Is the following well defined and allowed?

[Timon Gehr]

On 02.03.2018 16:05, Steven Schveighoffer wrote:
> On 3/2/18 10:00 AM, Timon Gehr wrote:
>> On 02.03.2018 15:39, Steven Schveighoffer wrote:
>>>
>>>
>>>> In this interpetation, -noboundscheck switches DMD to a different 
>>>> dialect of D. In that dialect, out-of-bounds accesses (and 
>>>> overlapping copies, apparently) always have UB, in both @system and 
>>>> @safe code. That defeats the purpose of @safe. Which is why I don't 
>>>> really care for that dialect.
>>>
>>> I agree, I think we should remove the option to disable bounds checks 
>>> on @safe code, in any way. It's too dangerous. If you want 
>>> performance that comes without bounds checks, use a trusted escape, 
>>> or write system code.
>>
>> I.e., the -release flag should not remove assertions in @safe code, or 
>> at the very least it should not turn them into sources of UB.
> 
> -release flag already operates this way.

That's not what the spec says:

https://dlang.org/dmd-linux.html#switch-release

"Compile release version, which means not emitting run-time checks for 
contracts and asserts. Array bounds checking is not done for system and 
trusted functions, and assertion failures are undefined behaviour."

Note that this is the only way to stop checking assertions. There is no 
option to just ignore them.

> It's the -noboundscheck or -boundscheck=off that causes problems.
> 
> -Steve

At least the specification advises to use caution:

"off: Bounds checks are disabled completely (even in @safe code). This 
option should be used with caution and as a last resort to improve 
performance. Confirm turning off @safe bounds checks is worthwhile by 
benchmarking."