Replace undefined with more descriptive error

data/AlexTemplate.hs

@@ -94,7 +94,7 @@ data AlexReturn a
 
 -- alexScan :: AlexInput -> StartCode -> AlexReturn a
 alexScan input__ IBOX(sc)
-  = alexScanUser undefined input__ IBOX(sc)
+  = alexScanUser (error "alex rule requiring context was invoked by alexScan; use alexScanUser instead?") input__ IBOX(sc)
 
 alexScanUser user__ input__ IBOX(sc)
   = case alex_scan_tkn user__ input__ ILIT(0) input__ sc AlexNone of

tests/Makefile

@@ -58,6 +58,7 @@ TESTS = \
         issue_119.x \
         issue_141.x \
         issue_197.x \
+        issue_262.x \
         monad_typeclass.x \
         monad_typeclass_bytestring.x \
         monadUserState_typeclass.x \

tests/issue_262.x

@@ -0,0 +1,26 @@
+{
+-- https://github.com/haskell/alex/pull/262
+-- https://gitlab.haskell.org/ghc/ghc/-/issues/25609
+--
+-- Error happens when using alexScan with a lexer that
+-- inspects the context.
+
+import Control.Exception
+import Data.List (isInfixOf)
+}
+
+%wrapper "basic"
+
+:-
+.* / { \state _ _ _ -> state == 'x' } { id }
+
+{
+main :: IO ()
+main = do
+  result <- try $ evaluate $ alexScan ('\n', [], "") 0 `seq` ()
+  case result of
+    Left (e :: SomeException)
+      | "use alexScanUser instead" `isInfixOf` show e
+      -> pure ()
+    _ -> error $ "Got unexpected result: " ++ show result
+}