ICE in trait normalization when specialization enabled

[insaneinside]

The following code (playground)

#![crate_type = "lib"]
#![feature(specialization)] // comment this line to get the expected behavior
trait X {}
trait Y: X {}
trait Z { type Assoc: Y; }
struct A<T>(T);

impl<T> Y for T where T: X {}
impl<T: X> Z for A<T> { type Assoc = T; }

// this impl is invalid, but causes an ICE anyway
impl<T> From<<A<T> as Z>::Assoc> for T {}

produces

error: internal compiler error: /checkout/src/librustc/traits/specialize/mod.rs:201: failed to fully normalize <T as std::convert::From<<A<T> as Z>::Assoc>>: [FulfillmentError(Obligation(predicate=Binder(TraitPredicate(<T as X>)),depth=1),Unimplemented)]

[steveklabnik]

Triage: still ICEs

[matthiaskrgr]

This ICEs since 1.33
@rustbot modify labels: +regression-from-stable-to-stable