As per the document, which says that[emphasis mine]:

template <class Fn, class... ArgTypes>
struct is_invocable; 

Determines whether Fn can be invoked with the arguments ArgTypes.... Formally, determines whether INVOKE(declval<Fn>(), declval<ArgTypes>()...) is well formed when treated as an unevaluated operand, where INVOKE is the operation defined in Callable.

How to understand the statement in bold? What's "an unevaluated operand"?

Maybe, a simple example helps to fully understand this matter.

CodePudding user response：

How to understand the statement in bold? What's "an unevaluated operand"?

An unevaluated operand is an operand that is not evaluated.

Maybe, a simple example helps to fully understand this matter.

void g(auto) requires false;
void f(auto x) { g(x); }

static_assert(is_invocable_v<decltype(f<int>), int>);
f(0); // ill-formed

Considering the above example, since is_invocable is not evaluated, g(x) in f will not actually be invoked either, so the static_assert will pass.

Since f(0) is actually evaluated, which will fail because g(0) fails to satisfy the constraints.