patch 8.1.1924: using empty string for current buffer is unexpected

Problem:    Using empty string for current buffer is unexpected.
Solution:   Make the argument optional for bufname() and bufnr().

runtime/doc/eval.txt

@@ -2335,8 +2335,8 @@ bufexists({expr})		Number	|TRUE| if buffer {expr} exists
 buflisted({expr})		Number	|TRUE| if buffer {expr} is listed
 bufload({expr})			Number	load buffer {expr} if not loaded yet
 bufloaded({expr})		Number	|TRUE| if buffer {expr} is loaded
-bufname({expr})			String	Name of the buffer {expr}
+bufname([{expr}])		String	Name of the buffer {expr}
-bufnr({expr} [, {create}])	Number	Number of the buffer {expr}
+bufnr([{expr} [, {create}]])	Number	Number of the buffer {expr}
 bufwinid({expr})		Number	window ID of buffer {expr}
 bufwinnr({expr})		Number	window number of buffer {expr}
 byte2line({byte})		Number	line number at byte count {byte}
@@ -3217,9 +3217,10 @@ bufloaded({expr})					*bufloaded()*
 		Can also be used as a |method|: >
 			let loaded = 'somename'->bufloaded()
 
-bufname({expr})						*bufname()*
+bufname([{expr}])					*bufname()*
 		The result is the name of a buffer, as it is displayed by the
 		":ls" command.
+		If {expr} is omitted the current buffer is used.
 		If {expr} is a Number, that buffer number's name is given.
 		Number zero is the alternate buffer for the current window.
 		If {expr} is a String, it is used as a |file-pattern| to match
@@ -3251,7 +3252,7 @@ bufname({expr})						*bufname()*
 		Obsolete name: buffer_name().
 
 							*bufnr()*
-bufnr({expr} [, {create}])
+bufnr([{expr} [, {create}]])
 		The result is the number of a buffer, as it is displayed by
 		the ":ls" command.  For the use of {expr}, see |bufname()|
 		above.
@@ -7201,7 +7202,7 @@ prompt_setcallback({buf}, {expr})			*prompt_setcallback()*
 		that was entered at the prompt.  This can be an empty string
 		if the user only typed Enter.
 		Example: >
-		   call prompt_setcallback(bufnr(''), function('s:TextEntered'))
+		   call prompt_setcallback(bufnr(), function('s:TextEntered'))
 		   func s:TextEntered(text)
 		     if a:text == 'exit' || a:text == 'quit'
 		       stopinsert
@@ -7227,7 +7228,7 @@ prompt_setprompt({buf}, {text})				*prompt_setprompt()*
 		{text} to end in a space.
 		The result is only visible if {buf} has 'buftype' set to
 		"prompt".  Example: >
-			call prompt_setprompt(bufnr(''), 'command: ')
+			call prompt_setprompt(bufnr(), 'command: ')
 <
 prop_ functions are documented here: |text-prop-functions|.
 

src/evalfunc.c

@@ -457,13 +457,13 @@ static funcentry_T global_functions[] =
     {"bufadd",		1, 1, FEARG_1,	  f_bufadd},
     {"bufexists",	1, 1, FEARG_1,	  f_bufexists},
     {"buffer_exists",	1, 1, FEARG_1,	  f_bufexists},	// obsolete
-    {"buffer_name",	1, 1, 0,	  f_bufname},	// obsolete
+    {"buffer_name",	0, 1, FEARG_1,	  f_bufname},	// obsolete
-    {"buffer_number",	1, 1, 0,	  f_bufnr},	// obsolete
+    {"buffer_number",	0, 1, FEARG_1,	  f_bufnr},	// obsolete
     {"buflisted",	1, 1, FEARG_1,	  f_buflisted},
     {"bufload",		1, 1, FEARG_1,	  f_bufload},
     {"bufloaded",	1, 1, FEARG_1,	  f_bufloaded},
-    {"bufname",		1, 1, FEARG_1,	  f_bufname},
+    {"bufname",		0, 1, FEARG_1,	  f_bufname},
-    {"bufnr",		1, 2, FEARG_1,	  f_bufnr},
+    {"bufnr",		0, 2, FEARG_1,	  f_bufnr},
     {"bufwinid",	1, 1, FEARG_1,	  f_bufwinid},
     {"bufwinnr",	1, 1, FEARG_1,	  f_bufwinnr},
     {"byte2line",	1, 1, FEARG_1,	  f_byte2line},
@@ -1820,15 +1820,20 @@ f_bufname(typval_T *argvars, typval_T *rettv)
 {
     buf_T	*buf;
 
-    (void)tv_get_number(&argvars[0]);	    /* issue errmsg if type error */
+    if (argvars[0].v_type == VAR_UNKNOWN)
+	buf = curbuf;
+    else
+    {
+	(void)tv_get_number(&argvars[0]);	// issue errmsg if type error
 	++emsg_off;
 	buf = tv_get_buf(&argvars[0], FALSE);
+	--emsg_off;
+    }
     rettv->v_type = VAR_STRING;
     if (buf != NULL && buf->b_fname != NULL)
 	rettv->vval.v_string = vim_strsave(buf->b_fname);
     else
 	rettv->vval.v_string = NULL;
-    --emsg_off;
 }
 
 /*
@@ -1841,13 +1846,18 @@ f_bufnr(typval_T *argvars, typval_T *rettv)
     int		error = FALSE;
     char_u	*name;
 
-    (void)tv_get_number(&argvars[0]);	    /* issue errmsg if type error */
+    if (argvars[0].v_type == VAR_UNKNOWN)
+	buf = curbuf;
+    else
+    {
+	(void)tv_get_number(&argvars[0]);    // issue errmsg if type error
 	++emsg_off;
 	buf = tv_get_buf(&argvars[0], FALSE);
 	--emsg_off;
+    }
 
-    /* If the buffer isn't found and the second argument is not zero create a
+    // If the buffer isn't found and the second argument is not zero create a
-     * new buffer. */
+    // new buffer.
     if (buf == NULL
 	    && argvars[1].v_type != VAR_UNKNOWN
 	    && tv_get_number_chk(&argvars[1], &error) != 0

src/testdir/test_arglist.vim

@@ -398,10 +398,10 @@ func Test_argedit()
   " make sure to use a new buffer number for x when it is loaded
   bw! x
   new
-  let a = bufnr('')
+  let a = bufnr()
   argedit x
-  call assert_equal(a, bufnr(''))
+  call assert_equal(a, bufnr())
-  call assert_equal('x', bufname(''))
+  call assert_equal('x', bufname())
   %argd
   bw! x
 endfunc

src/version.c

@@ -761,6 +761,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1924,
 /**/
     1923,
 /**/