Invalid use of non-static data member 'offset'

[springmeyer]

@ghoshkaj noticed that include/util/indexed_data.hpp will not compile with clang++ via xcode 7 on circleci's OS X infrastructure over at Project-OSRM/node-osrm#296 (comment). The workaround was to upgrade to xcode 8 on circle. This issue stands to track the underlying issue and decide how to avoid it in the future.

The problem appears that code which landed in #3585 will not compile on all versions of OS X.

I was able to replicate with this testcase:

#include <iostream>
#include <limits>

struct VariableGroupBlock
{

    struct BlockReference
    {
        std::uint32_t offset;
        std::uint32_t descriptor;
    };

    VariableGroupBlock() {}

    std::size_t max() const {
        return std::numeric_limits<decltype(BlockReference::offset)>::max();
    }
};

int main() {

    VariableGroupBlock bg;
    std::clog << bg.max() << "\n";
    
}

$ clang++ testcase.cpp
testcase.cpp:16:61: error: invalid use of non-static data member 'offset'
        return std::numeric_limits<decltype(BlockReference::offset)>::max();
                                            ~~~~~~~~~~~~~~~~^~~~~~
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/__config:605:36: note: expanded from macro 'decltype'
#  define decltype(__x) __decltype(__x)
                                   ^~~
1 error generated.

I can workaround by adding the -std=c++1y or -std=c++11 flag.

I see this behavior with this version of clang++ and xcode:

$ clang++ -v
Apple LLVM version 8.0.0 (clang-800.0.42.1)
Target: x86_64-apple-darwin16.3.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

$ xcodebuild -version
Xcode 8.2.1
Build version 8C1002

I presume what is happening is that despite the -std=c++1y being added in the osrm-backend builds, the clang++ version in xcode 7 must be missing some C++11 support.

Next steps:

• Run this test on older OS X, narrow down the issue
• Consider adding a cmake configure check to ensure a working clang++/xcode version is available, rather than suffering from a compiler error later on

/cc @danpat

[springmeyer]

I created a repo with the testcase (https://github.com/springmeyer/invalid-use-of-non-static-data-test) and tried testing on travis ci. So far I've found that xcode 7 on travis works fine (https://travis-ci.org/springmeyer/invalid-use-of-non-static-data-test) so this may be something specific to the circleci version.

[springmeyer]

The testcase also works on circleci with xcode 7.3 (https://circleci.com/gh/mapbox/invalid-use-of-non-static-data-test/3). So this is starting to feel like a bug not worth digging into more. Going to close without any action.