[MacPorts] #57576: llvm 8+ usuability

[MacPorts]

#57576: llvm 8+ usuability
-------------------------+------------------------
 Reporter:  RJVB         |      Owner:  (none)
     Type:  enhancement  |     Status:  new
 Priority:  Normal       |  Milestone:
Component:  ports        |    Version:
 Keywords:               |       Port:  llvm-devel
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 I just saw this

 {{{
 Starting with LLVM 8.0.0, users that wish to link together translation
 units built with different versions of libc++’s headers into the same
 final linked image MUST define the _LIBCPP_HIDE_FROM_ABI_PER_TU macro to 1
 when building those translation units. Not defining
 _LIBCPP_HIDE_FROM_ABI_PER_TU to 1 and linking translation units built with
 different versions of libc++’s headers together may lead to ODR violations
 and ABI issues. On the flipside, code size improvements should be expected
 for everyone not defining the macro.
 }}}
 (https://libcxx.llvm.org/docs/ReleaseNotes.html#what-s-new-in-libc-8-0-0)

 I interpret this as "define _LIBCPP_HIDE_FROM_ABI_PER_TU if you want to
 use the libc++ headers to build code that will link against system
 libraries".

 A really strange decision if that interpretation is correct but if it is
 correct wouldn't it be a least-worst idea revert the logic in
 `c++/v1/__config` to avoid having to add `-D_LIBCPP_HIDE_FROM_ABI_PER_TU`
 to just about every C++ compiler invocation?

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Ticket URL: <https://trac.macports.org/ticket/57576>
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