Fix usage string for invoke_without_command groups to mark subcommand as optional

repro.py

@@ -0,0 +1,17 @@
+import click
+
+
+@click.group(invoke_without_command=True)
+@click.pass_context
+def main(ctx):
+    if ctx.invoked_subcommand is not None:
+        return
+
+
+@main.command()
+def sub():
+    pass
+
+
+if __name__ == "__main__":
+    main()

src/click/core.py

@@ -1585,6 +1585,8 @@ def __init__(
         if subcommand_metavar is None:
             if chain:
                 subcommand_metavar = "COMMAND1 [ARGS]... [COMMAND2 [ARGS]...]..."
+            elif self.invoke_without_command:
+                subcommand_metavar = "[COMMAND] [ARGS]..."
             else:
                 subcommand_metavar = "COMMAND [ARGS]..."
 

tests/test_commands.py

@@ -299,6 +299,24 @@ def sync():
     assert result.output == "no subcommand, use default\nin subcommand\n"
 
 
+def test_invoke_without_command_usage_shows_optional_subcommand(runner):
+    import click
+
+    @click.group(invoke_without_command=True)
+    @click.pass_context
+    def cli(ctx):
+        if ctx.invoked_subcommand is not None:
+            return
+
+    @cli.command()
+    def sub():
+        pass
+
+    result = runner.invoke(cli, ["--help"])
+    assert result.exit_code == 0
+    assert "Usage: cli [OPTIONS] [COMMAND] [ARGS]..." in result.output
+
+
 def test_aliased_command_canonical_name(runner):
     class AliasedGroup(click.Group):
         def get_command(self, ctx, cmd_name):