Trait bounds are not checked on type aliases until they are used

[glandium]

Consider the following code:

#![crate_type = "lib"]

pub trait Trait {}

pub struct Foo<T: Trait>(T);

pub struct Qux;

pub type Bar<T=Qux> = Foo<T>;

This builds without an error although Qux doesn't respect the trait bound.

Build failure only occurs when one actually uses the type alias, like the following:

fn foo(_: Bar) {}

[nikomatsakis]

Compiler team pre-triage: Marking as P-medium as this is a rather old and long-standing issue that we have to resolve.