GAT ICE: impl item and trait item have different parameter counts

[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 = T>>(t: T) -> T::F<'a> {
    T::identity(t)
}


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

Backtrace:

error: internal compiler error: impl item and trait item have different parameter counts

thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', src/librustc_errors/lib.rs:357:17
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 as core::ops::drop::Drop>::drop
  14: core::ptr::drop_in_place
  15: <alloc::rc::Rc<T> as core::ops::drop::Drop>::drop
  16: core::ptr::drop_in_place
  17: rustc_interface::interface::run_compiler_in_existing_thread_pool
  18: scoped_tls::ScopedKey<T>::set
  19: syntax::with_globals
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

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 1.42.0-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: could not compile `playground`.

[casey]

I believe that I'm hitting this with very similar code. Is there a workaround, or different way of phrasing the same thing that avoids the ICE?

[TheRawMeatball]

I'm also running into this issue, but I can't see what the error in this code is. Can someone please explain why this code is wrong?

[DutchGhost]

Funny, it can trip up LLVM as well:

#![feature(generic_associated_types)]

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

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

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


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

Invalid bitcast
  %3 = bitcast i32 %0 to i32*, !dbg !184
Function return type does not match operand type of return inst!
  ret i32* %4, !dbg !185
 i32in function _ZN10playground3bug17h6ed92295a28c5703E
LLVM ERROR: Broken function found, compilation aborted!
error: could not compile `playground`