vim-patch:8.1.0573: cannot redefine user command without ! in same script

Problem:    Cannot redefine user command without ! in same script
Solution:   Allow redefining user command without ! in same script, like with
            functions.
55d4691308

runtime/doc/map.txt

@@ -1162,6 +1162,10 @@ See |:verbose-cmd| for more information.
 			attributes (see below) are {attr}.  If the command
 			already exists, an error is reported, unless a ! is
 			specified, in which case the command is redefined.
+			There is one exception: When sourcing a script again,
+			a command that was previously defined in that script
+			will be silently replaced.
+
 
 :delc[ommand] {cmd}				*:delc* *:delcommand* *E184*
 			Delete the user-defined command {cmd}.
@@ -1169,7 +1173,8 @@ See |:verbose-cmd| for more information.
 :comc[lear]						*:comc* *:comclear*
 			Delete all user-defined commands.
 
-Command attributes
+
+Command attributes ~
 
 User-defined commands are treated by Vim just like any other Ex commands.  They
 can have arguments, or have a range specified.  Arguments are subject to
@@ -1180,8 +1185,9 @@ There are a number of attributes, split into four categories: argument
 handling, completion behavior, range handling, and special cases.  The
 attributes are described below, by category.
 
-Argument handling				*E175* *E176* *:command-nargs*
 
+Argument handling ~
+						*E175* *E176* *:command-nargs*
 By default, a user defined command will take no arguments (and an error is
 reported if any are supplied).  However, it is possible to specify that the
 command can take arguments, using the -nargs attribute.  Valid cases are:
@@ -1257,9 +1263,9 @@ completion can be enabled:
 Note: That some completion methods might expand environment variables.
 
 
-Custom completion			*:command-completion-custom*
-					*:command-completion-customlist*
-					*E467* *E468*
+Custom completion ~
+				*:command-completion-custom*
+				*:command-completion-customlist* *E467* *E468*
 It is possible to define customized completion schemes via the "custom,{func}"
 or the "customlist,{func}" completion argument.  The {func} part should be a
 function with the following signature: >
@@ -1304,8 +1310,8 @@ the 'path' option: >
 This example does not work for file names with spaces!
 
 
-Range handling				*E177* *E178* *:command-range*
-							*:command-count*
+Range handling ~
+				*E177* *E178* *:command-range* *:command-count*
 By default, user-defined commands do not accept a line number range.  However,
 it is possible to specify that the command does take a range (the -range
 attribute), or that it takes an arbitrary count value, either in the line
@@ -1342,7 +1348,8 @@ Possible values are:
 	-addr=other		other kind of range
 
 
-Special cases				*:command-bang* *:command-bar*
+Special cases ~
+					*:command-bang* *:command-bar*
 					*:command-register* *:command-buffer*
 There are some special cases as well:
 
@@ -1360,7 +1367,8 @@ replacement text separately.
 Note that these arguments can be abbreviated, but that is a deprecated
 feature.  Use the full name for new scripts.
 
-Replacement text
+
+Replacement text ~
 
 The replacement text for a user defined command is scanned for special escape
 sequences, using <...> notation.  Escape sequences are replaced with values

src/nvim/ex_docmd.c

@@ -5026,8 +5026,13 @@ static int uc_add_command(char_u *name, size_t name_len, char_u *rep,
     }
 
     if (cmp == 0) {
-      if (!force) {
-        EMSG(_("E174: Command already exists: add ! to replace it"));
+      // Command can be replaced with "command!" and when sourcing the
+      // same script again, but only once.
+      if (!force
+          && (cmd->uc_script_ctx.sc_sid != current_sctx.sc_sid
+              || cmd->uc_script_ctx.sc_seq == current_sctx.sc_seq)) {
+        EMSG2(_("E174: Command already exists: add ! to replace it: %s"),
+              name);
         goto fail;
       }
 

src/nvim/testdir/test_usercommands.vim

@@ -184,6 +184,34 @@ func Test_Ambiguous()
   call assert_fails("\x4ei\041", 'E492: Not an editor command: Ni!')
 endfunc
 
+func Test_redefine_on_reload()
+  call writefile(['command ExistingCommand echo "yes"'], 'Xcommandexists')
+  call assert_equal(0, exists(':ExistingCommand'))
+  source Xcommandexists
+  call assert_equal(2, exists(':ExistingCommand'))
+  " Redefining a command when reloading a script is OK.
+  source Xcommandexists
+  call assert_equal(2, exists(':ExistingCommand'))
+
+  " But redefining in another script is not OK.
+  call writefile(['command ExistingCommand echo "yes"'], 'Xcommandexists2')
+  call assert_fails('source Xcommandexists2', 'E174:')
+  call delete('Xcommandexists2')
+
+  " And defining twice in one script is not OK.
+  delcommand ExistingCommand
+  call assert_equal(0, exists(':ExistingCommand'))
+  call writefile([
+	\ 'command ExistingCommand echo "yes"',
+	\ 'command ExistingCommand echo "no"',
+	\ ], 'Xcommandexists')
+  call assert_fails('source Xcommandexists', 'E174:')
+  call assert_equal(2, exists(':ExistingCommand'))
+
+  call delete('Xcommandexists')
+  delcommand ExistingCommand
+endfunc
+
 func Test_CmdUndefined()
   call assert_fails('Doit', 'E492:')
   au CmdUndefined Doit :command Doit let g:didit = 'yes'