Associated type as struct field causes type-checking error

[murarth]

The following causes an error (indicated in the comment):

#![feature(associated_types)]

pub trait Foo {
    type Bar;
}

impl Foo for int {
    type Bar = int;
}

struct Thing<F: Foo> {
    a: F,
    b: F::Bar,
}

fn main() {
    let _ = Thing{a: 1i, b: 2i};
    //                      ^~
    // error: mismatched types: expected `<int as Foo>::Bar`,
    // found `int` (expected associated type, found int)
}

And this closely related case causes an ICE:

#![feature(associated_types)]

pub trait Foo {
    type Bar;
}

impl Foo for int {
    type Bar = int;
}

struct Thing<F: Foo> {
    a: F,
    b: Option<F::Bar>,
}

fn main() {
    let _ = Thing{a: 1i, b: None};
}

Backtrace:

error: internal compiler error: trying to take the sizing type of <int as Foo>::Bar, an unsized type
note: the compiler unexpectedly panicked. this is a bug.
note: we would appreciate a bug report: http://doc.rust-lang.org/complement-bugreport.html
note: run with `RUST_BACKTRACE=1` for a backtrace
thread 'rustc' panicked at 'Box<Any>', /home/rustbuild/src/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libsyntax/diagnostic.rs:182

stack backtrace:
   1:     0x7f22c6fb32d0 - sys::backtrace::write::h8532e701ef86014f4it
   2:     0x7f22c6fd8b00 - failure::on_fail::h7532e1f79d134d5dzvz
   3:     0x7f22c6f3e1c0 - rt::unwind::begin_unwind_inner::h97b151606151d62deaz
   4:     0x7f22c1defc60 - rt::unwind::begin_unwind::h15809447133099964284
   5:     0x7f22c1df0510 - diagnostic::Handler::bug::h8818b567cf47e6a0DLF
   6:     0x7f22c512acf0 - session::Session::bug::h251da16737c526c0lrq
   7:     0x7f22c5dfd020 - trans::type_of::sizing_type_of::he06887b02c649aebXRo
   8:     0x7f22c5f1cb10 - trans::adt::represent_type_uncached::h6b884b95692379c1oxH
   9:     0x7f22c5df7b30 - trans::adt::represent_type::ha0c2ea2716678100YtH
  10:     0x7f22c5de2fb0 - trans::type_of::type_of::h7caa699265884e0bFYo
  11:     0x7f22c5de2fb0 - trans::type_of::type_of::h7caa699265884e0bFYo
  12:     0x7f22c5f22b20 - vec::Vec<T>.FromIterator<T>::from_iter::h11585473372288691594
  13:     0x7f22c5de2fb0 - trans::type_of::type_of::h7caa699265884e0bFYo
  14:     0x7f22c5de2fb0 - trans::type_of::type_of::h7caa699265884e0bFYo
  15:     0x7f22c5decf50 - trans::base::alloc_ty::h29690970a9e20515oft
  16:     0x7f22c5ee6800 - trans::_match::mk_binding_alloca::h2686137581982254550
  17:     0x7f22c5ddcb70 - trans::base::init_local::h9a5509ab5cb7df8eB1s
  18:     0x7f22c5dddd10 - trans::controlflow::trans_block::hfd89f3a5d0ddcbbdQ1d
  19:     0x7f22c5ea8f60 - trans::base::trans_closure::hbecde8c2bc5f7b2fsSt
  20:     0x7f22c5dc9120 - trans::base::trans_fn::hba312ea6d66cf757Z2t
  21:     0x7f22c5dc4300 - trans::base::trans_item::ha0e598b4af14d13fQpu
  22:     0x7f22c5eb1a60 - trans::base::trans_crate::hf12b6e99a3af9ab9blv
  23:     0x7f22c7513250 - driver::phase_4_translate_to_llvm::h40ce54ee4a878885vFa
  24:     0x7f22c74f2ae0 - driver::compile_input::h68b8602933aad8d7wba
  25:     0x7f22c75bdeb0 - thunk::F.Invoke<A, R>::invoke::h18029802347644288836
  26:     0x7f22c75bcc60 - rt::unwind::try::try_fn::h6518866316425934196
  27:     0x7f22c703f400 - rust_try_inner
  28:     0x7f22c703f3f0 - rust_try
  29:     0x7f22c75bcfb0 - thunk::F.Invoke<A, R>::invoke::h15513809553472565307
  30:     0x7f22c6fc4e40 - sys::thread::thread_start::h5ea7ba97235331d5a9v
  31:     0x7f22c16130c0 - start_thread
  32:     0x7f22c6be4ec9 - __clone
  33:                0x0 - <unknown>

[nikomatsakis]

The second example (ICE) is fixed by #20706