ICE with GATs

[Boscop]

What is the right way to do this?

pub trait ChanFx2: Clone + Default {
	type Ctx<'a> = ChannelProcessParams<'a>; // GAT with default type
	fn process<'a>(&mut self, msgs: Vec<MidiMsg>, params: &ChannelProcessParams<'a>, ctx: &Self::Ctx<'a>) -> Vec<MidiMsg>;
}

error: lifetime parameters are not allowed on this type

It's underlinining &Self::Ctx<'a> but how else am I supposed to instantiate the GAT with a lifetime?
Now I tried to omit the <'a>:

pub trait ChanFx2: Clone + Default {
	type Ctx<'a> = ChannelProcessParams<'a>;
	fn process<'a>(&mut self, msgs: Vec<MidiMsg>, params: &ChannelProcessParams<'a>, ctx: &Self::Ctx/*<'a>*/) -> Vec<MidiMsg>;
}

struct MidiMerger {}

struct SidechainCtx<'a> {
	inherited: ChannelProcessParams<'a>,
	sc_msgs: Vec<MidiMsg>,
}

impl ChanFx2 for MidiMerger {
	type Ctx<'a> = SidechainCtx<'a>;
	fn process<'a>(&mut self, mut msgs: Vec<MidiMsg>, params: &ChannelProcessParams<'a>, ctx: &Self::Ctx/*<'a>*/) -> Vec<MidiMsg> {
		msgs.extend(ctx.sc_msgs);
		msgs
	}
}

But I get this (ICE):

error: internal compiler error: src\main.rs:1: librustc\ty\subst.rs:427: Region parameter out of range when substituting in region 'a (root type=Some(fx::multichanfx::SidechainCtx<'a>)) (index=0)
thread 'rustc' panicked at 'Box<Any>', librustc_errors\lib.rs:482:9
stack backtrace:
   0: _rdl_shrink_in_place
   1: std::panicking::take_hook
   2: std::panicking::take_hook
   3: <unknown>
   4: std::panicking::rust_panic_with_hook
   5: <unknown>
   6: <unknown>
   7: rustc::session::bug_fmt
   8: rustc::ty::context::tls::span_debug
   9: rustc::ty::context::tls::span_debug
  10: <unknown>
  11: <unknown>
  12: rustc::ty::context::tls::span_debug
  13: rustc::ty::context::tls::span_debug
  14: rustc::session::bug_fmt
  15: rustc::session::bug_fmt
  16: <rustc::ty::subst::SubstFolder<'a, 'gcx, 'tcx> as rustc::ty::fold::TypeFolder<'gcx, 'tcx>>::fold_region
  17: rustc::ty::subst::<impl rustc::ty::Slice<rustc::ty::subst::Kind<'tcx>>>::truncate_to
  18: <rustc::ty::subst::SubstFolder<'a, 'gcx, 'tcx> as rustc::ty::fold::TypeFolder<'gcx, 'tcx>>::fold_ty
  19: rustc::traits::project::normalize_projection_type
  20: rustc::traits::project::normalize_projection_type
  21: rustc::traits::project::normalize_projection_type
  22: rustc::traits::project::normalize_projection_type
  23: <rustc::traits::project::AssociatedTypeNormalizer<'a, 'b, 'gcx, 'tcx> as rustc::ty::fold::TypeFolder<'gcx, 'tcx>>::fold_ty
  24: rustc::ty::subst::<impl rustc::ty::Slice<rustc::ty::subst::Kind<'tcx>>>::truncate_to
  25: rustc::ty::structural_impls::<impl rustc::ty::context::Lift<'tcx> for rustc::ty::layout::LayoutError<'a>>::lift_to_tcx
  26: rustc::traits::project::poly_project_and_unify_type
  27: rustc::ty::wf::predicate_obligations
  28: <unknown>
  29: <unknown>
  30: rustc::ty::wf::trait_obligations
  31: <rustc_typeck::check::upvar::InferBorrowKind<'a, 'gcx, 'tcx> as rustc::middle::expr_use_visitor::Delegate<'tcx>>::mutate
  32: <rustc_typeck::check::wfcheck::CheckTypeWellFormedVisitor<'a, 'tcx> as rustc::hir::intravisit::Visitor<'v>>::visit_item
  33: rustc_typeck::check_crate
  34: rustc_typeck::check_crate
  35: <rustc_driver::derive_registrar::Finder as rustc::hir::itemlikevisit::ItemLikeVisitor<'v>>::visit_impl_item
  36: rustc_driver::driver::compile_input
  37: rustc_driver::run_compiler
  38: <unknown>
  39: _rust_maybe_catch_panic
  40: <rustc_driver::derive_registrar::Finder as rustc::hir::itemlikevisit::ItemLikeVisitor<'v>>::visit_impl_item
  41: std::sys::windows::thread::Thread::new
  42: BaseThreadInitThunk
  43: RtlUserThreadStart

