[Issue 21565] @safe code allows modification of a scalar that overlaps with a pointer

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21565

--- Comment #11 from Paul Backus <snarwin+bugzilla at gmail.com> ---
> Read-only access is fine. Write access is not.

Again, on what grounds do you make this claim? Can writing to the integer
member cause undefined behavior in @safe-only code? If so, please provide an
example.

> I just don't know what the definition of "sensical" means, based on your
> prior messages. What rules do you have in mind?

What I have in mind is to change the definition of "unsafe value" for unions to
the following:

> A struct/union instance is safe when:
> 
> * the values of its accessible fields are safe, and
> * it does not introduce unsafe aliasing with unions that is accessible
>   from @safe code.

This change does not, as far as I can tell, introduce unsoundness into the
language. It does not allow undefined behavior to occur in @safe code. If you
believe I am mistaken about this, please correct me.

The reason I call this "sensical" is that *unnecessarily* excluding values from
the definition of "safe value" makes the language more difficult to use without
any benefit to soundness or memory-safety. Ideally, we would like @safe to
impose on the programmer only those restrictions that are truly necessary in
order to avoid undefined behavior.

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