TyEq:N assertion: only for saturated applications

The assertion that checked TyEq:N in canEqCanLHSFinish incorrectly
triggered in the case of an unsaturated newtype TyCon heading the RHS,
even though we can't unwrap such an application. Now, we only trigger
an assertion failure in case of a saturated application of a newtype
TyCon.

Fixes #22310

2 files changed, 24 insertions, 0 deletions

diff --git a/testsuite/tests/typecheck/should_compile/T22310.hs b/testsuite/tests/typecheck/should_compile/T22310.hs
new file mode 100644
index 0000000000..9605a674be
--- /dev/null
+++ b/testsuite/tests/typecheck/should_compile/T22310.hs
@@ -0,0 +1,23 @@
+{-# LANGUAGE GADTs, ScopedTypeVariables, StandaloneKindSignatures #-}
+
+module T22310 where
+
+import Data.Coerce ( coerce )
+import Data.Kind   ( Type )
+
+type Some :: (Type -> Type) -> Type
+data Some t where
+  Some :: t ex -> Some t
+
+type NT :: Type -> Type
+newtype NT f = MkNT ()
+
+oops :: Some NT -> Some NT
+oops = coerce (\(Some @NT x) -> Some x)
+  -- After canonicalisation of Wanted constraints,
+  -- we end up with:
+  --
+  -- [W] t[tau:0] ~R# NT
+  --
+  -- Note the newtype TyCon on the RHS.
+  -- Does not violate TyEq:N, because it is unsaturated!
diff --git a/testsuite/tests/typecheck/should_compile/all.T b/testsuite/tests/typecheck/should_compile/all.T
index 802eb9097d..c1b10ff6c8 100644
--- a/testsuite/tests/typecheck/should_compile/all.T
+++ b/testsuite/tests/typecheck/should_compile/all.T
@@ -856,3 +856,4 @@ test('DeepSubsumption09', normal, compile, [''])
 test('T21951a', normal, compile, ['-Wredundant-strictness-flags'])
 test('T21951b', normal, compile, ['-Wredundant-strictness-flags'])
 test('T21550', normal, compile, [''])
+test('T22310', normal, compile, [''])