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 1.26.0-nightly (2789b067d 2018-03-06) running on x86_64-pc-windows-msvc

note: compiler flags: -C opt-level=1 -C debuginfo=2 -C debug-assertions=on -C incremental -C target-cpu=native --crate-type bin

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

What is the correct way to instantiate the GAT with a lifetime here?

[Boscop]

Similar situation with this, what's the right way to instantiate the GAT (with types this time)?

pub trait ToOther {
	type Other<P: AsPitch, V: AsVal, B: AsPitchBend>;

	fn to_other<Pitch: AsPitch, Val: AsVal, Pb: AsPitchBend>(&self) -> Self::Other/*<Pitch, Val, Pb>*/;
}

impl<Pitch: AsPitch, Val: AsVal, Pb: AsPitchBend> ToOther for HighLvlMsg<Pitch, Val, Pb> {
	type Other<P, V, B> = HighLvlMsg<P, V, B>;

	fn to_other<Pitch2: AsPitch, Val2: AsVal, Pb2: AsPitchBend>(&self) -> Self::Other/*<Pitch2, Val2, Pb2>*/ {
		use HighLvlMsg::*;

		match *self {
			NoteOn { pitch, vel, len } => NoteOn { pitch: AsPitch::conv(pitch), vel, len },
			NoteOff { pitch, vel } => NoteOff { pitch: AsPitch::conv(pitch), vel },
			ControlChange { cc, val } => ControlChange { cc, val: AsVal::conv(val) },
			ProgramChange { prog } => ProgramChange { prog },
			PitchBend { semitones } => PitchBend { semitones: AsPitchBend::conv(semitones) },
			PolyphonicAftertouch { pitch, val } => PolyphonicAftertouch { pitch: AsPitch::conv(pitch), val: AsVal::conv(val) },
			ChannelAftertouch { val } => ChannelAftertouch { val: AsVal::conv(val) },
		}
	}
}

