Tagged unions not narrowed down correctly for TypedDicts

[klbostee]

Environment data

• Language Server version: 2021.5.0
• OS and version: macOS 10.15.7
• Python version (& distribution if applicable, e.g. Anaconda): CPython 2.7

Expected behaviour

The ability to discriminate between two TypedDicts using a Literal type, as explained in detail here: https://mypy.readthedocs.io/en/stable/literal_types.html#tagged-unions

Actual behaviour

The Union type doesn't get narrowed by a check on the Literal:

[erictraut]

Thanks for the feature request.

Pylance already supports type narrowing for tagged unions of classes that have literal-typed fields. It doesn't currently support type narrowing for tagged unions of typed dicts with literal-typed values. Since mypy already supports this case, I think pyright should as well. I'll add it to the backlog.

[erictraut]

I've added support for type narrowing of tagged unions of typed dicts for conditional expressions of the form x[K] == V and x[K] != V. This support will be in the next release.

Thanks again for the feature request.

[klbostee]

For what it's worth: the specific use case I had in mind would also benefit from the ability to narrow with a conditional expression of the form x[K] in (V, W), but I'm already very happy that we'll have == and != soon of course! 🙂

[jakebailey]

This issue has been fixed in version 2021.5.4, which we've just released. You can find the changelog here: https://github.com/microsoft/pylance-release/blob/main/CHANGELOG.md#202154-26-may-2021