`T: ?Sized` does not work in `where` clauses

[ftxqxd]

fn foo<T>(x: &T) where T: ?Sized {} // error: unexpected `?`

The RFC states that the syntax should work in where clauses.

[nrc]

Note that we don't currently accept where Sized? T, so there is no backwards compatibility hazard here. Although we should certainly fix this, it doesn't need to be done for 1.0 (it probably should be though).

[jroesch]

@nick29581 I will poke into this.

[ftxqxd]

There are a few edge cases here. In particular, specifying ?Sized in a where clause should probably only be allowed at the type parameter’s definition site; e.g., the following probably shouldn’t be allowed:

impl<T> Foo<T> {
    fn foo(&self) where T: ?Sized { ... }
}

Although allowing that could potentially be useful, I suspect it would complicate the implementation and be slightly unclear anyway.

Additionally, specifying ?Sized on an associated type is unnecessary. In general, specifying ?Sized on anything that isn’t a type parameter is somewhat nonsensical: where Option<T>: ?Sized doesn’t really make much sense either.

[aochagavia]

What needs to be done in order to get this working? Is it just a parsing issue?

[Gankra]

Triage: still a really annoying problem.

[jroesch]

@gankro I can fix this, probably my fault anyways.

[kamalmarhubi]

This is still a bit of a wart, and tripped me up the first time I wanted to use a ?Sized bound.

[ihrwein]

I hit this issue today.

[kamalmarhubi]

@jroesch although it was a few months ago, do you remember enough context to write a very quick summary of where changes would need to be made to fix this?

[tinyplasticgreyknight]

Ran into this one today. Is it just a parsing problem?