The polymorphic_allocator was added in C++17.
This issue was filed in 2022 so well after C++20. This issue adds an
operator==.
Starting with C++20 this adds a compiler generated operator!=. To have
the same behaviour in C++17 and C++20 (and later) a manual operator!= is
defined in C++17.
Implements
- LWG3683 operator== for polymorphic_allocator cannot deduce template
argument in common cases
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