constexpr with untouched non-constexpr arguments:

问题:

I have 4 test cases and I believe that all of them are valid:

constexpr int f(int const& /*unused*/){
    return 1;
}

void g(int const& p){
    constexpr int a = f(p); // clang error, gcc valid

    int v = 0;
    constexpr int b = f(v); // clang valid, gcc valid

    int const& r = v;
    constexpr int c = f(r); // clang error, gcc error

    int n = p;
    constexpr int d = f(n); // clang valid, gcc valid
}

int main(){
    int p = 0;
    g(p);
}

Clang and GCC differ only in the first test case.

I tested with clang 4 & 5 (20170319) and with GCC 7.0.1 (20170221).

If I'm right it would massively simplify the usage of boost::hana in static_assert's.

回答1:

[expr.const]/2:

An expression e is a core constant expression unless the evaluation of e, following the rules of the abstract machine, would evaluate one of the following expressions:

• [...]

• an id-expression that refers to a variable or data member of reference type unless the reference has a preceding initialization and either

  • it is initialized with a constant expression or

  • its lifetime began within the evaluation of e;

• [...]

Neither condition is satisfied for p or r. Therefore neither f(p) nor f(r) is a core constant expression and hence neither can be used to initialize a constexpr variable. Clang is correct.