Say we have a function that uses both const generics and takes an impl Trait argument

fn const_add<const ARG: u64>(addable: impl std::ops::Add<u64>){ addable + ARG; }

The function itself compiles just fine... but there is no way to use it.

Specifying the argument like so

const_add::<5>(10);

does not work since one cannot provide explicit arguments when impl Trait is used [E0632]

error[E0632]: cannot provide explicit generic arguments when `impl Trait` is used in argument position --> src/main.rs:6:17 | 6 | const_add::<5>(10); | ^ explicit generic argument not allowed 

and if we remove the explicit arguments

const_add(10);

the compiler fairly complains that it doesn't know what value to use for the const generic [E0282]

error[E0282]: type annotations needed --> src/main.rs:6:5 | 6 | const_add(10); | ^^^^^^^^^ cannot infer the value of const parameter `ARG` declared on the function `const_add` 

Workaround: Use explicit arguments instead of impl Trait

fn const_add<A, const ARG: u64>(addable: A) where A: std::ops::Add<u64> { addable + ARG; }

and infer them with _

const_add::<_, 5>(10);

TLDR: fn with both const generic and impl Trait itself compiles, but is not useable in any way. Rust Playground

Meta

Tested on both Stable 1.52.1 and Nightly 1.54.0-nightly (2021-05-18 4e3e6db011c5b482d2be) channels, with same results.