Fix copying text properties by 'format'

* src/editfns.c (styled_format): Fix copying text properties from
the format specification to the produced string representation.
(Bug#23730)
(Fformat) Doc fix.

* doc/lispref/strings.texi (Formatting Strings): Document that
text properties from the format specifiers are also copied to the
produced string.

doc/lispref/strings.texi

@@ -833,7 +833,8 @@ arguments @var{objects} are the computed values to be formatted.
 
 The characters in @var{string}, other than the format specifications,
 are copied directly into the output, including their text properties,
-if any.
+if any.  Any text properties of the format specifications are copied
+to the produced string representations of the argument @var{objects}.
 @end defun
 
 @defun format-message string &rest objects

src/editfns.c

@@ -3883,6 +3883,9 @@ precision specifier says how many decimal places to show; if zero, the
 decimal point itself is omitted.  For %s and %S, the precision
 specifier truncates the string to the given width.
 
+Text properties, if any, are copied from the format-string to the
+produced text.
+
 usage: (format STRING &rest OBJECTS)  */)
   (ptrdiff_t nargs, Lisp_Object *args)
 {
@@ -4170,6 +4173,7 @@ styled_format (ptrdiff_t nargs, Lisp_Object *args, bool message)
 	      convbytes += padding;
 	      if (convbytes <= buf + bufsize - p)
 		{
+                  info[n].start = nchars;
 		  if (! minus_flag)
 		    {
 		      memset (p, ' ', padding);
@@ -4188,9 +4192,7 @@ styled_format (ptrdiff_t nargs, Lisp_Object *args, bool message)
 				  nbytes,
 				  STRING_MULTIBYTE (args[n]), multibyte);
 
-                  info[n].start = nchars;
 		  nchars += nchars_string;
-		  info[n].end = nchars;
 
 		  if (minus_flag)
 		    {
@@ -4198,6 +4200,7 @@ styled_format (ptrdiff_t nargs, Lisp_Object *args, bool message)
 		      p += padding;
 		      nchars += padding;
 		    }
+		  info[n].end = nchars;
 
 		  /* If this argument has text properties, record where
 		     in the result string it appears.  */
@@ -4415,6 +4418,7 @@ styled_format (ptrdiff_t nargs, Lisp_Object *args, bool message)
 			exponent_bytes = src + sprintf_bytes - e;
 		    }
 
+                  info[n].start = nchars;
 		  if (! minus_flag)
 		    {
 		      memset (p, ' ', padding);
@@ -4437,9 +4441,7 @@ styled_format (ptrdiff_t nargs, Lisp_Object *args, bool message)
 		  memcpy (p, src, exponent_bytes);
 		  p += exponent_bytes;
 
-                  info[n].start = nchars;
 		  nchars += leading_zeros + sprintf_bytes + trailing_zeros;
-		  info[n].end = nchars;
 
 		  if (minus_flag)
 		    {
@@ -4447,6 +4449,7 @@ styled_format (ptrdiff_t nargs, Lisp_Object *args, bool message)
 		      p += padding;
 		      nchars += padding;
 		    }
+		  info[n].end = nchars;
 
 		  continue;
 		}