const generic: unclear why "type annotations needed" error

[tspiteri]

I'm experimenting with replacing typenum with const generics, and replacing constraints involving constant generics being compared to other constant generics. I have gotten a little further recently, probably because of the recent merging of lazy normalization PRs. I have just stumbled upon a new issue.

#![feature(const_generics)]

pub struct IsLessOrEqual<const LHS: u32, const RHS: u32>;
pub struct Condition<const CONDITION: bool>;
pub trait True {}

impl<const LHS: u32, const RHS: u32> True for IsLessOrEqual<LHS, RHS> where
    Condition<{ LHS <= RHS }>: True
{
}
impl True for Condition<true> {}

struct S<const I: u32, const J: u32>;
impl<const I: u32, const J: u32> S<I, J>
where
    IsLessOrEqual<I, 8>: True,
    IsLessOrEqual<J, 8>: True,
    IsLessOrEqual<{ 8 - I }, { 8 - J }>: True,
    // Condition<{ 8 - I <= 8 - J }>: True,
{
    fn print() {
        println!("I {} J {}", I, J);
    }
}

This gives the error

error[E0283]: type annotations needed
  --> src/lib.rs:16:26
   |
5  | pub trait True {}
   | -------------- required by this bound in `True`
...
16 |     IsLessOrEqual<I, 8>: True,
   |                          ^^^^ cannot infer type for struct `IsLessOrEqual<I, 8u32>`
   |
   = note: cannot satisfy `IsLessOrEqual<I, 8u32>: True

In this specific case, replacing IsLessOrEqual<{ 8 - I }, { 8 - J }>: True with the commented-out Condition<{ 8 - I <= 8 - J }>: True makes the compilation work. However that is still not very robust: changing all three IsLessOrEqual constraints to the equivalent Condition constraints would again lead to compilation failure.

[lcnr]

This now fails with

warning: the feature `const_generics` is incomplete and may not be safe to use and/or cause compiler crashes
 --> src/lib.rs:1:12
  |
1 | #![feature(const_generics)]
  |            ^^^^^^^^^^^^^^
  |
  = note: `#[warn(incomplete_features)]` on by default
  = note: see issue #44580 <https://github.com/rust-lang/rust/issues/44580> for more information

error: constant expression depends on a generic parameter
 --> src/lib.rs:8:32
  |
8 |     Condition<{ LHS <= RHS }>: True
  |                                ^^^^
  |
  = note: this may fail depending on what value the parameter takes

error: constant expression depends on a generic parameter
  --> src/lib.rs:18:42
   |
18 |     IsLessOrEqual<{ 8 - I }, { 8 - J }>: True,
   |                                          ^^^^
   |
   = note: this may fail depending on what value the parameter takes

error: aborting due to 2 previous errors; 1 warning emitted

We should add this as a regression test to check the error messages once const wf bounds are implemented, marking E-needs-test