Incoherent impls are allowed on opaque types

[matthewjasper]

I tried this code:

#![feature(type_alias_impl_trait)]

type X<T> = impl Sized;

fn f<T>() -> X<T> {}

trait Y {
    fn g(&self) {}
}

impl Y for X<()> {}
impl Y for X<i32> {}

fn main() {
    f::<()>().g();
}

I expected to see this happen: error for potentially overlapping impls

Instead, this happened: ICEs in codegen

Meta

rustc --version --verbose:

rustc 1.46.0-nightly (346aec9b0 2020-07-11)
binary: rustc
commit-hash: 346aec9b02f3c74f3fce97fd6bda24709d220e49
commit-date: 2020-07-11
host: x86_64-pc-windows-msvc
release: 1.46.0-nightly
LLVM version: 10.0

Backtrace

error: internal compiler error: src\librustc_trait_selection\traits\codegen\mod.rs:62:17: Encountered error `Unimplemented` selecting `Binder(<() as Y>)` during codegen

thread 'rustc' panicked at 'Box<Any>', src\librustc_errors\lib.rs:916:9
stack backtrace:
   0: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
   1: core::fmt::write
   2: <std::io::IoSliceMut as core::fmt::Debug>::fmt
   3: std::panicking::take_hook
   4: std::panicking::take_hook
   5: rustc_driver::report_ice
   6: std::panicking::rust_panic_with_hook
   7: <rustc_errors::styled_buffer::StyledBuffer as core::fmt::Debug>::fmt
   8: rustc_errors::HandlerInner::err_count
   9: rustc_errors::Handler::bug
  10: rustc_middle::util::bug::bug_fmt
  11: <rustc_middle::ty::consts::ConstInt as core::fmt::Debug>::fmt
  12: <rustc_middle::ty::consts::ConstInt as core::fmt::Debug>::fmt
  13: rustc_middle::util::bug::bug_fmt
  14: rustc_middle::util::bug::bug_fmt
  15: unicode_normalization::normalize::is_hangul_syllable
  16: rustc_trait_selection::traits::codegen::codegen_fulfill_obligation
  17: rustc_ty::provide
  18: crc32fast::Hasher::finalize
  19: crc32fast::Hasher::finalize
  20: crc32fast::Hasher::finalize
  21: rustc_ty::provide
  22: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::features_query>::hash_result
  23: <rustc_middle::ty::binding::BindingMode as rustc_middle::ty::context::Lift>::lift_to_tcx
  24: rustc_middle::ty::structural_impls::<impl rustc_middle::ty::context::Lift for rustc_ast::ast::InlineAsmTemplatePiece>::lift_to_tcx
  25: <rustc_middle::ty::subst::UserSubsts as core::fmt::Debug>::fmt
  26: rustc_middle::ty::instance::Instance::resolve
  27: <rustc_mir::monomorphize::collector::MirNeighborCollector as rustc_middle::mir::visit::Visitor>::visit_terminator
  28: <rustc_mir::monomorphize::collector::RootCollector as rustc_hir::itemlikevisit::ItemLikeVisitor>::visit_impl_item
  29: rustc_mir::monomorphize::collector::collect_crate_mono_items
  30: <rustc_mir::dataflow::impls::borrows::BorrowIndex as core::fmt::Debug>::fmt
  31: rustc_mir::monomorphize::collector::collect_crate_mono_items
  32: rustc_mir::monomorphize::partitioning::compute_codegen_unit_name
  33: <rustc_target::abi::TyAndLayout<&rustc_middle::ty::TyS> as rustc_codegen_llvm::type_of::LayoutLlvmExt>::pointee_info_at
  34: <rustc_codegen_llvm::llvm_::ffi::debuginfo::DISPFlags as core::fmt::Debug>::fmt
  35: rustc_codegen_llvm::type_::<impl rustc_codegen_ssa::traits::type_::LayoutTypeMethods for rustc_codegen_llvm::context::CodegenCx>::reg_backend_type
  36: rustc_codegen_llvm::type_::<impl rustc_codegen_ssa::traits::type_::LayoutTypeMethods for rustc_codegen_llvm::context::CodegenCx>::reg_backend_type
  37: <rustc_target::abi::TyAndLayout<&rustc_middle::ty::TyS> as rustc_codegen_llvm::type_of::LayoutLlvmExt>::pointee_info_at
  38: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_ssa::traits::backend::CodegenBackend>::codegen_crate
  39: rustc_interface::passes::QueryContext::print_stats
  40: rustc_interface::passes::BoxedResolver::complete
  41: rustc_interface::queries::Queries::ongoing_codegen
  42: <rustc_codegen_ssa::back::linker::MsvcLinker as rustc_codegen_ssa::back::linker::Linker>::partial_relro
  43: <env_logger::filter::inner::Filter as core::fmt::Display>::fmt
  44: <rustc_driver::DEFAULT_HOOK as core::ops::deref::Deref>::deref
  45: <rustc_driver::DEFAULT_HOOK as core::ops::deref::Deref>::deref
  46: <(&rustc_middle::ty::sty::Const,rustc_middle::mir::Field) as rustc_middle::ty::query::keys::Key>::default_span
  47: std::sys::windows::thread::Thread::new
  48: BaseThreadInitThunk
  49: RtlUserThreadStart
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 1.46.0-nightly (346aec9b0 2020-07-11) running on x86_64-pc-windows-msvc

query stack during panic:
#0 [codegen_fulfill_obligation] checking if `Y` fulfills its obligations
#1 [resolve_instance] resolving instance `<() as Y>::g`
#2 [collect_and_partition_mono_items] collect_and_partition_mono_items
end of query stack
error: aborting due to previous error

[Aaron1011]

These impls shouldn't be allowed at all, so #76940 should fix this