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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
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Area: Constant evaluation, covers all const contexts (static, const fn, ...)Area: const generics (parameters and arguments)Category: This is a bug.`#![feature(const_generics)]`Issue: The compiler panicked, giving an Internal Compilation Error (ICE) ❄️Relevant to the compiler team, which will review and decide on the PR/issue.ICE tracked in rust-lang/glacier.This issue requires a nightly compiler in some way.
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