[swift-dev] SILNode versus C++

[David Zarzycki]

Hi Adrian,

Feel free to file a bug if you’re motivated. Personally speaking, I don’t expect this bug to be on anybody’s priority list. The workaround of using named structs/unions has no downside for most projects, and Swift is no exception.

Dave

-- 
Sent from my iPad

> On Jan 4, 2018, at 20:11, Adrian Prantl <aprantl at apple.com> wrote:
> 
> 
> 
>>> On Dec 16, 2017, at 10:31 AM, John McCall via swift-dev <swift-dev at swift.org> wrote:
>>> 
>>> 
>>> On Dec 16, 2017, at 9:08 AM, David Zarzycki via swift-dev <swift-dev at swift.org> wrote:
>>> 
>>> Hello,
>>> 
>>> I’m trying to improve SILNode memory layout density by adopting the AST bitfield macros. Unfortunately, multiple inheritance doesn’t seem to get along with anonymous/unnamed unions. Here is a distillation of the problem:
>>> 
>>> class B {
>>> protected:
>>>  int i;
>>>  union { int j; };
>>> };
>>> 
>>> class X : public B { };
>>> class Y : public B { };
>>> 
>>> class Z : public X, public Y {
>>>  int a() { return X::i; } // works
>>>  int b() { return X::j; } // fails
>>> };
>>> 
>>> Is this expected C++ behavior? I can certainly workaround this by naming the unnamed union, but before I do, I thought that I should check here first.
>> 
>> This seems like a bug; anonymous unions are just supposed to inject their member names into the containing scope as if there was an ordinary member there, and the explicit scope-qualification should resolve which subobject is meant for such injected names the same it resolves them for ordinary fields.  But if it's a bug in all existing clangs, it's a bug we're going to have to work around.
> 
> Which should not prevent Dave from still reporting it on https://bugs.llvm.org/enter_bug.cgi?product=clang :-)
> 
> -- adrian