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

Consider the following code:

```rust
#![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:

```rust
fn foo(_: Bar) {}
```