https://issues.dlang.org/show_bug.cgi?id=13626

          Issue ID: 13626
           Summary: core.exception.AssertError@runnable\testthread2.d(85):
                    Assertion failure
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Windows
            Status: NEW
          Severity: normal
          Priority: P1
         Component: druntime
          Assignee: nobody@puremagic.com
          Reporter: bugzilla@digitalmars.com

This only happens on Win64, and appears to be a heisenbug.

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