Allow to document user-defined aliases.

Compared to the previous version, this version fixes the comments reported by
Tom Tromey and ensures that the 'help some-user-documented-alias'
shows the alias definition to ensure the user understands this is an
alias even if specifically documented.

When using 'help ALIASNAME', GDB shows the help of the aliased command.
This is a good default behaviour.

However, GDB alias command allows to define aliases with arguments
possibly changing or tuning significantly the behaviour of
the aliased command.  In such a case, showing the help of the aliased
command might not be ideal.

This is particularly true when defining an alias as a set of
nested 'with' followed by a last command to launch, such as:
  (gdb) alias pp10 = with print pretty -- with print elements 10 -- print
Asking 'help pp10' shows the help of the 'with' command, which is
not particularly useful:
  (gdb) help pp10
  with, pp10, w
    alias pp10 = with print pretty -- with print elements 10 -- print
  Temporarily set SETTING to VALUE, run COMMAND, and restore SETTING.
  Usage: with SETTING [VALUE] [-- COMMAND]
  ....

Such an alias can now be documented by the user:
  (gdb) document pp10
  >Pretty printing an expressiong, printing 10 elements.
  >Usage: pp10 [PRINT-COMMAND-OPTIONS] EXP
  >See 'help print' for more information.
  >end
  (gdb) help pp10
    alias pp10 = with print pretty -- with print elements 10 -- print
  Pretty printing an expressiong, printing 10 elements.
  Usage: pp10 [PRINT-COMMAND-OPTIONS] EXP
  See 'help print' for more information.
  (gdb)

When a user-defined alias is documented specifically, help and apropos
use the provided alias documentation instead of the documentation of
the aliased command.

Such a documented alias is also not shown anymore in the help of the
aliased command, and the alias is not listed anymore in the help
of the aliased command.  In particular for cases such as pp10 example above,
indicating that pp10 is an alias of the 'with' command is confusing.

gdb/NEWS

@@ -99,6 +99,16 @@ maintenance print frame-id [ LEVEL ]
 
 * Changed commands
 
+document user-defined
+  It is now possible to document user-defined aliases.
+  When a user-defined alias is documented, the help and apropos commands
+  use the provided documentation instead of the documentation of the
+  aliased command.
+  Documenting a user-defined alias is particularly useful when the alias
+  is a set of nested 'with' commands to avoid showing the help of
+  the with command for an alias that will in fact launch the
+  last command given in the nested commands.
+
 maintenance info line-table
   Add a PROLOGUE-END column to the output which indicates that an
   entry corresponds to an address where a breakpoint should be placed

gdb/cli/cli-decode.c

@@ -1349,6 +1349,18 @@ fput_command_name_styled (const cmd_list_element &c, struct ui_file *stream)
 		  prefixname.c_str (), c.name);
 }
 
+/* True if ALIAS has a user-defined documentation.  */
+
+static bool
+user_documented_alias (const cmd_list_element &alias)
+{
+  gdb_assert (alias.is_alias ());
+  /* Alias is user documented if it has an allocated documentation
+     that differs from the aliased command.  */
+  return (alias.doc_allocated
+	  && strcmp (alias.doc, alias.alias_target->doc) != 0);
+}
+
 /* Print the definition of alias C using title style for alias
    and aliased command.  */
 
@@ -1364,20 +1376,22 @@ fput_alias_definition_styled (const cmd_list_element &c,
   gdb_printf (stream, " %s\n", c.default_args.c_str ());
 }
 
-/* Print the definition of the aliases of CMD that have default args.  */
+/* Print the definition of CMD aliases not deprecated and having default args
+   and not specifically documented by the user.  */
 
 static void
 fput_aliases_definition_styled (const cmd_list_element &cmd,
 				struct ui_file *stream)
 {
   for (const cmd_list_element &alias : cmd.aliases)
-    if (!alias.cmd_deprecated && !alias.default_args.empty ())
+    if (!alias.cmd_deprecated
+	&& !user_documented_alias (alias)
+	&& !alias.default_args.empty ())
       fput_alias_definition_styled (alias, stream);
 }
 
-
+/* If C has one or more aliases, style print the name of C and the name of its
-/* If C has one or more aliases, style print the name of C and
+   aliases not documented specifically by the user, separated by commas.
-   the name of its aliases, separated by commas.
    If ALWAYS_FPUT_C_NAME, print the name of C even if it has no aliases.
    If one or more names are printed, POSTFIX is printed after the last name.
 */
@@ -1389,11 +1403,11 @@ fput_command_names_styled (const cmd_list_element &c,
 {
   /* First, check if we are going to print something.  That is, either if
      ALWAYS_FPUT_C_NAME is true or if there exists at least one non-deprecated
-     alias.  */
+     alias not documented specifically by the user.  */
 
   auto print_alias = [] (const cmd_list_element &alias)
     {
-      return !alias.cmd_deprecated;
+      return !alias.cmd_deprecated && !user_documented_alias (alias);
     };
 
   bool print_something = always_fput_c_name;
@@ -1474,11 +1488,11 @@ apropos_cmd (struct ui_file *stream,
   /* Walk through the commands.  */
   for (c=commandlist;c;c=c->next)
     {
-      if (c->is_alias ())
+      if (c->is_alias () && !user_documented_alias (*c))
 	{
-	  /* Command aliases/abbreviations are skipped to ensure we print the
+	  /* Command aliases/abbreviations not specifically documented by the
-	     doc of a command only once, when encountering the aliased
+	     user are skipped to ensure we print the doc of a command only once,
-	     command.  */
+	     when encountering the aliased command.  */
 	  continue;
 	}
 
