Use transSuperClasses in TcErrors

Code in TcErrors was recursively using immSuperClasses,
which loops in the presence of UndecidableSuperClasses.

Better to use transSuperClasses instead, which has a loop-breaker
mechanism built in.

Fixes issue #16414.

3 files changed, 31 insertions, 0 deletions

diff --git a/testsuite/tests/typecheck/should_fail/T16414.hs b/testsuite/tests/typecheck/should_fail/T16414.hs
new file mode 100644
index 0000000000..27807e82bf
--- /dev/null
+++ b/testsuite/tests/typecheck/should_fail/T16414.hs
@@ -0,0 +1,17 @@
+{-# LANGUAGE FlexibleContexts, FlexibleInstances, AllowAmbiguousTypes #-}
+{-# LANGUAGE UndecidableSuperClasses #-}
+module T16414 where
+
+data I = I
+
+class All2 x => All x
+class All x => All2 x
+
+class AllZip2 f
+instance AllZip2 f
+
+f1 :: (All x, AllZip2 I) => x -> ()
+f1 = f2
+
+f2 :: AllZip2 f => x -> ()
+f2 _ = ()
diff --git a/testsuite/tests/typecheck/should_fail/T16414.stderr b/testsuite/tests/typecheck/should_fail/T16414.stderr
new file mode 100644
index 0000000000..5cfbf7549a
--- /dev/null
+++ b/testsuite/tests/typecheck/should_fail/T16414.stderr
@@ -0,0 +1,13 @@
+
+T16414.hs:14:6: error:
+    • Overlapping instances for AllZip2 f0 arising from a use of ‘f2’
+      Matching givens (or their superclasses):
+        AllZip2 I
+          bound by the type signature for:
+                     f1 :: forall x. (All x, AllZip2 I) => x -> ()
+          at T16414.hs:13:1-35
+      Matching instances:
+        instance AllZip2 f -- Defined at T16414.hs:11:10
+      (The choice depends on the instantiation of ‘f0’)
+    • In the expression: f2
+      In an equation for ‘f1’: f1 = f2
diff --git a/testsuite/tests/typecheck/should_fail/all.T b/testsuite/tests/typecheck/should_fail/all.T
index a5d1847bb0..b3c25eabe9 100644
--- a/testsuite/tests/typecheck/should_fail/all.T
+++ b/testsuite/tests/typecheck/should_fail/all.T
@@ -510,3 +510,4 @@ test('T16059e', [extra_files(['T16059b.hs'])], multimod_compile_fail,
     ['T16059e', '-v0'])
 test('T16255', normal, compile_fail, [''])
 test('T16204c', normal, compile_fail, [''])
+test('T16414', normal, compile_fail, [''])