old solver incompletely prefers projection and object candidates #27
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Description
the following example compiles in the old solver and errors in the new solver. This happens because the old solver makes an arbitrary choice here https://github.com/rust-lang/rust/blob/617d3d6d722c432cdcbf210e6db55c3bdeafe381/compiler/rustc_trait_selection/src/traits/select/mod.rs#L1967-L1972
trait Trait<'t> {}
impl<'t> Trait<'t> for () {}
fn foo<'x, 'y>() -> impl Trait<'x> + Trait<'y> {}
fn impls_trait<'x, T: Trait<'x>>(_: T) {}
fn bar<'x, 'y>() {
impls_trait::<'y, _>(foo::<'_, '_>());
}
fn main() {}changing this example a bit we get a MIR typeck ICE with the new solver
trait Trait<'t> {}
impl<'t> Trait<'t> for () {}
fn foo<'x, 'y>() -> impl Trait<'x> + Trait<'y> {}
fn impls_trait<'x, T: Trait<'x>>(_: T) {}
fn bar<'x, 'y>() {
impls_trait::<'y, _>(foo::<'x, 'y>());
}
fn main() {}error: internal compiler error: errors selecting obligation during MIR typeck: [FulfillmentError(Obligation(predicate=Binder(TraitPredicate(<Opaque(DefId(0:17 ~ main[db48]::foo::{opaque#0}), ['?16, '?17, '?18, '?19]) as Trait<ReFree(DefId(0:13 ~ main[db48]::bar), BrNamed(DefId(0:15 ~ main[db48]::bar::'y), 'y))>>, polarity:Positive), []), cause=ObligationCause { span: src/main.rs:6:23: 6:32 (#0), body_id: DefId(0:0 ~ main[db48]), code: AscribeUserTypeProvePredicate(src/main.rs:6:23: 6:32 (#0)) }, param_env=ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst }, depth=0),Ambiguity), FulfillmentError(Obligation(predicate=Binder(WellFormed(fn(?0t) {impls_trait::<ReFree(DefId(0:13 ~ main[db48]::bar), BrNamed(DefId(0:15 ~ main[db48]::bar::'y), 'y)), ?0t>}), []), cause=ObligationCause { span: no-location (#0), body_id: DefId(0:0 ~ main[db48]), code: MiscObligation }, param_env=ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst }, depth=0),Ambiguity)]
This feels like it may be caused by the theoretical issue discussed in https://rust-lang.zulipchat.com/#narrow/stream/364551-t-types.2Ftrait-system-refactor/topic/mir.20typeck.20and.20relying.20on.20region.20equality
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