@@ -1571,11 +1585,24 @@ help_cmd (const char *command, struct ui_file *stream)
      number of this class so that the commands in the class will be
      listed.  */
 
-  /* If the user asked 'help somecommand' and there is no alias,
+  if (alias == nullptr || !user_documented_alias (*alias))
-     the false indicates to not output the (single) command name.  */
+    {
-  fput_command_names_styled (*c, false, "\n", stream);
+      /* Case of a normal command, or an alias not explictly
-  fput_aliases_definition_styled (*c, stream);
+	 documented by the user.  */
-  gdb_puts (c->doc, stream);
+      /* If the user asked 'help somecommand' and there is no alias,
+	 the false indicates to not output the (single) command name.  */
+      fput_command_names_styled (*c, false, "\n", stream);
+      fput_aliases_definition_styled (*c, stream);
+      gdb_puts (c->doc, stream);
+    }
+  else
+    {
+      /* Case of an alias explictly documented by the user.
+	 Only output the alias definition and its explicit documentation.  */
+      fput_alias_definition_styled (*alias, stream);
+      fput_command_names_styled (*alias, false, "\n", stream);
+      gdb_puts (alias->doc, stream);
+    }
   gdb_puts ("\n", stream);
 
   if (!c->is_prefix () && !c->is_command_class_help ())

gdb/cli/cli-script.c

@@ -1500,39 +1500,49 @@ define_command (const char *comname, int from_tty)
   do_define_command (comname, from_tty, nullptr);
 }
 
-/* Document a user-defined command.  If COMMANDS is NULL, then this is a
+/* Document a user-defined command or user defined alias.  If COMMANDS is NULL,
-   top-level call and the document will be read using read_command_lines.
+   then this is a top-level call and the document will be read using
-   Otherwise, it is a "document" command in an existing command and the
+   read_command_lines.  Otherwise, it is a "document" command in an existing
-   commands are provided.  */
+   command and the commands are provided.  */
 static void
 do_document_command (const char *comname, int from_tty,
                      const counted_command_line *commands)
 {
-  struct cmd_list_element *c, **list;
+  struct cmd_list_element *alias, *prefix_cmd, *c;
-  const char *tem;
   const char *comfull;
 
   comfull = comname;
-  list = validate_comname (&comname);
+  validate_comname (&comname);
 
-  tem = comname;
+  lookup_cmd_composition (comfull, &alias, &prefix_cmd, &c);
-  c = lookup_cmd (&tem, *list, "", NULL, 0, 1);
 
-  if (c->theclass != class_user)
+  if (c->theclass != class_user
-    error (_("Command \"%s\" is built-in."), comfull);
+      && (alias == nullptr || alias->theclass != class_alias))
+    {
+      if (alias == nullptr)
+	error (_("Command \"%s\" is built-in."), comfull);
+      else
+	error (_("Alias \"%s\" is built-in."), comfull);
+    }
+
+  /* If we found an alias of class_alias, the user is documenting this
+     user-defined alias.  */
+  if (alias != nullptr)
+    c = alias;
 
   counted_command_line doclines;
 
   if (commands == nullptr)
     {
-      std::string prompt 
+      std::string prompt
         = string_printf ("Type documentation for \"%s\".", comfull);
       doclines = read_command_lines (prompt.c_str (), from_tty, 0, 0);
     }
   else
     doclines = *commands;
 
-  xfree ((char *) c->doc);
+  if (c->doc_allocated)
+    xfree ((char *) c->doc);
 
   {
     struct command_line *cl1;
@@ -1553,6 +1563,7 @@ do_document_command (const char *comname, int from_tty,
       }
 
     c->doc = doc;
+    c->doc_allocated = 1;
   }
 }
 
@@ -1681,8 +1692,8 @@ _initialize_cli_script ()
      its prefixes.  */
   document_cmd_element = add_com ("document", class_support, document_command,
                                   _("\
-Document a user-defined command.\n\
+Document a user-defined command or user-defined alias.\n\
-Give command name as argument.  Give documentation on following lines.\n\
+Give command or alias name as argument.  Give documentation on following lines.\n\
 End with a line of just \"end\"."));
   set_cmd_completer (document_cmd_element, command_completer);
   define_cmd_element = add_com ("define", class_support, define_command, _("\

gdb/doc/gdb.texinfo

@@ -2265,11 +2265,20 @@ one or more aliases, @value{GDBN} will display a first line with
 the command name and all its aliases separated by commas.
 This first line will be followed by the full definition of all aliases
 having default arguments.
+When asking the help for an alias, the documentation for the aliased
+command is shown.
+
+A user-defined alias can optionally be documented using the
+@code{document} command (@pxref{Define, document}).  @value{GDBN} then
+considers this alias as different from the aliased command: this alias
+is not listed in the aliased command help output, and asking help for
+this alias will show the documentation provided for the alias instead of
+the documentation of the aliased command.
 
