GAT ICE: FulfillmentError

[DutchGhost]

The following code ICE's:

#![feature(generic_associated_types)]

trait Fun {
    type F<'a>: Fn() -> u32;
    
    fn callme<'a>(f: Self::F<'a>) -> u32 {
        f()
    }
}

impl <T> Fun for T {
    type F<'a> = Self;
}

fn main() {
    <&dyn Iterator<Item = u8>>::callme(&std::iter::once(1));
}

Backtrace:

error: internal compiler error: src/librustc/traits/codegen/mod.rs:104: Encountered errors `[FulfillmentError(Obligation(predicate=Binder(TraitPredicate(<dyn std::iter::Iterator<Item = u8> as std::ops::Fn<()>>)), depth=1),Unimplemented)]` resolving bounds after type-checking

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:883:9
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/git.luolix.top-1ecc6299db9ec823/backtrace-0.3.40/src/backtrace/libunwind.rs:88
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/git.luolix.top-1ecc6299db9ec823/backtrace-0.3.40/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print_fmt
             at src/libstd/sys_common/backtrace.rs:77
   3: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
             at src/libstd/sys_common/backtrace.rs:59
   4: core::fmt::write
             at src/libcore/fmt/mod.rs:1052
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1426
   6: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:62
   7: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:49
   8: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:204
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:224
  10: rustc_driver::report_ice
  11: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:476
  12: std::panicking::begin_panic
  13: rustc_errors::HandlerInner::bug
  14: rustc_errors::Handler::bug
  15: rustc::util::bug::opt_span_bug_fmt::{{closure}}
  16: rustc::ty::context::tls::with_opt::{{closure}}
  17: rustc::ty::context::tls::with_opt
  18: rustc::util::bug::opt_span_bug_fmt
  19: rustc::util::bug::bug_fmt
  20: rustc::traits::codegen::<impl rustc::infer::InferCtxt>::drain_fulfillment_cx_or_panic
  21: rustc::ty::context::GlobalCtxt::enter_local
  22: rustc::traits::codegen::codegen_fulfill_obligation
  23: rustc::ty::query::__query_compute::codegen_fulfill_obligation
  24: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::codegen_fulfill_obligation>::compute
  25: rustc::dep_graph::graph::DepGraph::with_task_impl
  26: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  27: rustc::ty::instance::Instance::resolve
  28: <rustc_mir::monomorphize::collector::MirNeighborCollector as rustc::mir::visit::Visitor>::visit_terminator_kind
  29: rustc_mir::monomorphize::collector::collect_items_rec
  30: rustc_mir::monomorphize::collector::collect_items_rec
  31: rustc_session::utils::<impl rustc_session::session::Session>::time
  32: rustc_mir::monomorphize::collector::collect_crate_mono_items
  33: rustc_mir::monomorphize::partitioning::collect_and_partition_mono_items
  34: rustc::ty::query::__query_compute::collect_and_partition_mono_items
  35: rustc::dep_graph::graph::DepGraph::with_task_impl
  36: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  37: rustc_codegen_ssa::base::codegen_crate
  38: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_utils::codegen_backend::CodegenBackend>::codegen_crate
  39: rustc_session::utils::<impl rustc_session::session::Session>::time
  40: rustc_interface::passes::QueryContext::enter
  41: rustc_interface::queries::Queries::ongoing_codegen
  42: rustc_interface::interface::run_compiler_in_existing_thread_pool
  43: scoped_tls::ScopedKey<T>::set
  44: syntax::with_globals
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.42.0-nightly (3761dcd34 2020-01-28) running on x86_64-unknown-linux-gnu

note: compiler flags: -C codegen-units=1 -C debuginfo=2 --crate-type bin

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
#0 [codegen_fulfill_obligation] checking if `std::ops::Fn` fulfills its obligations
#1 [collect_and_partition_mono_items] collect_and_partition_mono_items
end of query stack
error: aborting due to previous error

error: could not compile `playground`.

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