Suggest using PEP 604-style union types

[hoefling]

Humble suggestion: add new code to suggest a replacement of Union[...] types with the | chain, e.g. Y093 Use PEP 604 union types. A simple implementation start could be e.g.

class PyVisitor(ast.NodeVisitor):
    ...
     def visit_FunctionDef(self, node):
        if node.returns.value is not None and node.returns.value.id in ("Optional", "Union"):
            self.error(node.returns, Y093)
        ...

    def visit_arguments(self, node):
        if arg.annotation.value.id in ("Optional", "Union"):
            self.error(arg, Y093)
        ...

[srittau]

Another way to do this is to check Union and Optional imports. This would generate only one warning per file.

[AlexWaygood]

In my view, we should probably not bother with trying to port the PEP 604 logic in check_new_syntax to flake8-pyi for now. It would be non-trivial to do so, and hopefully, full mypy support for PEP 604 type aliases is just around the corner. Once it is, we can just ban importing Union and Optional, as @srittau suggests. That should be much easier.

[hoefling]

Hmm, but I didn't propose building replacement snippets 8-) A simple Use PEP 604 union types would already suffice IMO.

[AlexWaygood]

Hmm, but I didn't propose building replacement snippets 8-) A simple Use PEP 604 union types would already suffice IMO.

Well, feel free to submit a PR, if you like ;)