[dcl.attr.annotation]

Let E be the expression std::meta::reflect_constant(constant-expression).

E shall be a constant expression; the result of E is the underlying constant of the annotation.

Substituting into an annotation is not in the immediate context.

[Example 1:

[[=1] void f(); [[=2, =3, =2] void g(); void g [[=4, =2] (); f has one annotation and g has five annotations.

These can be queried with metafunctions such as std::meta::annotations_of ([meta.reflection.annotation]).

— end example]

[Example 2: template<class T> [[=T::type()] void f(T t); void f(int); void g() { f(0); / OK f('0'); / error, substituting into the annotation results in an invalid expression } — end example]