```cpp
namespace std {

// unconditionally trigger undefined behavior
// (c) 2020 by Justin Basset
[[noreturn]]
inline void unreachable() {}

} // std
```

SF +1


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On Mon, 26 Oct 2020 at 06:26, Justin Bassett via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> Technically, yes, *get_if is enough, but most compilers can't recognize it
> yet. I think gcc can under specific circumstances, but the rest I've tested
> failed to do so.
>
> On Sun, Oct 25, 2020 at 6:25 PM Tony V E <tvaneerd_at_[hidden]> wrote:
>
>> I don't think you need unreachable.
>> *get_if should be enough. Unconditionally dereference the result.
>>
>> Sent from my BlackBerry portable Babbage Device
>> *From: *Justin Bassett via Std-Proposals
>> *Sent: *Sunday, October 25, 2020 9:16 PM
>> *To: *Std-Proposals
>> *Reply To: *std-proposals_at_[hidden]
>> *Cc: *Justin Bassett
>> *Subject: *Re: [std-proposals] std::as
>>
>> I'm glad to see that clang can also optimize this get_if access; it used
>> to be unable to do so. MSVC is also unable to optimize this when using
>> __assume (
>> https://docs.microsoft.com/en-us/cpp/intrinsics/assume?view=vs-2019 )
>> (which is the same as Clang's __builtin_assume()
>> https://clang.llvm.org/docs/LanguageExtensions.html#builtin-assume )
>> instead of __builtin_unreachable().
>>
>> In theory, a portable __builtin_unreachable() would be:
>>
>> [[noreturn]]
>> inline void unreachable() {} // basically, unconditionally trigger
>> undefined behavior
>>
>> In practice, only GCC seems to recognize this.
>>
>> --Justin Bassett
>>
>> On Sun, Oct 25, 2020 at 5:59 PM Ryan P. Nicholl via Std-Proposals <
>> std-proposals_at_[hidden]> wrote:
>>
>>>
>>> [build] C:\Users\Ryan\rpnx-core\private\sources\all\test3.cpp(119,5): error C3861: '__builtin_unreachable': identifier not found [C:\Users\Ryan\rpnx-core\build\rpnx-core-test3.vcxproj]
>>>
>>> Great idea, except that __builtin_unreachable() is a GCC specific
>>> extension, and is not part of standard C++. (It would be nice to be able to
>>> do this in a cross platform way though! But that is for another discussion.)
>>>
>>> --
>>> Ryan P. Nicholl
>>> Tel: (678)-358-7765
>>> Personal Email: rnicholl_at_[hidden]
>>> Work Email: ryan.nicholl_at_[hidden]
>>> Tox:
>>> 52B3A61CDAF88BCC854F568933F071E3530600CDBA3D5354AADC9AD179575D68021AE959719D
>>>
>>>
>>> ‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
>>> On Sunday, October 25, 2020 8:50 PM, Barry Revzin <
>>> barry.revzin_at_[hidden]> wrote:
>>>
>>>
>>>
>>> On Sun, Oct 25, 2020 at 7:41 PM Ryan P. Nicholl via Std-Proposals <
>>> std-proposals_at_[hidden]> wrote:
>>>
>>>> I decided to implement a new class based on std::variant. I call it
>>>> "rpnx::derivator", but it's basically "allocating_variant". I tried to make
>>>> it as similar to std::variant as possible. When looking at this, I noticed
>>>> something weird about std::variant. There is no "zero overhead" way to get
>>>> the element contained by the variant, as std::get<I> checks for invalid
>>>> access and throws an exception if invalid. To solve this issue, I would
>>>> like to propose std::as, which works the same as std::get, but accessing
>>>> the wrong type is undefined behavior instead of throwing an exception.
>>>>
>>>> --
>>>> Ryan P. Nicholl
>>>> Tel: (678)-358-7765
>>>> Personal Email: rnicholl_at_[hidden]
>>>> Work Email: ryan.nicholl_at_[hidden]
>>>> Tox:
>>>> 52B3A61CDAF88BCC854F568933F071E3530600CDBA3D5354AADC9AD179575D68021AE959719D
>>>>
>>>
>>> You can achieve this by using std::get_if() and marking the nullptr case
>>> as unreachable. For example:
>>>
>>> auto f(std::variant<int, double>& v) -> int* {
>>> return std::get_if<int>(&v);
>>> }
>>>
>>> auto g(std::variant<int, double>& v) -> int* {
>>> auto p = std::get_if<int>(&v);
>>> if (not p) __builtin_unreachable();
>>> return p;
>>> }
>>>
>>> On -O1, f emits a check but g does not: https://godbolt.org/z/9G9fd5.
>>>
>>> Barry
>>>
>>>
>>> --
>>> Std-Proposals mailing list
>>> Std-Proposals_at_[hidden]
>>> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>>>
>>
>> --
> Std-Proposals mailing list
> Std-Proposals_at_[hidden]
> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>

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