[Issue 1432] Bogus "overlapping initialization" error with structs, unions, and member initializers

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=1432



--- Comment #3 from Kenji Hara <k.hara.pg at gmail.com> 2013-10-01 07:16:17 PDT ---
(In reply to comment #1)
> Another variation:
> 
> union S
> {
>     float[2] a;
>     struct
>     {
>         union { float i = 1; float x; }
>         union { float j = 2; float y; }
>     }
> }
> 
> $ dmd bar.d -c
> bar.d(2): Error: union bar.S overlapping initialization for struct S.i
> bar.d(2): Error: union bar.S overlapping initialization for struct S.j
> 
> If 'a' is moved below the anonymous struct it works. The spec does not mention
> this special case as far as I can tell. Explicit initialization of aggregate
> fields inside anonymous struct/union combinations should be documented.

More reduced test case.

union U1
{
    int a = 1;  // OK
    string b;
}
union U2
{
    int a;
    string b = "s";  // overlapping initialization for struct U2.b
}

I discovered this behavior while debugging other compiler bug. If there's
overlapped fields, and the non-first field is explicitly initialized, compiler
rejects it by "overlapping initialization".

As far as I see, the behavior seems to be intended by the current compiler
code. However, normally D language semantics does not rely on the lexical order
of declarations. I can agree that the behavior is contrary to the basic
principle.

---

I'd propose more better semantics:

If exactly one field has an explicit initializer between overlapped fields,
compiler should prefer it for default initialization.

U1 u1;  assert(u1.a == 1);
U2 u2;  assert(u2.b == "s");

If there's no explicitly initialized field between overlapped siblings, the
first one is initialized at the default initialization.

union U3 { int a; char c; }    U3 u3;  assert(u3.a == 0);
union U4 { char c; int a; }    U4 u4;  assert(u4.c == 0xff);

If two or more fields have explicit initializers between overlapped siblings,
it would cause compilation error.

union U5 { int a = 1, string b = "s" }
U5 u5;   // Error: overlapped initialization between U5.a and U5.b

How about that?

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