[PATCH] libstdc++: add initializer_list constructor to std::span (P2447)

[Giuseppe D'Angelo]

Hello,

The attached patch adds the span(initializer_list) constructor, added by 
P2447R6 for C++26.

It seems that GCC is somewhat aggressive with its -Winit-list-lifetime 
warning, which actively interferes with this feature. The idea of the 
new constructor is to allow calls like:

   void f(std::span<const int>);
   f({1, 2, 3});

which is completely fine as the lifetime of the initializer_list 
encompasses the one of the std::span parameter. However GCC complains 
about the risk of dangling here. I've therefore disabled the warning for 
the new constructor.

Thanks,
-- 
Giuseppe D'Angelo
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