patch: avoid ubsan errors in implicit copy constructors

[Eli Friedman]

On Mon, Sep 9, 2013 at 3:03 PM, Nick Lewycky <nlewycky at google.com> wrote:

> The attached patch disables the bool and enum sanitizers when emitting the
> implicitly-defined copy constructor.
>
> To start with an example:
>   struct X { X(); X(const X&); };
>   struct Y { X x; bool b; };
> if you don't initialize Y::b then try to copy an object of Y type, ubsan
> will complain. Technically, with the standard as written, ubsan is correct.
> However, this is a useful thing to do -- you may have a discriminator which
> decides which elements are not interesting and therefore never initialize
> or read them. Secondly, it's a departure from the rules in C, making
> well-defined C code have undefined behaviour in C++ (structs are never trap
> values, see C99 6.2.6.1p6). Thirdly, it's checked incompletely right now --
> if you make subtle changes (f.e. add an "int i;" member to Y) ubsan will
> stop complaining. The semantic I'm implementing is as if the implicit copy
> constructor is copying the value representation (the bytes) not the object
> representation (the meaning of those bytes).
>

Does copy assignment have the same issue?

Being more consistent seems like a good idea.  And I agree that performing
this checking in defaulted copy-constructors probably isn't productive.

-Eli
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