Existential types for traits without generic type parameters make rustc panic

I tried this code:

#![feature(existential_type)]

trait Meow {
    type MeowType;
    fn meow(self) -> Self::MeowType;
}

impl<T, I> Meow for I
    where I: Iterator<Item = T>
{
    existential type MeowType: Iterator<Item = T>;
    fn meow(self) -> Self::MeowType {
        self
    }
}

fn main() {
}

I expected to be able to compile this code and to use it like this:

let result: Vec<i32> = vec![1, 2, 3].into_iter().meow().collect();

Instead, rustc crashes (see backtrace in the end of the issue).

However, the code compiles fine when I add generic type parameter to Meow trait definition:

#![feature(existential_type)]

trait Meow<T> {
    type MeowType;
    fn meow(self) -> Self::MeowType;
}

impl<T, I> Meow<T> for I
    where I: Iterator<Item = T>
{
    existential type MeowType: Iterator<Item = T>;
    fn meow(self) -> Self::MeowType {
        self
    }
}


fn main() {
}

Meta

rustc --version --verbose:

rustc 1.35.0-nightly (82e2f3ec2 2019-03-20)
binary: rustc
commit-hash: 82e2f3ec25a316c7536c33a6b6704366b14cbf2a
commit-date: 2019-03-20
host: x86_64-apple-darwin
release: 1.35.0-nightly
LLVM version: 8.0

Backtrace:

thread 'rustc' panicked at 'called `Option::unwrap()` on a `None` value', src/libcore/option.rs:345:21
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
   1: std::sys_common::backtrace::_print
   2: std::panicking::default_hook::{{closure}}
   3: std::panicking::default_hook
   4: rustc::util::common::panic_hook
   5: std::panicking::rust_panic_with_hook
   6: std::panicking::continue_panic_fmt
   7: rust_begin_unwind
   8: core::panicking::panic_fmt
   9: core::panicking::panic
  10: rustc_typeck::check::writeback::WritebackCx::visit_opaque_types
  11: rustc_typeck::check::writeback::<impl rustc_typeck::check::FnCtxt>::resolve_type_vars_in_body
  12: rustc::ty::context::GlobalCtxt::enter_local
  13: rustc_typeck::check::typeck_tables_of
  14: rustc::ty::query::__query_compute::typeck_tables_of
  15: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::typeck_tables_of>::compute
  16: rustc::dep_graph::graph::DepGraph::with_task_impl
  17: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  18: rustc_typeck::collect::find_existential_constraints::ConstraintLocator::check
  19: rustc::hir::intravisit::Visitor::visit_nested_impl_item
  20: rustc::hir::intravisit::walk_item
  21: rustc_typeck::collect::find_existential_constraints
  22: rustc_typeck::collect::type_of
  23: rustc::ty::query::__query_compute::type_of
  24: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::type_of>::compute
  25: rustc::dep_graph::graph::DepGraph::with_task_impl
  26: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  27: <rustc_typeck::collect::CollectItemTypesVisitor as rustc::hir::intravisit::Visitor>::visit_impl_item
  28: rustc::hir::map::Map::visit_item_likes_in_module
  29: rustc_typeck::collect::collect_mod_item_types
  30: rustc::ty::query::__query_compute::collect_mod_item_types
  31: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::collect_mod_item_types>::compute
  32: rustc::dep_graph::graph::DepGraph::with_task_impl
  33: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  34: rustc_typeck::check_crate::{{closure}}::{{closure}}
  35: rustc::util::common::time
  36: rustc_typeck::check_crate
  37: rustc_interface::passes::analysis
  38: rustc::ty::query::__query_compute::analysis
  39: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::analysis>::compute
  40: rustc::dep_graph::graph::DepGraph::with_task_impl
  41: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  42: rustc::ty::context::tls::enter_global
  43: rustc_interface::passes::BoxedGlobalCtxt::access::{{closure}}
  44: rustc_interface::passes::create_global_ctxt::{{closure}}
  45: rustc_interface::passes::BoxedGlobalCtxt::enter
  46: rustc_interface::interface::run_compiler_in_existing_thread_pool
  47: std::thread::local::LocalKey<T>::with
  48: scoped_tls::ScopedKey<T>::set
  49: syntax::with_globals
query stack during panic:
#0 [typeck_tables_of] processing `<I as Meow>::meow`
#1 [type_of] processing `<I as Meow>::MeowType`
#2 [collect_mod_item_types] collecting item types in top-level module
#3 [analysis] running analysis passes on this crate
end of query stack

error: internal compiler error: unexpected panic

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 (82e2f3ec2 2019-03-20) running on x86_64-apple-darwin

[Arnavion]

@a-rodin Note that it's better to not lift the assoc type I::Item into a type parameter in the first place:

impl<I> Meow for I
    where I: Iterator
{
    existential type MeowType: Iterator<Item = I::Item>;
    fn meow(self) -> Self::MeowType {
        self
    }
}

@Arnavion
But what if I want to have separate implementations for T = f32 and T = i32?

[Aaron1011]

This now compiles on the latest nightly.

@Aaron1011 It looks like it doesn't crash, but returns an error now: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=bc01d4e3f91ad7a350955891d2ad84d4.