Fix OptCoercion

In the presence of -fdefer-type-errors, OptCoercion can
encounter a mal-formed coerercion with type
    T a ~ T a b
and that was causing a subsequent Lint error.

This caused Trac #14607.  Easily fixed by turning an ASSERT
into a guard.

1 files changed, 23 insertions, 0 deletions

diff --git a/testsuite/tests/typecheck/should_fail/T14607.hs b/testsuite/tests/typecheck/should_fail/T14607.hs
new file mode 100644
index 0000000000..891d3ccc18
--- /dev/null
+++ b/testsuite/tests/typecheck/should_fail/T14607.hs
@@ -0,0 +1,23 @@
+{-# OPTIONS_GHC -fdefer-type-errors #-}
+    -- This line is crucial to the bug
+
+{-# Language GADTs #-}
+{-# Language InstanceSigs #-}
+{-# Language KindSignatures #-}
+{-# Language TypeFamilies #-}
+{-# Language DataKinds #-}
+{-# Language FlexibleInstances #-}
+
+module T14607 where
+
+import Data.Kind
+
+data LamCons :: Type -> Type -> () -> Type where
+  C :: LamCons a a '()
+
+class Mk a where
+  mk :: LamCons a a '()
+
+instance Mk a where
+  mk :: LamCons a '()
+  mk = mk