Support dict(name=x, ...)

[JukkaL]

Support dict(name=x, ...) in the type checker. Infer key type as str and value type based on keyword argument types.

We can't represent the signature of dict precisely using current functionality alone, i.e. we need some new type checker functionality.

[JukkaL]

The common case should actually be pretty easy to implement by just special casing this in the semantic analyzer and using the analyzed attribute of the dict(...) CallExpr to store a DictExpr when the call only uses keyword arguments. The dict literal keys should be string literals generated from the keyword argument names, and the dict values are just the argument expressions.

The **args case is trickier, but this seems rare so we don't need to support it yet.

[JukkaL]

Increasing priority as this is actually a pretty common issue.

[gvanrossum]

Yeah, I see this a lot in our latest demo project.

[JukkaL]

This now works but for heterogeneous dictionaries the inferred type will likely be Dict[str, object] unless there is an explicit type annotation, so this isn't super useful yet.