vim-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().
a8eee21e75

runtime/doc/eval.txt

@@ -2023,8 +2023,8 @@ 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}
@@ -2759,9 +2759,10 @@ bufloaded({expr})					*bufloaded()*
 		{expr} exists and is loaded (shown in a window or hidden).
 		The {expr} argument is used like with |bufexists()|.
 
-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
@@ -2788,7 +2789,7 @@ bufname({expr})						*bufname()*
 	bufname("file2")	name of buffer where "file2" matches.
 
 							*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.
@@ -2796,7 +2797,7 @@ bufnr({expr} [, {create}])
 		{create} argument is present and not zero, a new, unlisted,
 		buffer is created and its number is returned.
 		bufnr("$") is the last buffer: >
-	:let last_buffer = bufnr("$")
+			:let last_buffer = bufnr("$")
 <		The result is a Number, which is the highest buffer number
 		of existing buffers.  Note that not all buffers with a smaller
 		number necessarily exist, because ":bwipeout" may have removed

src/nvim/eval.c

@@ -7353,14 +7353,19 @@ static buf_T * get_buf_arg(typval_T *arg)
  */
 static void f_bufname(typval_T *argvars, typval_T *rettv, FunPtr fptr)
 {
+  const buf_T *buf;
   rettv->v_type = VAR_STRING;
   rettv->vval.v_string = NULL;
-  if (!tv_check_str_or_nr(&argvars[0])) {
+  if (argvars[0].v_type == VAR_UNKNOWN) {
-    return;
+    buf = curbuf;
+  } else {
+    if (!tv_check_str_or_nr(&argvars[0])) {
+      return;
+    }
+    emsg_off++;
+    buf = tv_get_buf(&argvars[0], false);
+    emsg_off--;
   }
-  emsg_off++;
-  const buf_T *const buf = tv_get_buf(&argvars[0], false);
-  emsg_off--;
   if (buf != NULL && buf->b_fname != NULL) {
     rettv->vval.v_string = (char_u *)xstrdup((char *)buf->b_fname);
   }
@@ -7371,15 +7376,21 @@ static void f_bufname(typval_T *argvars, typval_T *rettv, FunPtr fptr)
  */
 static void f_bufnr(typval_T *argvars, typval_T *rettv, FunPtr fptr)
 {
+  const buf_T *buf;
   bool error = false;
 
   rettv->vval.v_number = -1;
-  if (!tv_check_str_or_nr(&argvars[0])) {
+
-    return;
+  if (argvars[0].v_type == VAR_UNKNOWN) {
+    buf = curbuf;
+  } else {
+    if (!tv_check_str_or_nr(&argvars[0])) {
+      return;
+    }
+    emsg_off++;
+    buf = tv_get_buf(&argvars[0], false);
+    emsg_off--;
   }
-  emsg_off++;
-  const buf_T *buf = tv_get_buf(&argvars[0], false);
-  emsg_off--;
 
   // If the buffer isn't found and the second argument is not zero create a
   // new buffer.

src/nvim/eval.lua

@@ -44,13 +44,13 @@ return {
     bufadd={args=1},
     bufexists={args=1},
     buffer_exists={args=1, func='f_bufexists'},  -- obsolete
-    buffer_name={args=1, func='f_bufname'},  -- obsolete
+    buffer_name={args={0, 1}, func='f_bufname'},  -- obsolete
-    buffer_number={args=1, func='f_bufnr'},  -- obsolete
+    buffer_number={args={0, 1}, func='f_bufnr'},  -- obsolete
     buflisted={args=1},
     bufload={args=1},
     bufloaded={args=1},
-    bufname={args=1},
+    bufname={args={0, 1}},
-    bufnr={args={1, 2}},
+    bufnr={args={0, 2}},
     bufwinid={args=1},
     bufwinnr={args=1},
     byte2line={args=1},

src/nvim/testdir/test_arglist.vim

@@ -379,10 +379,10 @@ 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