Why is this not a constexpr?

Question

#include <iostream>

union gc_bits {
    size_t value;
    struct {
        size_t arena : 2;
    } bits;

    constexpr gc_bits(size_t value_) : value(value_) {
    }
};

static constexpr size_t get_max_arenas() {
    return gc_bits(~0ULL).bits.arena;
}

size_t current_colour[get_max_arenas()]; // error

int main() {
    std::cout << get_max_arenas() << std::endl;
}

The array declaration errors out because get_max_arenas is not a constexpr. I'm not clear on why this should be so.

Answer 1

Slightly rephrasing your program:

static constexpr auto gma = get_max_arenas();

size_t current_colour[gma]; // error

gives the Clang error:

read of member 'bits' of union with active member 'value' is not allowed in a constant expression

The reason you get this error is that the constructor sets the value, and then you try to read bits. This is not allowed, as commented by @gurka.

Standard quote:

[expr.const]

2 A conditional-expression e is a core constant expression unless the evaluation of e, following the rules of the abstract machine (1.9), would evaluate one of the following expressions:

(2.8) — an lvalue-to-rvalue conversion (4.1) or modification (5.18, 5.2.6, 5.3.2) that is applied to a glvalue that refers to a non-active member of a union or a subobject thereof;