(From submission #716.)

The following note added to 9.13.1 [dcl.attr.grammar] by issue 2695 embodies the design intent that attributes can be ignored by implementations:

[Note 5: The attributes specified in 9.13 [dcl.attr] have optional semantics: given a well-formed program, removing all instances of any one of those attributes results in a program whose set of possible executions (4.1.2 [intro.abstract]) for a given input is a subset of those of the original program for the same input, absent implementation-defined guarantees with respect to that attribute. -- end note]

However, the note is incorrect for attributes that contain a potentially-evaluated expression such as [[assume]] (9.13.3 [dcl.attr.assume]). For example,

  template<int N>
  struct reader {
    friend auto flag(reader);
  };

  template<int N>
  struct setter {
    friend auto flag(reader<N>) {}
  };

  template<int N = 0, auto DifferenceMaker = [] {}>
  consteval int next() {
    if constexpr (requires { flag(reader<N>{}); }) {
     return next<N + 1>();
   } else {
     (void) setter<N>{};
     return N;
   }
  }

  int main() {
    static_assert(next() == 0);
    [[assume(true || next())]];
    static_assert(next() == 2);
  }

While this particular example using stateful friend injection is addressed by issue 2118, similar examples can be constructed using mainstream facilities introduced by P2996R13 (Reflection); see the compile-time ticket counter example therein.

There are also counterexamples for [[no_unique_address]]; see issue 2866.

Forwarded to EWG with paper issue 2407, by decision of the CWG chair.