Plugins and overloads don't interact well

[ilevkivskyi]

While working on our own plugin for self-checking proper types and isinstance() I have tried to use get_function_hook() to emit some additional errors for overloaded functions. However this didn't work for three reasons:

• The hook is called for every item in the overload, not for the overload as a whole.
• The overload logic relies on empty Messages as a sign of successful match
• The errors emitted during overload selection are discarded afterwards, thus discarding the plugin generated errors

This issue can be fixed in three ways:

• Calling the hook on the overload as a whole after selecting the overload variant and type checking it (this however will be a breaking API change)
• Use newly added error codes to only abandon a variant if the error is due to bad argument type (this is however still bad because check_call() may have an accept() call possibly triggering almost arbitrary error including a nested function call with bad argument type).
• Use a boolean attribute somewhere that will be set by check_argument_types() and will be reset after return from every accept().

I am leaning towards the last option, since it is most principled IMO.

cc @Michael0x2a

[tapaswenipathak]

Hi @ilevkivskyi: I would like closing the issue. Can I PR?

[ilevkivskyi]

Sure, you can try, but note that this may not be the best issue for a new contributor. If you would like to try few simpler issues first we have a special label https://github.com/python/mypy/issues?q=is%3Aopen+is%3Aissue+label%3Agood-first-issue