[swift-dev] SILNode versus C++

John McCall rjmccall at apple.com
Sat Dec 16 12:31:48 CST 2017

> 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.


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