Temporary fix to Trac #14380

This fix replaces an utterly bogus error message with a decent one,
rejecting a pattern synonym with a list pattern and rebindable syntax.

Not hard to fix properly, but I'm going to wait for a willing volunteer
and/or more user pressure.

3 files changed, 18 insertions, 0 deletions

diff --git a/testsuite/tests/patsyn/should_fail/T14380.hs b/testsuite/tests/patsyn/should_fail/T14380.hs
new file mode 100644
index 0000000000..aec398590d
--- /dev/null
+++ b/testsuite/tests/patsyn/should_fail/T14380.hs
@@ -0,0 +1,8 @@
+{-# LANGUAGE OverloadedLists #-}
+{-# LANGUAGE PatternSynonyms #-}
+
+module T14380 where
+
+data Foo = Foo [Int]
+pattern Bar :: Foo
+pattern Bar = Foo []
diff --git a/testsuite/tests/patsyn/should_fail/T14380.stderr b/testsuite/tests/patsyn/should_fail/T14380.stderr
new file mode 100644
index 0000000000..4228d2955c
--- /dev/null
+++ b/testsuite/tests/patsyn/should_fail/T14380.stderr
@@ -0,0 +1,9 @@
+
+T14380.hs:8:15: error:
+    Invalid right-hand side of bidirectional pattern synonym ‘Bar’:
+      Pattern ‘[]’ is not invertible
+      Suggestion: instead use an explicitly bidirectional pattern synonym, e.g.
+        pattern Bar <- Foo [] where Bar = ...
+      Reason: rebindable syntax is on.
+      This is fixable: add use-case to Trac #14380
+    RHS pattern: Foo []
diff --git a/testsuite/tests/patsyn/should_fail/all.T b/testsuite/tests/patsyn/should_fail/all.T
index 8a098d9d1f..388e67b27b 100644
--- a/testsuite/tests/patsyn/should_fail/all.T
+++ b/testsuite/tests/patsyn/should_fail/all.T
@@ -38,3 +38,4 @@ test('T13349', normal, compile_fail, [''])
 test('T13470', normal, compile_fail, [''])
 test('T14112', normal, compile_fail, [''])
 test('T14114', normal, compile_fail, [''])
+test('T14380', normal, compile_fail, [''])