Fix #18145 and also avoid needless work with implicit vars

 - `forAllOrNothing` now is monadic, so we can trace whether we bind
   an explicit `forall` or not.

 - #18145 arose because the free vars calculation was needlessly
   complex. It is now greatly simplified.

 - Replaced some other implicit var code with `filterFreeVarsToBind`.

Co-authored-by: Ryan Scott <ryan.gl.scott@gmail.com>

3 files changed, 24 insertions, 0 deletions

diff --git a/testsuite/tests/rename/should_fail/T18145.hs b/testsuite/tests/rename/should_fail/T18145.hs
new file mode 100644
index 0000000000..f991f3b19c
--- /dev/null
+++ b/testsuite/tests/rename/should_fail/T18145.hs
@@ -0,0 +1,17 @@
+{-# LANGUAGE ExplicitForAll #-}
+{-# LANGUAGE TypeApplications #-}
+{-# LANGUAGE PolyKinds #-}
+{-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE DataKinds #-}
+
+module T18145 where
+
+type family A :: k
+type instance forall. A = Nothing :: Maybe a -- 'a' should be out of scope
+
+class Foo x where
+  type B x :: Maybe a
+  type forall x. B x = Nothing :: Maybe a -- 'a' should be out of scope
+
+instance Foo [x] where
+  type forall. B [x] = Nothing :: Maybe a -- 'a' should be out of scope
diff --git a/testsuite/tests/rename/should_fail/T18145.stderr b/testsuite/tests/rename/should_fail/T18145.stderr
new file mode 100644
index 0000000000..606ad0f695
--- /dev/null
+++ b/testsuite/tests/rename/should_fail/T18145.stderr
@@ -0,0 +1,6 @@
+
+T18145.hs:10:44: error: Not in scope: type variable ‘a’
+
+T18145.hs:14:41: error: Not in scope: type variable ‘a’
+
+T18145.hs:17:41: error: Not in scope: type variable ‘a’
diff --git a/testsuite/tests/rename/should_fail/all.T b/testsuite/tests/rename/should_fail/all.T
index 179ff13560..27b359dec1 100644
--- a/testsuite/tests/rename/should_fail/all.T
+++ b/testsuite/tests/rename/should_fail/all.T
@@ -153,3 +153,4 @@ test('T16504', normal, compile_fail, [''])
 test('T14548', normal, compile_fail, [''])
 test('T16610', normal, compile_fail, [''])
 test('T17593', normal, compile_fail, [''])
+test('T18145', normal, compile_fail, [''])