error: internal compiler error: src\lib.rs:1: librustc\ty\subst.rs:481: Type parameter `P/#3` (P/3) out of range when substituting (root type=Some(highlvl::HighLvlMsg<P, V, B>)) substs=[_, _, _]
thread 'rustc' panicked at 'Box<Any>', librustc_errors\lib.rs:482:9
stack backtrace:
   0: _rdl_shrink_in_place
   1: std::panicking::take_hook
   2: std::panicking::take_hook
   3: <unknown>
   4: std::panicking::rust_panic_with_hook
   5: <unknown>
   6: <unknown>
   7: rustc::session::bug_fmt
   8: rustc::ty::context::tls::span_debug
   9: rustc::ty::context::tls::span_debug
  10: <unknown>
  11: <unknown>
  12: rustc::ty::context::tls::span_debug
  13: rustc::ty::context::tls::span_debug
  14: rustc::session::bug_fmt
  15: rustc::session::bug_fmt
  16: <rustc::ty::subst::SubstFolder<'a, 'gcx, 'tcx> as rustc::ty::fold::TypeFolder<'gcx, 'tcx>>::fold_ty
  17: rustc::ty::subst::<impl rustc::ty::Slice<rustc::ty::subst::Kind<'tcx>>>::truncate_to
  18: <rustc::ty::subst::SubstFolder<'a, 'gcx, 'tcx> as rustc::ty::fold::TypeFolder<'gcx, 'tcx>>::fold_ty
  19: rustc::traits::project::normalize_projection_type
  20: rustc::traits::project::normalize_projection_type
  21: rustc::traits::project::normalize_projection_type
  22: rustc::traits::project::normalize_projection_type
  23: <rustc::traits::project::AssociatedTypeNormalizer<'a, 'b, 'gcx, 'tcx> as rustc::ty::fold::TypeFolder<'gcx, 'tcx>>::fold_ty
  24: rustc::ty::subst::<impl rustc::ty::Slice<rustc::ty::subst::Kind<'tcx>>>::truncate_to
  25: rustc::ty::structural_impls::<impl rustc::ty::context::Lift<'tcx> for rustc::ty::layout::LayoutError<'a>>::lift_to_tcx
  26: rustc::traits::project::poly_project_and_unify_type
  27: rustc::ty::wf::predicate_obligations
  28: <unknown>
  29: <unknown>
  30: rustc::ty::wf::trait_obligations
  31: <rustc_typeck::check::upvar::InferBorrowKind<'a, 'gcx, 'tcx> as rustc::middle::expr_use_visitor::Delegate<'tcx>>::mutate
  32: <rustc_typeck::check::wfcheck::CheckTypeWellFormedVisitor<'a, 'tcx> as rustc::hir::intravisit::Visitor<'v>>::visit_item
  33: rustc_typeck::check_crate
  34: rustc_typeck::check_crate
  35: <rustc_driver::derive_registrar::Finder as rustc::hir::itemlikevisit::ItemLikeVisitor<'v>>::visit_impl_item
  36: rustc_driver::driver::compile_input
  37: rustc_driver::run_compiler
  38: <unknown>
  39: _rust_maybe_catch_panic
  40: <rustc_driver::derive_registrar::Finder as rustc::hir::itemlikevisit::ItemLikeVisitor<'v>>::visit_impl_item
  41: std::sys::windows::thread::Thread::new
  42: BaseThreadInitThunk
  43: RtlUserThreadStart

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 1.26.0-nightly (2789b067d 2018-03-06) running on x86_64-pc-windows-msvc

note: compiler flags: -C debuginfo=2 -C incremental --crate-type lib

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

[pietroalbini]

Thanks for reporting this! Can you provide a small example to reproduce the ICE? With the custom types you used it's difficult for us to bisect/reproduce ourself :)

[Boscop]

#![feature(generic_associated_types)]
#![feature(associated_type_defaults)]

struct Foo;
struct Bar;
struct Baz;

struct A<'a> {
	foo: &'a Foo,
}

struct B<'a> {
	inherited: A<'a>,
	own: Bar,
}

trait Trait: Clone + Default {
	type Ctx<'a> = A<'a>;
	fn process<'a>(&mut self, ctx: &Self::Ctx/*<'a>*/);
}

impl Trait for Baz {
	type Ctx<'a> = B<'a>;
	fn process<'a>(&mut self, ctx: &Self::Ctx/*<'a>*/) {}
}

fn main() {}

https://play.rust-lang.org/?gist=5fc07e14a633f3ec4db4de1c421053ff&version=nightly

[sinkuu]

This seems to be a known issue. GAT (#44265) is a WIP feature and probably not supposed to be usable yet.

rust/src/test/ui/rfc1598-generic-associated-types/streaming_iterator.rs

Lines 13 to 14 in a10bb6b

	//FIXME(#44265): "lifetime parameter not allowed on this type" errors will be addressed in a
	// follow-up PR

[dtolnay]

Closing as a duplicate of #49362.