Overlapping field initialization in CTFE-generated union literal

[ibuclaw]

Reduced from dlang/phobos#9064

union U
{
    struct S
    {
        long l;
    }

    int i;
    S s;
}

enum ctfeInit = {
    U u;
    u.s.l = 1;
    return u;
}();

void main()
{
    static a = ctfeInit;
    auto b = ctfeInit;
    assert(a is b);
}

Assert fails because what CTFE generates is:

enum U ctfeInit = U(0, S(1L));

And dmd backend codegen for static initializers ignores overlapping fields, resulting in the value becoming U(0), while the runtime initializer sets each field individually as b.i = 0; b.s.l = 1;.

---

Workaround is to declare variable as being void initialized.

enum ctfeInit = {
    U u = void;
    u.s.l = 1;
    return u;
}();

This results in the correct value being generated.

enum U ctfeInit = U(, S(1L));