DM-29563: add compilation flag_LIBCPP_DISABLE_AVAILABILITY=1 to allow use of st…

[mwittgen]

…d::variant/filesystem on pre 10.13 or 10.15 macos deployment platforms

[mwittgen]

@TallJimbo any objections to this change to unlock {{C++17}} features in osx?

[TallJimbo]

Sorry, I thought I'd hit "submit review" on this a while ago, but apparently it was still waiting for me.

I will take another look at the C++ changes in afw on this ticket as soon as I can, but that may not be until tomorrow.

[TallJimbo]

So it's not needed for 10.13 and 10.14? I assume it does no harm for those versions?

I gather this forces linking against a particular libstdc++ (conda vs. xcode?). In either case, it would be good to extend the comment clarifying what this does to make std::variant and std::filesystem work. Or maybe that's a question for @erykoff ?

[erykoff]

So this is the right flag to set when building against conda libraries on macos. The point is that we don't want to check against availability of features against the system libraries because we don't use them. So the comment should be something like This flag is required to use new features like std::variant and std::filesystem on older macos. We build against conda libraries so we do not need to check if features are supported by system libraries.

[timj]

So what we are really saying is that the deployment target is irrelevant when running inside conda.

[erykoff]

Almost. (And our number has to be at least what conda-forge specifies which is 10.9). But when building for osx-arm64 (not the subject of this ticket!) it does actually matter.

[mwittgen]

This is the proposed change for ARM osx from another ticket

import os
import platform
import re
import shlex

@@ -89,6 +90,10 @@ def _initVariables():
        files.append("buildOpts.py")
    global opts
    opts = SCons.Script.Variables(files)
    if(platform.mac_ver()[2] == 'arm64'):
        macos_deployment_target='11.0'
    else:
        macos_deployment_target='10.9'