Switching cxx_stdlib for C++11 on legacy macOS

[db]

On 4 Sep 2017, at 20:16, Ken Cunningham <ken.cunningham.webuse at gmail.com> wrote:
> switch to libc++:
> Benefits:
> 1. most builds are identical to 10.9+, most portfiles need no changes.
> Drawbacks:
> 1. needs changes in macports infrastructure to support new buildbots and pre-built binaries
> 2. poorly-written software or Portfiles do not spec the stdlib on the compile line. Clang adds wrong default stdlib if we want libc++. Need to be manually fixed, usually be adding the -stdlib to the compiler spec. (see botan).
> 3. bootstrapping issues - no getting around the LibCxxOnOlderSystems instructions. Might be beyond some users.

Couldn't this be implemented with a transition phase in which broken ports are reported for users who depend on them? When I tested using libc++ it worked for the most part, except for 2 out of ~450 installed ports.