Backport function.

src/lib/sedlexing.ml

@@ -7,8 +7,21 @@ exception MalFormed
 
 module Uchar = struct
   (* This for compatibility with ocaml < 4.14.0 *)
-  let utf_8_byte_length _ = 1
-  let utf_16_byte_length _ = 1
+  let utf_8_byte_length u =
+    match Uchar.to_int u with
+      | u when u < 0 -> assert false
+      | u when u <= 0x007F -> 1
+      | u when u <= 0x07FF -> 2
+      | u when u <= 0xFFFF -> 3
+      | u when u <= 0x10FFFF -> 4
+      | _ -> assert false
+
+  let utf_16_byte_length u =
+    match Uchar.to_int u with
+      | u when u < 0 -> assert false
+      | u when u <= 0xFFFF -> 2
+      | u when u <= 0x10FFFF -> 4
+      | _ -> assert false
 
   let () =
     ignore utf_8_byte_length;

src/lib/sedlexing.mli

@@ -91,9 +91,7 @@ val lexeme_start : lexbuf -> int
 
 (** [Sedlexing.lexeme_start lexbuf] returns the offset in the
         input stream of the first byte of the matched string.
-        The first code point of the stream has offset 0. Returned
-        value is the same as its code point equivalent when compiled with
-         OCaml < 4.14.0 *)
+        The first code point of the stream has offset 0. *)
 val lexeme_bytes_start : lexbuf -> int
 
 (** [Sedlexing.lexeme_end lexbuf] returns the offset in the input
@@ -105,8 +103,7 @@ val lexeme_end : lexbuf -> int
 (** [Sedlexing.lexeme_end lexbuf] returns the offset in the input
         stream of the byte following the last code point of the
         matched string. The first character of the stream has offset
-        0. Returned value is the same as its code point equivalent when
-        compiled with OCaml < 4.14.0 *)
+        0. *)
 val lexeme_bytes_end : lexbuf -> int
 
 (** [Sedlexing.loc lexbuf] returns the pair
@@ -140,8 +137,7 @@ val lexing_positions : lexbuf -> Lexing.position * Lexing.position
 (** [Sedlexing.lexing_bytes_positions lexbuf] returns the start and end
         positions, in bytes, of the current token, using a record of type
         [Lexing.position]. This is intended for consumption
-        by parsers like those generated by [Menhir]. Returned value is the
-        same as its code point equivalent when compiled with OCaml < 4.14.0 *)
+        by parsers like those generated by [Menhir]. *)
 val lexing_bytes_positions : lexbuf -> Lexing.position * Lexing.position
 
 (** [Sedlexing.new_line lexbuf] increments the line count and