[PATCH] D46218: PR37275 packed attribute should not apply to base classes

[Richard Smith - zygoloid via Phabricator via cfe-commits]

rsmith added a comment.

In https://reviews.llvm.org/D46218#1082834, @probinson wrote:

> In https://reviews.llvm.org/D46218#1081933, @rjmccall wrote:
>
> > I wonder if that was originally just an oversight that got turned into official policy.  Anyway, if it's the policy, it's what we have to do; LGTM.
>
>
> I think it's actually correct behavior.  Why would an attribute on a derived class be allowed to alter the layout of a base class?  It would mean you can't convert Derived* to Base* safely.


With Clang's old behavior, base classes were treated exactly like fields -- their layout was not changed per se, but their minimum alignment was reduced. Yes, that means you can't convert `Derived*` to `Base*` safely, but the packed attribute means you can't use `&object.member` to convert `Class*` to `Member*` safely either, so that's not an entirely unique problem. Probably more serious is that GCC doesn't permit a `T&` to bind to a packed field, so treating base classes as packed would interfere with passing a `Derived` lvalue to a `Base&`, which does seem like a much more severe restriction for base classes than for fields.


Repository:
  rL LLVM

https://reviews.llvm.org/D46218