`const_eval_select` cannot be used to call `#[rustc_comptime]` function from const

@oli-obk said this should work and I should open an issue if it didn't https://hachyderm.io/@oli/116733007519699753
Took me some time until I found the time to try.

I tried this code:

#![feature(rustc_attrs, core_intrinsics)]
#![allow(internal_features)]

use std::intrinsics::const_eval_select;

fn main() {
    const VAL: u64 = use_appropriate_impl(42);
    let val = use_appropriate_impl(42);

    println!("Comptime result: {VAL}, runtime result: {val}");
}

const fn use_appropriate_impl(i: u32) -> u64 {
    const_eval_select((i,), comptime_compatible_but_slow, runtime_only_impl)
}

#[rustc_comptime]
fn comptime_compatible_but_slow(mut i: u32) -> u64 {
    let mut sum = 0;

    while i > 0 {
        i -= 1;
        sum += (i * i) as u64
    }

    sum
}

fn runtime_only_impl(i: u32) -> u64 {
    (0..i).map(|i| u64::from(i * i)).sum()
}

I expected to see this happen: Compiles and prints "Comptime result: 23821, runtime result: 23821"

Instead, this happened: Compilation fails with the following error

   Compiling comptime-const-eval-select v0.1.0 (E:\Git\comptime-const-eval-select)
error[E0277]: the trait bound `fn(u32) -> u64 {comptime_compatible_but_slow}: const FnOnce(u32)` is not satisfied                                                                                                                                                                                       
    --> src\main.rs:14:29
     |
  14 |     const_eval_select((i,), comptime_compatible_but_slow, runtime_only_impl)
     |     -----------------       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     |     |
     |     required by a bound introduced by this call
     |
note: required by a bound in `const_eval_select`
    --> C:\Users\Bennet\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib/rustlib/src/rust\library\core\src\intrinsics\mod.rs:2408:8
     |
2401 | pub const fn const_eval_select<ARG: Tuple, F, G, RET>(
     |              ----------------- required by a bound in this function
...
2408 |     F: const FnOnce<ARG, Output = RET>;
     |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `const_eval_select`

For more information about this error, try `rustc --explain E0277`.                                                                                                                                                                                                                                     
error: could not compile `comptime-const-eval-select` (bin "comptime-const-eval-select") due to 1 previous error    

Meta

rustc --version --verbose:

rustc 1.98.0-nightly (3daae5e42 2026-06-14)
binary: rustc
commit-hash: 3daae5e42ec9ba435212987331af1b7b8634fa90
commit-date: 2026-06-14
host: x86_64-pc-windows-msvc
release: 1.98.0-nightly
LLVM version: 22.1.6

Reproduce the example from the issue with the nightly compiler, then inspect core/src/intrinsics/mod.rs around const_eval_select and its const FnOnce bound. Compare the handling of the #[rustc_comptime] function with the reported call, and consider the issue done when the example compiles and prints matching comptime and runtime results.