[Issue 19903] postblit called for uninitialised elements of unions

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19903

RazvanN <razvan.nitu1305 at gmail.com> changed:

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                 CC|                            |razvan.nitu1305 at gmail.com

--- Comment #3 from RazvanN <razvan.nitu1305 at gmail.com> ---
(In reply to Manu from comment #0)

> This assignment calls the T postblit, which is a `void` initialised member
> of a union.
> This is just a hard crash waiting to happen!
> 
> This pattern should be supplanted with copy-ctors.
> I suggest a reasonable resolution is:
>   1. no such implicit non-trivial assignment is attempted inside a union
>   2. if a union contains an element with a non-trivial copy, then a
> copy-ctor must be defined otherwise user receives a compile error informing
> them such.
>     2a. perhaps rather than emitting a compile error, it might be better to
> emit an automatic @disable copy constructor for that object.
> 
> I think it's the case that any union with elaborate copy semantics can only
> be correctly composed by the author of the union.

Spec:

"Unions may have fields that have postblits. However, a union itself never has
a postblit. Copying a union does not result in postblit calls for any fields.
If those calls are desired, they must be inserted explicitly by the
programmer."

It seems that it has a problem with anonymous unions. Slightly changing your
program results in corect behavior:

import std.stdio;

    struct T
    {
        this(this) { writeln("dun postblit\n"); }
        float f = 0;
    }

    struct S
    {
        union U {    // the union is named now
            int x = 0;
            T y = void; // <- void initialised
        }
        U u;  // and we have a field
        int which = 0;
    }

    S a;
    S b = a; // <- assignment calls postblit

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