This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 119a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2025-12-20

3145. Uniqueness of annotations

(From submission #825.)

Consider:

  template<int> int x [[=1]];
  template<int A> int y [[=A]];
  static_assert(annotations_of(^^x<0>) == annotations_of(^^x<1>)); // #1
  static_assert(annotations_of(^^y<0>) == annotations_of(^^y<1>)); // #2

Subclause 9.13.12 [dcl.attr.annotation] paragraph 3 specifies:

Each annotation produces a unique annotation.

It is unclear whether each instantiation of a template creates a distinct annotation.

Suggested resolution:

Change in 9.13.12 [dcl.attr.annotation] paragraph 3 and paragraph 4 as follows:

(3) Each annotation produces a unique annotation.

Substituting into an annotation is not in the immediate context.

[Example:

  [[=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 (21.4.12 [meta.reflection.annotation]). —end example]

[Example:

  template<int> int x [[=1]];
  static_assert(annotations_of(^^x<0>) != annotations_of(^^x<1>));   // OK

-- end example]

(4) Substituting into an annotation is not in the immediate context.

[Example:

  template<class T>
  [[=T::type()]] void f(T t);
  void f(int);
  void g() {
    f(0);       // OK
    f('0');     // error, ...
  }

-- end example]