[Issue 12024] [REG DMD2.065-b2] template instantiation for swap(SysTime, SysTime) fails

[d-bugmail at puremagic.com]

https://d.puremagic.com/issues/show_bug.cgi?id=12024



--- Comment #9 from Kenji Hara <k.hara.pg at gmail.com> 2014-01-29 05:56:38 PST ---
(In reply to comment #3)
> > Caused by https://github.com/D-Programming-Language/phobos/pull/1603
> 
> Indeed. However, as I explained there, I think it is legit behavior:
> 
> SysTime contains a rebindable, and Rebindable contains an immutable, and swap
> can't make the choice to mutate immutable.
> 
> That said, the issue could be in Rebindable to being with: It stores an
> immutable member, but obviously mutates it all the time. In that case, why
> bother storing an immutable at all? I think the fix is there.

If a non-mutable field has one or more overlapped union _mutable_ fields, the
whole struct is treated as modifiable.

import std.traits : Unqual;
struct Rebindable(T)
{
    union
    {
        T origin;
        Unqual!T stripped;  // overlapped union mutable field of 'origin'
    }
}
void main()
{
    Rebindable!int r1 = {origin:10};
    Rebindable!int r2 = {origin:20};
    r1 = r2;   // Rebindable!int is modifiable (assignable)
               // even if non-mutable field `T origin;` exists.
    assert(r1.origin == 20);
}

If all of overlapped union fields are non-mutable, the whole struct is not also
modifiable.

struct S
{
    union
    {
        immutable int x;
        immutable int y;
    }
}
void main()
{
    S s1 = {x:10};
    S s2 = {x:20};
    s1 = s2;  // cannot modify struct s1 S with immutable members
}

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