests/quantified-constraints/all.T
+++ b/testsuite/tests/quantified-constraints/all.T
@@ -29,3 +29,4 @@ test('T17267c', normal, compile_fail, [''])
 test('T17267d', normal, compile_and_run, [''])
 test('T17267e', normal, compile_fail, [''])
 test('T17458', normal, compile_fail, [''])
+test('T18432', normal, compile, [''])