Shell completion fails with for arguments starting with "/"

[samschott]

After enabling shell completion support, arguments which start with a leading forward slash "/" fail to complete.

After a bit of debugging, it appears that if the argument starts with a "/", the completions are generated for the command itself instead of for the argument. For example, my_app open /fold⇥ would generate completions for open instead of /fold whereas my_app open fold⇥ correctly generates completions for fold.

Edit: To clarify the above, in both cases either "/fold" or "fold" is correctly picked up as the incomplete string. But in the first case, obj.shell_complete(ctx, "/fold") is called with the "open" command as the obj.

Environment:

• Python version: 3.9
• Click version: 8.0.1
• Shell: zsh, bash

[samschott]

Further debugging shows that an argument starting with "/" is incorrectly identified as the start of an option by click.shell_completion._start_of_option():

click/src/click/shell_completion.py

Lines 449 to 451 in af0af57

	def _start_of_option(value: str) -> bool:
	"""Check if the value looks like the start of an option."""
	return not value[0].isalnum() if value else False

This causes the wrong object to be chosen to provide the completion in _resolve_incomplete():

click/src/click/shell_completion.py

Lines 555 to 556 in af0af57

	if "--" not in args and _start_of_option(incomplete):
	return ctx.command, incomplete