https://issues.dlang.org/show_bug.cgi?id=22945

          Issue ID: 22945
           Summary: [Conditional Compilation] support invariant version
                    flag
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody@puremagic.com
          Reporter: jlourenco5691@gmail.com

This is a follow-up from https://issues.dlang.org/show_bug.cgi?id=22278. An
existent Phobos unit test that can benefit from this is:
https://github.com/dlang/phobos/blob/12b78bdebfe5ba55f3717f5ae1a5c81be8bac337/std/sumtype.d#L1275-L1290

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