Consider this concept, which has a default template parameter.

I’ve got a thread-queue with a generic callback that takes the queue elements. This is the minimally reduced code:

I am trying to find the most efficient and clear way to define a C++20 concept to check that all types in a parameter pack are unique.

I know about std::convertible_to<From, To> but it includes also explicit conversions (static/const).

I need to select between two types for a variable at compile time. One of these types have a concept requirement which is not always satisfied and that is when I want to select the other type. I am trying to achieve this with std::conditional_t, but both types to std::conditional are evaluated causing a template constraint failure.

Is there a way – and if yes, how – to create a C++ concept that includes a parameter pack for the checked type and is then called recursively, such that more matches yield a better match for overloading?

Given the following concept