Complaint about trait not being implemented

[est31]

Rustc complains about the trait not being implemented for [(); _], but actually it's given a [(); 1] literal and for that one, the trait is implemented. Turbofishing foo::<[(); 0]>([]) doesn't help.

cc @varkor @rodrimati1992

see also #61383

Code ↓

#![feature(const_generics)]
trait Foo {}

impl<const N: usize> Foo for [(); N]
where
    Self:FooImpl<{N==0}>
{}

trait FooImpl<const IS_ZERO: bool>{}

impl FooImpl<{0u8==0u8}> for [();0] {}

impl<const N:usize> FooImpl<{0u8!=0u8}> for [();N] {}

fn foo<T: Foo>(v: T) {}

fn main() {
    foo([]);
    foo([()]);
}

Error ↓

error[E0277]: the trait bound `[(); _]: FooImpl<{N==0}>` is not satisfied
  --> src/lib.rs:18:5
   |
18 |     foo([]);
   |     ^^^ the trait `FooImpl<{N==0}>` is not implemented for `[(); _]`
   |
   = help: the following implementations were found:
             <[(); 0] as FooImpl<true>>
             <[(); _] as FooImpl<false>>
   = note: required because of the requirements on the impl of `Foo` for `[(); 0]`
note: required by `foo`
  --> src/lib.rs:15:1
   |
15 | fn foo<T: Foo>(v: T) {}
   | ^^^^^^^^^^^^^^^^^^^^

error[E0277]: the trait bound `[(); _]: FooImpl<{N==0}>` is not satisfied
  --> src/lib.rs:19:5
   |
19 |     foo([()]);
   |     ^^^ the trait `FooImpl<{N==0}>` is not implemented for `[(); _]`
   |
   = help: the following implementations were found:
             <[(); 0] as FooImpl<true>>
             <[(); _] as FooImpl<false>>
   = note: required because of the requirements on the impl of `Foo` for `[(); 1]`
note: required by `foo`
  --> src/lib.rs:15:1
   |
15 | fn foo<T: Foo>(v: T) {}
   | ^^^^^^^^^^^^^^^^^^^^

error: aborting due to 2 previous errors

[est31]

Checking this example again gives me an ICE similar to #62879.

[est31]

ICE has been downgraded to an error since: #62879 (comment)

[Dylan-DPC-zz]

The error is now:

error[E0391]: cycle detected when const-evaluating + checking `<impl at src/main.rs:4:1: 7:3>::{{constant}}#0`
 --> src/main.rs:6:18
  |
6 |     Self:FooImpl<{N==0}>
  |                  ^^^^^^
  |
note: ...which requires const-evaluating + checking `<impl at src/main.rs:4:1: 7:3>::{{constant}}#0`...
 --> src/main.rs:6:18
  |
6 |     Self:FooImpl<{N==0}>
  |                  ^^^^^^
note: ...which requires const-evaluating `<impl at src/main.rs:4:1: 7:3>::{{constant}}#0`...
 --> src/main.rs:6:18
  |
6 |     Self:FooImpl<{N==0}>
  |                  ^^^^^^
note: ...which requires type-checking `<impl at src/main.rs:4:1: 7:3>::{{constant}}#0`...
 --> src/main.rs:6:18
  |
6 |     Self:FooImpl<{N==0}>
  |                  ^^^^^^
note: ...which requires processing `<impl at src/main.rs:4:1: 7:3>::{{constant}}#0`...
 --> src/main.rs:6:18
  |
6 |     Self:FooImpl<{N==0}>
  |                  ^^^^^^
  = note: ...which again requires const-evaluating + checking `<impl at src/main.rs:4:1: 7:3>::{{constant}}#0`, completing the cycle
note: cycle used when processing `<impl at src/main.rs:4:1: 7:3>`
 --> src/main.rs:4:1
  |
4 | / impl<const N: usize> Foo for [(); N]
5 | | where
6 | |     Self:FooImpl<{N==0}>
7 | | {}
  | |__^

error: aborting due to previous error

[est31]

@Dylan-DPC yeah also see the comment above yours as well as eddyb's reply in that thread.

[Dylan-DPC-zz]

@est31 i'm aware was just updating so it is easier to check with later

[est31]

Due to this reddit post I tested it again and now the compile error is gone. Good job guys and girls!

[varkor]

Let's make sure we add a regression test.

[est31]

Now there's a different error:

error: constant expression depends on a generic parameter
 --> src/main.rs:6:10
  |
6 |     Self:FooImpl<{N==0}>
  |          ^^^^^^^^^^^^^^^
  |
  = note: this may fail depending on what value the parameter takes

[lcnr]

This error is expected fallout of #70107

We also already have a regression test for some reason 😅
https://github.com/rust-lang/rust/blob/master/src/test/ui/const-generics/issues/issue-61935.rs

[est31]

@lcnr is there an issue that tracks the fallout of that PR? I'd like the code I posted to work so I'm curious.

[lcnr]

Not really, it might be a good idea to open an issue for this though 🤔

#![feature(const_generics)]
fn foo<const B: bool>() {}

fn bar<const N: usize>() {
    foo::<{ N == 0 }>();
}

I am not really sure how much design + implementation is still needed here, so
I can't give you a good estimate on when we will be able to soundly enable this again.

[est31]

Filed an issue: #73899