[support.signal] paragraph 2 specifies that a signal handler invocation has undefined behaviour if it includes an evaluation that is not signal-safe.

Such an included evaluation (consistent with CWG's understanding; see CWG 3162) may occur within a manifestly constant-evaluated context, in which case, the specified undefined behaviour is gratuitous.

[Example from Jens Maurer:] The issue here is:

constexpr int f()
{
  try { throw 0; } catch (...) {}  // not signal-safe [support.signal] bullets 3.4 and 3.5
  return 0;
}

void handler(int)  // signal handler
{
  constexpr int x = f();  // ok?
}

The evaluation of "handler" includes the evaluation of `f`, which is not signal-safe. But that's not really a good state of affairs, because the constant evaluation of `f` certainly has no runtime impact on the signal handler.