If infection is given a directory instead of a file for its configuration, it should search that directory

[Bilge]

In line with PHPUnit's behaviour, if the custom configuration file given is the path to a directory, rather than a path to a file, it should look for the same filenames it would by default, e.g. infection.json, infection.json.dist inside that directory instead of failing with an error because the directory itself is not a configuration file.

[theofidry]

The downside of that is that:

• You cannot use a more exotic name if you want to
• You cannot force infection to pick infection.json.dist over infection.json in any way
• It is easier to just check that the passed configuration file exists and is a file over checking that the passed config exists, is a directory and has a configuration file

[Bilge]

I am suggesting to support directories in addition to files, not instead of. This invalidates your first two points. I cannot refute that supporting directories as well would increase code complexity but that's not a particularly valid point in my view.

[theofidry]

Maybe an easier way to approach this would be to allow to change the working directory like composer does:

$ composer --working-dir=/path/to/...