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.

1 files changed, 6 insertions, 0 deletions

diff --git a/compiler/GHC/Hs/Type.hs b/compiler/GHC/Hs/Type.hs
index 7e88324b8a..3d8a90c752 100644
--- a/compiler/GHC/Hs/Type.hs
+++ b/compiler/GHC/Hs/Type.hs
@@ -1628,6 +1628,12 @@ instance types, which makes things like the instance above become illegal.
 For the sake of consistency, we also disallow nested contexts, even though they
 don't have the same strange interaction with ScopedTypeVariables.
 
+Just as we forbid nested `forall`s and contexts in normal instance
+declarations, we also forbid them in SPECIALISE instance pragmas (#18455).
+Unlike normal instance declarations, ScopedTypeVariables don't have any impact
+on SPECIALISE instance pragmas, but we use the same validity checks for
+SPECIALISE instance pragmas anyway to be consistent.
+
 -----
 -- Wrinkle: Derived instances
 -----