GAT ICE: unsize_thin_ptr: called on bad types

[DutchGhost]

The following ICE's:

#![feature(generic_associated_types)]

trait Fun {
    type F<'a>;
    
    fn identity<'a>(t: Self::F<'a>) -> Self::F<'a> { t }
}

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

fn bug<'a, T: Fun<F = &'a dyn Send>>(t: T) -> T::F<'a> {
    T::identity(&())
}


fn main() {
    let x = 10;
    
    bug(x);
}

Backtrace:

error: internal compiler error: src/librustc_codegen_ssa/base.rs:220: unsize_thin_ptr: called on bad types

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:883:9
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.40/src/backtrace/libunwind.rs:88
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-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_codegen_ssa::base::unsize_thin_ptr
  21: rustc_codegen_ssa::mir::rvalue::<impl rustc_codegen_ssa::mir::FunctionCx<Bx>>::codegen_rvalue_operand
  22: rustc_codegen_ssa::mir::codegen_mir
  23: <rustc::mir::mono::MonoItem as rustc_codegen_ssa::mono_item::MonoItemExt>::define
  24: rustc_codegen_llvm::base::compile_codegen_unit::module_codegen
  25: rustc::dep_graph::graph::DepGraph::with_task
  26: rustc_codegen_llvm::base::compile_codegen_unit
  27: rustc_codegen_ssa::base::codegen_crate
  28: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_utils::codegen_backend::CodegenBackend>::codegen_crate
  29: rustc_session::utils::<impl rustc_session::session::Session>::time
  30: rustc_interface::passes::QueryContext::enter
  31: rustc_interface::queries::Queries::ongoing_codegen
  32: rustc_interface::interface::run_compiler_in_existing_thread_pool
  33: scoped_tls::ScopedKey<T>::set
  34: 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 opt-level=3 -C codegen-units=1 --crate-type bin

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

query stack during panic:
end of query stack
error: aborting due to previous error

error: could not compile `playground`.