[JoshTriplett]

That kind of automatic sum-typing seems like an interesting idea. You might consider bringing it to the Rust internals forum, posting it as a pre-RFC, and discussing it as compared to some of the alternatives. That might lead to either a change in the direction you hope for, or the unearthing of other ergonomic approaches.

[eddyb]

This idea has been waved around a bit, but in the form of `impl Error` where `_` is. That is, inside the function the `E1 | E2 | ...` type is being built, and if you have automatic dispatch for `Error`'s methods then it will work with `impl Error`.

Actual global inference has never been on the table and still isn't.

[masklinn]

I believe there are conversations of being able to promote enum variants to full-blown types, though I don't know if there's any project for lightweight enums of types.