trivially-copyable optional<T> for trivial T

doc/91_relnotes.qbk

@@ -11,6 +11,11 @@
 
 [section:relnotes Release Notes]
 
+[heading Boost Release 1.66]
+
+* On newer compilers `optional` is now trivially-copyable for trivial `T`s. This uses a different storage (just `T` rather than `aligned_storage`). We require the compiler to support defaulted functions and type traits. Otherwise, we still use the the plain storage for scalar types.
+
+
 [heading Boost Release 1.63]
 * Added two new in-place constructors. They work similarly to `emplace()` functions: they initialize the contained value by perfect-forwarding the obtained arguments. One constructor always initializes the contained value, the other based on a boolean condition.
 * Syntax `o = {}` now correctly un-initializes optional, just like in `std::optional`.