From 4957497d022ace56f54d8c3b49f289be5a09a6d4 Mon Sep 17 00:00:00 2001
From: Joseph Ferano <joseph@ferano.io>
Date: Sat, 12 Sep 2026 03:56:40 +0700
Subject: [PATCH] A mutation run found the four-byte overlong nobody was
 testing
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Relaxing 0xf0's second-byte floor from 0x90 to 0x80 left the whole suite
green: every other row of the table had a case pinning it and that one did
not, so f0 80 80 af decoded happily as "/". The same smuggled slash the
two- and three-byte cases exist to catch, missed in the fourth width.

Seven mutations verified red after this: the lead-byte floor at 0xc2 and the
second-byte bounds on 0xe0, 0xed, 0xf0 and 0xf4, the truncated-sequence
width, and the split cursor dropping its trailing empty field. An eighth,
lower-ascii written as a bit-xor, is red on the bytes either side of the
letters — which is why those are in the table and the letters alone are not.
---
 test/programs/utf8.flan | 4 ++++
 test/test_acceptance.ml | 2 +-
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/test/programs/utf8.flan b/test/programs/utf8.flan
index 7e662a8..db5b269 100644
--- a/test/programs/utf8.flan
+++ b/test/programs/utf8.flan
@@ -28,6 +28,7 @@
 
 (defconst overlong2 [2 u8] [0xc0 0xaf])
 (defconst overlong3 [3 u8] [0xe0 0x80 0xaf])
+(defconst overlong4 [4 u8] [0xf0 0x80 0x80 0xaf])
 (defconst surrogate [3 u8] [0xed 0xa0 0x80])
 (defconst above-max [4 u8] [0xf4 0x90 0x80 0x80])
 (defconst lead-f5   [4 u8] [0xf5 0x80 0x80 0x80])
@@ -101,6 +102,9 @@
   (show-dec (slice lone-cont 0 1))    ; a continuation byte leading
   (show-dec (slice overlong2 0 2))    ; overlong "/"
   (show-dec (slice overlong3 0 3))    ; overlong "/" again, three bytes
+  (show-dec (slice overlong4 0 4))    ; and four. Added after a mutation run:
+                                      ; relaxing 0xf0's floor to 0x80 left the
+                                      ; whole suite green without this line.
   (show-dec (slice surrogate 0 3))    ; U+D800
   (show-dec (slice above-max 0 4))        ; U+110000
   (show-dec (slice lead-f5 0 4))          ; 0xf5 leads nothing
diff --git a/test/test_acceptance.ml b/test/test_acceptance.ml
index 1fb74f2..63e5ea8 100644
--- a/test/test_acceptance.ml
+++ b/test/test_acceptance.ml
@@ -904,7 +904,7 @@ ERR@7 unexpected token: not the kind the caller was reading
        shared. *)
     let utf8_out =
       "0/0/f 65/1/t 233/2/t 26085/3/t 128512/4/t \n\
-       0/1/f 0/1/f 0/1/f 0/1/f 0/1/f 0/1/f \n\
+       0/1/f 0/1/f 0/1/f 0/1/f 0/1/f 0/1/f 0/1/f \n\
        0/1/f 0/1/f 0/1/f 0/1/f \n\
        tft\n\
        0 3 8 13 3\n\