Can attributes be applied to constructor parameters?

Question

Clang 8.0.0 and GCC 9.1.0 seem to disagree as to whether this is valid code.

struct Foo {
  Foo([[maybe_unused]] int x) {}
};
int main() {}

Clang produces no warnings (even with -Wall -Wextra -Wpedantic) but GCC produces this error:

test.cpp:2:7: error: expected unqualified-id before '[' token
    2 |   Foo([[maybe_unused]] int x) {}
      |       ^
test.cpp:2:7: error: expected ')' before '[' token
    2 |   Foo([[maybe_unused]] int x) {}
      |      ~^
      |       )

So which compiler has a bug?

Answer 1

Yes, they can be applied. The standard allows this.

10.6.6 Maybe unused attribute [dcl.attr.unused]
...
2 The attribute may be applied to the declaration of a class, a typedef-name, a variable, a non-static data member, a function, an enumeration, or an enumerator.

So Clang is correct here and this is a GCC bug. A bug report has already been filed for this titled: maybe_unused attribute triggers syntax error when used on first argument to a constructor

Answer 2

Your code is valid

The [[maybe_unused]] attribute can be applied to the declaration of a struct, enum, union, typedef, variable (including member variables), function, or enumerator. Implementations are encouraged to not emit a diagnostic when such an entity is unused or when the entity is used despite being marked as [[maybe_unused]].

However there is already a bug report for this in gcc maybe_unused attribute triggers syntax error when used on first argument to a constructor . gcc probably is not able to parse it correctly.