qt59-qtlocation: fatal error: 'experimental/optional' file not found

[chrstphrchvz (Christopher Chavez)]

​Build fails on 10.15:

/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang++ -c -pipe -stdlib=libc++ -DNDEBUG -DQT_IMAGE_DECODERS -DRAPIDJSON_HAS_STDSTRING=1 -DMBGL_USE_GLES2 -D__QT__ -O3 -ftemplate-depth=1024 -fvisibility-inlines-hidden -fvisibility=hidden -DMBGL_VERSION_REV=\"qt-v1.2.0\" -Os -fPIC -std=c++1y  -arch x86_64 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk -mmacosx-version-min=10.15 -w -fvisibility=hidden -fvisibility-inlines-hidden -DQT_NO_NARROWING_CONVERSIONS_IN_CONNECT -DQT_NO_DEBUG -DQT_NETWORK_LIB -DQT_GUI_LIB -DQT_SQL_LIB -DQT_CORE_LIB -D_LARGEFILE64_SOURCE -D_LARGEFILE_SOURCE -I. -Ideps/boost/1.62.0 -Ideps/boost/1.62.0/include -Ideps/cheap-ruler/2.5.3 -Ideps/cheap-ruler/2.5.3/include -Ideps/earcut/0.12.4 -Ideps/earcut/0.12.4/include -Ideps/geojson/0.4.2 -Ideps/geojson/0.4.2/include -Ideps/geojsonvt/6.3.0 -Ideps/geojsonvt/6.3.0/include -Ideps/geometry/0.9.2 -Ideps/geometry/0.9.2/include -Ideps/kdbush/0.1.1-1 -Ideps/kdbush/0.1.1-1/include -Ideps/pixelmatch/0.10.0 -Ideps/pixelmatch/0.10.0/include -Ideps/polylabel/1.0.3 -Ideps/polylabel/1.0.3/include -Ideps/protozero/1.5.2 -Ideps/protozero/1.5.2/include -Ideps/rapidjson/1.1.0 -Ideps/rapidjson/1.1.0/include -Ideps/shelf-pack/2.1.1 -Ideps/shelf-pack/2.1.1/include -Ideps/supercluster/0.2.2 -Ideps/supercluster/0.2.2/include -Ideps/unique_resource/cba309e -Ideps/unique_resource/cba309e/include -Ideps/variant/1.1.4 -Ideps/variant/1.1.4/include -Ideps/vector-tile/1.0.0-rc7 -Ideps/vector-tile/1.0.0-rc7/include -Ideps/wagyu/0.4.3 -Ideps/wagyu/0.4.3/include -Iinclude -Iplatform/default -Iplatform/qt -Iplatform/qt/include -Isrc -I/opt/local/libexec/qt5/lib/QtNetwork.framework/Headers/5.9.9 -I/opt/local/libexec/qt5/lib/QtNetwork.framework/Headers/5.9.9/QtNetwork -I/opt/local/libexec/qt5/lib/QtGui.framework/Headers/5.9.9 -I/opt/local/libexec/qt5/lib/QtGui.framework/Headers/5.9.9/QtGui -I/opt/local/libexec/qt5/lib/QtSql.framework/Headers/5.9.9 -I/opt/local/libexec/qt5/lib/QtSql.framework/Headers/5.9.9/QtSql -I/opt/local/libexec/qt5/lib/QtCore.framework/Headers/5.9.9 -I/opt/local/libexec/qt5/lib/QtCore.framework/Headers/5.9.9/QtCore -I/opt/local/libexec/qt5/lib/QtNetwork.framework/Headers -I/opt/local/libexec/qt5/lib/QtGui.framework/Headers -I/opt/local/libexec/qt5/lib/QtSql.framework/Headers -I/opt/local/libexec/qt5/lib/QtCore.framework/Headers -I.moc -I/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk/System/Library/Frameworks/OpenGL.framework/Headers -I/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk/System/Library/Frameworks/AGL.framework/Headers -I/opt/local/libexec/qt5/mkspecs/macx-clang -F/opt/local/libexec/qt5/lib -o .obj/platform/qt/src/http_file_source.o platform/qt/src/http_file_source.cpp

In file included from platform/qt/src/http_request.cpp:1:
In file included from platform/qt/src/http_request.hpp:3:
In file included from src/mbgl/storage/http_file_source.hpp:3:
In file included from include/mbgl/storage/file_source.hpp:3:
In file included from include/mbgl/storage/response.hpp:4:
include/mbgl/util/optional.hpp:3:10: fatal error: 'experimental/optional' file not found
#include <experimental/optional>
         ^~~~~~~~~~~~~~~~~~~~~~~

According to ​Xcode 11 release notes:

The <experimental/optional> and <experimental/variant> headers are removed. Use <optional> and <variant> from C++17 instead.

(The ​Xcode 10.2 release notes had warned about these headers' deprecation and pending removal.)

Not sure if other older Qt5 versions' ports are affected.