 @kindex apropos
 @item apropos [-v] @var{regexp}
 The @code{apropos} command searches through all of the @value{GDBN}
-commands, and their documentation, for the regular expression specified in
+commands and aliases, and their documentation, for the regular expression specified in
 @var{args}.  It prints out all matches found.  The optional flag  @samp{-v},
 which stands for @samp{verbose}, indicates to output the full documentation
 of the matching commands and highlight the parts of the documentation
@@ -27967,6 +27976,13 @@ You may use the @code{document} command again to change the
 documentation of a command.  Redefining the command with @code{define}
 does not change the documentation.
 
+It is also possible to document user-defined aliases.  The alias documentation
+will then be used by the @code{help} and @code{apropos} commands
+instead of the documentation of the aliased command.
+Documenting a user-defined alias is particularly useful when defining
+an alias as a set of nested @code{with} commands
+(@pxref{Command aliases default args}).
+
 @kindex define-prefix
 @item define-prefix @var{commandname}
 Define or mark the command @var{commandname} as a user-defined prefix
@@ -28641,6 +28657,14 @@ by the user.
 For more information about the @code{with} command usage,
 see @ref{Command Settings}.
 
+By default, asking the help for an alias shows the documentation of
+the aliased command.  When the alias is a set of nested commands, @code{help}
+of an alias shows the documentation of the first command.  This help
+is not particularly useful for an alias such as @code{pp10}.
+For such an alias, it is useful to give a specific documentation
+using the @code{document} command (@pxref{Define, document}).
+
+
 @c Python docs live in a separate file.
 @include python.texi
 

gdb/testsuite/gdb.base/help.exp

@@ -128,8 +128,48 @@ gdb_test "apropos handle signal" "handle -- Specify how to handle signals\."
 gdb_test "apropos apropos" "apropos -- Search for commands matching a REGEXP.*"
 
 # Test apropos for commands having aliases.
+gdb_test_no_output "alias mybt = backtrace" "define mybt alias"
+gdb_test_no_output "alias mybt10 = backtrace 10" "define mybt10 alias"
+
 gdb_test "apropos Print backtrace of all stack frames, or innermost COUNT frames\." \
-    "backtrace, where, bt -- Print backtrace of all stack frames, or innermost COUNT frames\."
+    "backtrace, mybt10, mybt, where, bt -- Print backtrace of all stack frames, or innermost COUNT frames\.\[\r\n\]+  alias mybt10 = backtrace 10"
 
 # Test help for commands having aliases.
-gdb_test "help bt" "backtrace, where, bt\[\r\n\]+Print backtrace of all stack frames, or innermost COUNT frames\..*"
+gdb_test "help bt" "backtrace, mybt10, mybt, where, bt\[\r\n\]+  alias mybt10 = backtrace 10\[\r\n\]+Print backtrace of all stack frames, or innermost COUNT frames\..*"
+
+# Document the aliases.  The apropos and help commands should then consider them
+# as "standalone" commands.
+gdb_test_multiple "document mybt" "document alias: mybt" {
+    -re "Type documentation for \"mybt\".\r\nEnd with a line saying just \"end\".\r\n>$" {
+	gdb_test "An alias of command backtrace without any args.\nend" \
+	    "" \
+	    "document alias: mybt"
+    }
+}
+gdb_test_multiple "document mybt10" "document alias: mybt10" {
+    -re "Type documentation for \"mybt10\".\r\nEnd with a line saying just \"end\".\r\n>$" {
+	gdb_test "An alias of command backtrace with arg 10.\nend" \
+	    "" \
+	    "document alias: mybt10"
+    }
+}
+
+# As the aliases are now documented, they do not appear in apropos/help backtrace output anymore.
+gdb_test "apropos Print backtrace of all stack frames, or innermost COUNT frames\." \
+    "backtrace, where, bt -- Print backtrace of all stack frames, or innermost COUNT frames\." \
+    "apropos after documenting aliases"
+
+gdb_test "help bt" "backtrace, where, bt\[\r\n\]+Print backtrace of all stack frames, or innermost COUNT frames\..*" \
+    "help after documenting aliases"
+
+# Check apropos and help use the alias documentation.
+gdb_test "apropos An alias of command backtrace with arg 10" \
+    "mybt10 -- An alias of command backtrace with arg 10\." \
+    "apropos after documenting aliases showing mybt10 doc"
+
+gdb_test "help mybt" " alias mybt = backtrace \[\r\n\]+An alias of command backtrace without any args\." \
+    "help mybt after documenting aliases showing mybt doc"
+
+# Check pre-defined aliases cannot be documented.
+gdb_test "document where" "Alias \"where\" is built-in.*" \
+    "documenting builtin where alias disallowed"