A requires-expression similarly to a function can introduce local parameters using a parameter list. And lambda expressions defined at block scope may have captures without initializers. Is it allo...

A requires-expression can introduce local parameters using a parameter list. If any of these parameters has void type, will the requires expression just yield false or shall it be hard compilation ...

I'm writing a concept that checks if a type can be used in an expression that composes 2 functions: template<typename T> concept C = requires(T t) { f(g(t)); }; i.e., I want to check if for...

I want to test whether a type can be passed to some function, but I'd like to use ADL on the function lookup and include a function from a certain namespace. Consider this code: #include <utilit...

I try to define a concept with a requires expression which checks some member function existence on the type. The problem I run into is, that a single concept has to be plugged in for the return ty...

#include <type_traits> template<typename T> struct IsComplete final : std::bool_constant<requires{sizeof(T);}> {}; int main() { struct A; static_assert(!IsComplete<A>::...

I am learning the newly implemented concepts of C++20 standard using g++ 10. I am stuck with a simple type requirement. Namely I want to implement a requirement for a template argument T to have T:...

Let's say, given C++17's if constexpr and Concepts TS (for instance, in recent gcc versions), we'd like to check if a type in a template function has a nested type: #include <iostream> stru...

In the following example, the function arguments are used to test with a requires expression whether an expression using them is well formed. The requires expression takes no arguments; it uses the...

Look at this simple concept example: template <typename T> requires requires(T t) { { t+t } -> bool; } void fn() { } int main() { fn<bool>(); } Here, I used bool as the type-const...

One of the corners of C++20 constraints is that there are certain situations in which you have to write requires requires. For instance, this example from [expr.prim.req]/3: A requires-expression ...