Well-formedness of type parameter defaults is not checked

[leoyvens]

This compiles:

struct Foo<T:Copy=String>(T);

fn main() { }

This does not:

struct Foo<T:Copy=String>(T);

fn main() {
    let a: Foo;
}

Even though it's stable, we should probably phase out writing badly formed defaults in declarations.

We also don't check that a dependent default actually implements a given trait when using associated items:

struct Bar<A, T=<A as Iterator>::Item>(A, T);

Will compile. The default being applied is conditional on A implementing Iterator. May be useful for some hacks but seems bogus.

Edit:
We don't even check totally bogus stuff such as:

struct Foo<T, U: FromIterator<T>>(T, U);
struct Bar<Z = Foo<i32, i32>>(Z);