Fix #15954 by rejigging check_type's order

Summary:
Previously, `check_type` (which catches illegal uses of
unsaturated type synonyms without enabling `LiberalTypeSynonyms`,
among other things) always checks for uses of polytypes before
anything else. There is a problem with this plan, however:
checking for polytypes requires decomposing `forall`s and other
invisible arguments, an action which itself expands type synonyms!
Therefore, if we have something like:

```lang=haskell
type A a = Int
type B (a :: Type -> Type) = forall x. x -> x
type C = B A
```

Then when checking `B A`, `A` will get expanded to `forall x. x -> x`
before `check_type` has an opportunity to realize that `A` is an
unsaturated type synonym! This is the root cause of #15954.

This patch fixes the issue by moving the case of `check_type` that
detects polytypes to be //after// the case that checks for
`TyConApp`s. That way, the `TyConApp` case will properly flag things
like the unsaturated use of `A` in the example above before we ever
attempt to check for polytypes.

Test Plan: make test TEST=T15954

Reviewers: simonpj, bgamari, goldfire

Reviewed By: simonpj

Subscribers: rwbarton, carter

GHC Trac Issues: #15954

Differential Revision: https://phabricator.haskell.org/D5402

2 files changed, 2 insertions, 1 deletions

diff --git a/testsuite/tests/dependent/should_fail/T15859.hs b/testsuite/tests/dependent/should_fail/T15859.hs
index e8ffdf4ae2..e7adc5fc98 100644
--- a/testsuite/tests/dependent/should_fail/T15859.hs
+++ b/testsuite/tests/dependent/should_fail/T15859.hs
@@ -1,6 +1,7 @@
 {-# Language PolyKinds          #-}
 {-# Language TypeApplications   #-}
 {-# Language ImpredicativeTypes #-}
+{-# Language LiberalTypeSynonyms #-}
 
 module T15859 where
 
diff --git a/testsuite/tests/dependent/should_fail/T15859.stderr b/testsuite/tests/dependent/should_fail/T15859.stderr
index e4794048b7..c4dc1ef086 100644
--- a/testsuite/tests/dependent/should_fail/T15859.stderr
+++ b/testsuite/tests/dependent/should_fail/T15859.stderr
@@ -1,5 +1,5 @@
 
-T15859.hs:13:5: error:
+T15859.hs:14:5: error:
     • Cannot apply expression of type ‘forall k -> k -> *’
       to a visible type argument ‘Int’
     • In the expression: (undefined :: KindOf A) @Int