what is a requires requires constraint?

[JUANDENT]

Not sure if I understand the use of requires requires as in this code:

template<typename T>
requires requires (T x) { x + x; } // ad-hoc constraint, note keyword used twice
T add(T a, T b) { return a + b; }

the requires expression

requires (T x) { x + x; }

is an unnamed concept right?

So this syntax allows us to write a concept "inline" without placing a name to it. Right? Is this the purpose of the requires requires constraint?


Regards,
Juan

[mbozzi]

The requires expression is an unnamed concept right?

It's an expression of type bool.
requires (T x) { x + x; }
Can be read as "For an object of type T named x, the constraint x + x is satisfied?"

In the requires-clause

template<typename T>
requires constraint
T add(T a, T b) { return a + b; }

constraint is some constant expression which says whether the constraints are satisfied. The predicate
requires (T x) { x + x; }
is that constant expression.