[libstdc++] Assertion in optional

[Jonathan Wakely]

On 14/05/17 09:19 +0200, Marc Glisse wrote:
>Hello,
>
>this patch adds 2 simple __glibcxx_assert in optional that match the 
>precondition in the comment above. I am not sure if there was a reason 
>the author wrote that comment instead of the assertion, but constexpr 
>use still seems to work.

Yes, in a constexpr context we get the following when it's not
engaged:

In file included from /home/jwakely/gcc/8/include/c++/8.0.0/utility:68:0,
                 from /home/jwakely/gcc/8/include/c++/8.0.0/optional:36,
                 from opt.cc:1:
/home/jwakely/gcc/8/include/c++/8.0.0/optional: In function ‘int main()’:
opt.cc:11:22:   in constexpr expansion of ‘f()’
opt.cc:6:11:   in constexpr expansion of ‘o.std::optional<int>::operator*()’
/home/jwakely/gcc/8/include/c++/8.0.0/optional:708:29:   in constexpr expansion of ‘((std::optional<int>*)this)->std::optional<int>::<anonymous>.std::_Optional_base<int>::_M_get()’
/home/jwakely/gcc/8/include/c++/8.0.0/optional:390:2: error: call to non-constexpr function ‘void std::__replacement_assert(const char*, int, const char*, const char*)’
  __glibcxx_assert(_M_is_engaged());
  ^

I think that's an improvement over what we have now:

opt.cc: In function ‘int main()’:
opt.cc:11:22:   in constexpr expansion of ‘f()’
opt.cc:11:23: error: accessing ‘std::_Optional_payload<int, true, true>::<unnamed union>::_M_payload’ member instead of initialized ‘std::_Optional_payload<int, true, true>::<unnamed union>::_M_empty’ member in constant expression
   constexpr int i = f();
                       ^


So the patch is OK for trunk, thanks.