[Issue 20355] undefined identifier U in core.atomic

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20355

berni44 <bugzilla at d-ecke.de> changed:

           What    |Removed                     |Added
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                 CC|                            |bugzilla at d-ecke.de
          Component|phobos                      |druntime

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