9 Declarations [dcl.dcl]

9.12 Attributes [dcl.attr]

9.12.7 Maybe unused attribute [dcl.attr.unused]

1
#
The attribute-token maybe_­unused indicates that a name or entity is possibly intentionally unused.
It shall appear at most once in each attribute-list and no attribute-argument-clause shall be present.
2
#
The attribute may be applied to the declaration of a class, a typedef-name, a variable (including a structured binding declaration), a non-static data member, a function, an enumeration, or an enumerator.
3
#
A name or entity declared without the maybe_­unused attribute can later be redeclared with the attribute and vice versa.
An entity is considered marked after the first declaration that marks it.
4
#
Recommended practice: For an entity marked maybe_­unused, implementations should not emit a warning that the entity or its structured bindings (if any) are used or unused.
For a structured binding declaration not marked maybe_­unused, implementations should not emit such a warning unless all of its structured bindings are unused.
5
#
[Example
: 
[[maybe_unused]] void f([[maybe_unused]] bool thing1,
                        [[maybe_unused]] bool thing2) {
  [[maybe_unused]] bool b = thing1 && thing2;
  assert(b);
}
Implementations should not warn that b is unused, whether or not NDEBUG is defined.
— end example
]