The following code is compiled by gcc with -std=c++17



constexpr bool f() {
    int const a = 0;
    const_cast<int &>(a) = 2;
    return a == 0;
}

static_assert(f());


This should not be allowed because we modify a const value in a constant expression.

Here is another example test case that does not use const_cast (and gives a different answer in the return statement, but I don't think that's especially relevant because it's undefined behavior anyway):


struct S {
    int a = 1;
    int * ptr = &a;
};

constexpr bool f() {
    auto const s = S{};
    *s.ptr = 2;
    return s.a == 2;
}

static_assert(f());