9 Declarations [dcl]

9.13 Attributes [dcl.attr]

9.13.12 Annotations [dcl.attr.annotation]

1
#
An annotation may be applied to any declaration of a type, type alias, variable, function, namespace, enumerator, base-specifier, or non-static data member.
2
#
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.
3
#
Each annotation produces a unique annotation.
4
#
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]