https://issues.dlang.org/show_bug.cgi?id=24207

          Issue ID: 24207
           Summary: std.parallelism: AbstractTask private data is
                    inadvertently available
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: major
          Priority: P1
         Component: phobos
          Assignee: nobody@puremagic.com
          Reporter: schveiguy@gmail.com

In std.parallelism, `Task` has a "base" type `AbstractTask`, which is alias this'd to the type.

However, the base is accessible because it's public (yet undocumented). It looks like it was made public to workaround an issue in the compiler, see https://github.com/dlang/phobos/commit/9d151ac1f9e45d4d57a0b74508b41b303a6a293d

This should be reversed, and tested, because this allows access to the internal workings of the task system, in a completely mutable and unsafe way, including the `runTask`, the `prev` and `next` pointers, etc.

Given that the original field was private, and the type itself is private and undocumented, it's clear that this was never intended to be a public interface.

If this cannot be reversed, it should be redesigned so as to prevent unfettered thread-unsafe access to these fields.

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