Disallow use of is_flag and multiple in options

[amyreese]

Raises a TypeError similar to when specifying both count=True and is_flag=True.

• fixes Flags with multiple=True trigger TypeError #2246

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.

[0xDEC0DE]

This ends up being a breaking change, .

For instance:

@click.command
@click.option('-p', '--project', 'project', envvar='PROJECT', multiple=True)
@click.option('--all-projects', '--all', 'project', envvar='PROJECT', flag_value='all', multiple=True)
def main(project):
    ...

...has been a convenient way to provide an "all projects" shortcut in my software for years, and now it's broken with little-to-no notice.

[juledwar]

Agreed.

[davidism]

Removed the error and set default=True instead in #2550. The example from @0xDEC0DE and other issues should be addressed by that.