Valid to assign to field of struct in union?

[Timon Gehr via Digitalmars-d]

On 06.09.2016 14:56, Johan Engelen wrote:
> Hi all,
>   I have a question about the validity of this code:
> ```
> void main()
> {
>     struct A {
>         int i;
>     }
>     struct S
>     {
>         union U
>         {
>             A first;
>             A second;
>         }
>         U u;
>
>         this(A val)
>         {
>             u.second = val;
>             assign(val);
>         }
>
>         void assign(A val)
>         {
>             u.first.i = val.i+1;
>         }
>     }
>     enum a = S(A(1));
>
>     assert(a.u.first.i == 2);
> }
> ```
>
> My question is: is it allowed to assign to a field of a struct inside a
> union, without there having been an assignment to the (full) struct before?
> ...

I don't think so (although your case could be made to work easily 
enough). This seems to be accepts-invalid. Another case, perhaps 
demonstrating more clearly what is going on in the compiler:

float foo(){
     union U{
         int a;
         float b;
     }
     U u;
     u.b=1234;
     u.a=3;
     return u.b; // error
}
pragma(msg, foo());


float bar(){
     struct A{ int a; }
     struct B{ float b; }
     union U{
         A f;
         B s;
     }
     U u;
     u.s.b=1234;
     u.f.a=0;
     return u.s.b; // ok
}
pragma(msg, bar()); // 1234.00F

> The compiler allows it, but it leads to a bug with CTFE of this code:
> the assert fails.
> (changing `enum` to `auto` moves the evaluation to runtime, and all
> works fine)
>
> Reported here: https://issues.dlang.org/show_bug.cgi?id=16471.
>