Incorrect compilation/ICE using default associated type with specialization

[N-Maas]

With specialization, it is possible to define a default associated type that does not fulfill its trait bounds. Here is a minimal example, that surely should not compile, but does with rustc 1.35.0-nightly:

#![feature(specialization)]

struct MyStruct {}

trait MyTrait {
    type MyType: Default;
}

impl MyTrait for i32 {
    default type MyType = MyStruct;
}

Unsurprisingly, adding the main function

fn main() {
   let _x: <i32 as MyTrait>::MyType = <i32 as MyTrait>::MyType::default();
}

causes an ICE. Error message and backtrace (compiled at the playground):

error: internal compiler error: src/librustc/traits/codegen/mod.rs:58: Encountered error `Unimplemented` selecting `Binder(<MyStruct as std::default::Default>)` during codegen

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:634:9
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:39
   1: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:71
   2: std::panicking::default_hook::{{closure}}
             at src/libstd/sys_common/backtrace.rs:59
             at src/libstd/panicking.rs:197
   3: std::panicking::default_hook
             at src/libstd/panicking.rs:211
   4: rustc::util::common::panic_hook
   5: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:478
   6: std::panicking::begin_panic
   7: rustc_errors::Handler::bug
   8: rustc::util::bug::opt_span_bug_fmt::{{closure}}
   9: rustc::ty::context::tls::with_opt::{{closure}}
  10: rustc::ty::context::tls::with_context_opt
  11: rustc::ty::context::tls::with_opt
  12: rustc::util::bug::opt_span_bug_fmt
  13: rustc::util::bug::bug_fmt
  14: rustc::ty::context::GlobalCtxt::enter_local
  15: rustc::traits::codegen::codegen_fulfill_obligation
  16: rustc::ty::query::__query_compute::codegen_fulfill_obligation
  17: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::codegen_fulfill_obligation>::compute
  18: rustc::dep_graph::graph::DepGraph::with_task_impl
  19: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  20: rustc::ty::instance::Instance::resolve
  21: <rustc_mir::monomorphize::collector::MirNeighborCollector as rustc::mir::visit::Visitor>::visit_terminator_kind
  22: rustc_mir::monomorphize::collector::collect_items_rec
  23: rustc_mir::monomorphize::collector::collect_crate_mono_items::{{closure}}
  24: rustc::util::common::time
  25: rustc_mir::monomorphize::collector::collect_crate_mono_items
  26: rustc::util::common::time
  27: rustc_mir::monomorphize::partitioning::collect_and_partition_mono_items
  28: rustc::ty::query::__query_compute::collect_and_partition_mono_items
  29: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::collect_and_partition_mono_items>::compute
  30: rustc::dep_graph::graph::DepGraph::with_task_impl
  31: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  32: rustc_codegen_ssa::base::codegen_crate
  33: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_utils::codegen_backend::CodegenBackend>::codegen_crate
  34: rustc::util::common::time
  35: rustc_interface::passes::start_codegen
  36: rustc::ty::context::tls::enter_global
  37: rustc_interface::passes::BoxedGlobalCtxt::access::{{closure}}
  38: rustc_interface::passes::create_global_ctxt::{{closure}}
  39: rustc_interface::passes::BoxedGlobalCtxt::enter
  40: rustc_interface::queries::Query<T>::compute
  41: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::ongoing_codegen
  42: rustc_interface::interface::run_compiler_in_existing_thread_pool
  43: std::thread::local::LocalKey<T>::with
  44: scoped_tls::ScopedKey<T>::set
  45: syntax::with_globals
query stack during panic:
#0 [codegen_fulfill_obligation] checking if `std::default::Default` fulfills its obligations
#1 [collect_and_partition_mono_items] collect_and_partition_mono_items
end of query stack
error: aborting due to previous error


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 1.35.0-nightly (4c27fb19b 2019-03-25) 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

[JohnTitor]

Triage: the latest nightly rejects the code, marking as E-needs-test.