mac ligature fix from railwaycat emacs

Stops "Attempt to shape unibyte text" errors when using ligatures

Taken from https://github.com/railwaycat/mirror-emacs-mac/commit/e7a046987d66136da55ae10a9ef90728d1442cbb#diff-9114056b032516fa2de10ffaaa8e8ea7R1769

src/character.h

@@ -97,6 +97,7 @@ enum
 /* Return the raw 8-bit byte for character C.  */
 #define CHAR_TO_BYTE8(c) (CHAR_BYTE8_P (c) ? (c) - 0x3FFF00 : (c & 0xFF))
 
+
 /* Return the raw 8-bit byte for character C,
    or -1 if C doesn't correspond to a byte.  */
 #define CHAR_TO_BYTE_SAFE(c)						\
@@ -121,6 +122,9 @@ enum
 /* This is the maximum byte length of multibyte form.  */
 #define MAX_MULTIBYTE_LENGTH 5
 
+/* Nonzero iff C is an ASCII byte.  */
+#define ASCII_BYTE_P(c) UNSIGNED_CMP (c, <, 0x80)
+
 /* Nonzero iff X is a character.  */
 #define CHARACTERP(x) (NATNUMP (x) && XFASTINT (x) <= MAX_CHAR)
 

src/composite.c

@@ -1727,20 +1727,45 @@ should be ignored.  */)
 
   if (NILP (string))
     {
-      if (NILP (BVAR (current_buffer, enable_multibyte_characters)))
-	error ("Attempt to shape unibyte text");
+
       validate_region (&from, &to);
       frompos = XFASTINT (from);
       topos = XFASTINT (to);
-      frombyte = CHAR_TO_BYTE (frompos);
+      if (!NILP (BVAR (current_buffer, enable_multibyte_characters)))
+	frombyte = CHAR_TO_BYTE (frompos);
+      else
+	{
+	  ptrdiff_t pos;
+
+	  /* fill_gstring_header below uses
+	     FETCH_CHAR_ADVANCE_NO_CHECK that assumes the current
+	     buffer is multibyte, but it is safe as long as it only
+	     fetches ASCII chars.  */
+	  for (pos = frompos; pos < topos; pos++)
+	    if (!ASCII_BYTE_P (*(BYTE_POS_ADDR (pos))))
+	      error ("Attempt to shape non-ASCII part of unibyte text");
+	  frombyte = frompos;
+	}
     }
   else
     {
       CHECK_STRING (string);
       validate_subarray (string, from, to, SCHARS (string), &frompos, &topos);
-      if (! STRING_MULTIBYTE (string))
-	error ("Attempt to shape unibyte text");
-      frombyte = string_char_to_byte (string, frompos);
+      if (STRING_MULTIBYTE (string))
+	frombyte = string_char_to_byte (string, frompos);
+      else
+	{
+	  ptrdiff_t pos;
+
+	  /* fill_gstring_header below uses
+	     FETCH_STRING_CHAR_ADVANCE_NO_CHECK that assumes the
+	     string is multibyte, but it is safe as long as it only
+	     fetches ASCII chars.  */
+	  for (pos = frompos; pos < topos; pos++)
+	    if (!ASCII_BYTE_P (SREF (string, pos)))
+	      error ("Attempt to shape non-ASCII part of unibyte text");
+	  frombyte = frompos;
+	}
     }
 
   header = fill_gstring_header (Qnil, frompos, frombyte,