ICE: eval_const_to_op: Unexpected ConstKind #74634
Description
#![feature(const_generics)] #![allow(incomplete_features)] trait If<const COND: bool> {} impl If<true> for () {} trait IsZero<const N: u8> { type Answer; } struct True; struct False; impl<const N: u8> IsZero<N> for () where (): If<{N == 0}> { type Answer = True; } trait Foobar<const N: u8> {} impl<const N: u8> Foobar<N> for () where (): IsZero<N, Answer = True> {} impl<const N: u8> Foobar<N> for () where (): IsZero<N, Answer = False> {}Errors:
Compiling playground v0.0.1 (/playground) error: internal compiler error: src/librustc_mir/interpret/operand.rs:569:17: eval_const_to_op: Unexpected ConstKind Const { ty: u8, val: Bound(DebruijnIndex(0), 0) } --> src/lib.rs:15:15 | 15 | where (): If<{N == 0}> { | ^^^^^^ thread 'rustc' panicked at 'Box<Any>', /rustc/f9a3086363f214f2b56bef30f0ac572e1a9127f1/src/libstd/macros.rs:13:23 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace note: the compiler unexpectedly panicked. this is a bug. note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md note: rustc 1.47.0-nightly (f9a308636 2020-07-20) running on x86_64-unknown-linux-gnu note: compiler flags: -C embed-bitcode=no -C codegen-units=1 -C debuginfo=2 --crate-type lib note: some of the compiler flags provided by cargo are hidden error: aborting due to previous error error: could not compile `playground`. To learn more, run the command again with --verbose. Probably related: #72819
Removing (at least) one of the impls for Foobar makes the panic go away, and the compiler correctly reports the error:
Compiling playground v0.0.1 (/playground) error: constant expression depends on a generic parameter --> src/lib.rs:15:11 | 15 | where (): If<{N == 0}> { | ^^^^^^^^^^^^ | = note: this may fail depending on what value the parameter takes error: aborting due to previous error error: could not compile `playground`. To learn more, run the command again with --verbose. @rustbot modify labels: I-ICE, T-compiler, A-const-eval, A-const-generics, C-bug, F-const_generics, requires-nightly