Weird interaction with projections in trait bounds

[sgrif]

Reproduction case here: https://is.gd/KJTI1j.

This code should compile fine, as it's simply stating a constraint and then something attempting to use it. Regardless of whether the projection is unified with the real type or not, this code should be able to compile. Replacing the projection on line 22 with a concrete type will cause the issue to go away.

[scalexm]

It seems like in the requirement T: AsExpression<<Self as Expression>::SqlType>, rustc is confused with the impl impl<T: Expression> AsExpression<T::SqlType> for T . It becomes more apparent when you replace the impl with:

impl<T> DoStuff<T> for Foo where
    T: AsExpression<<Self as Expression>::SqlType>,
    T::Expression: Expression,
    T: Expression
{}

(just added T: Expression), the compiler yields the error
type mismatch resolving `<T as Expression>::SqlType == Foo`.

However, the impl:

impl<T> DoStuff<T> for Foo where
    T: AsExpression<<T as Expression>::SqlType>,
    T::Expression: Expression,
    T: Expression
{}

compiles fine.

Coming back to your code, the main problem is that rustc tries to evaluate T as Expression (whereas it should not) which explains the error and the proposed fix ("consider adding a where T: Expression bound").

I'm investigating.

[scalexm]

So I removed the impl impl<T: Expression> AsExpression<T::SqlType> for T in order to have a bit less confusion. The error is also more informative:

the trait `AsExpression<Foo>` is not implemented for `T`

So basically, the error comes from opt_normalize_projection_type not being able to project <T as AsExpression<Foo>>::Expression, and thus adding an obligation T as AsExpression<Foo> which it is unable to prove.

This seems related to the caching mechanism...

[scalexm]

Ok the real culprit probably is select::match_poly_trait_ref, or rather select::match_where_clause_trait_ref which is calling the former, the call:

match_poly_trait_ref: obligation=Obligation(predicate=Binder(TraitPredicate(<T as AsExpression<Foo>>)),depth=0) poly_trait_ref=Binder(<T as AsExpression<<Foo as Expression>::SqlType>>)

fails, probably because poly_trait_ref is not normalized.

[Spoonbender]

Triage: no change