Merge branch 'do-not-require-property-valuetype-default-constructible' into 'master'

This breaks LHCb compilation with clang:

In file included from /build/jenkins-build-new/workspace/nightly-builds/build/build/LHCB/LHCB_HEAD/Velo/VeloDAQ/src/VeloClusterFilter.cpp:4:
In file included from /build/jenkins-build-new/workspace/nightly-builds/build/build/LHCB/LHCB_HEAD/Det/VeloDet/VeloDet/DeVelo.h:10:
In file included from /build/jenkins-build-new/workspace/nightly-builds/build/build/LHCB/LHCB_HEAD/Det/VeloDet/VeloDet/DeVeloSensor.h:17:
In file included from /build/jenkins-build-new/workspace/nightly-builds/build/build/LHCB/LHCB_HEAD/Det/DetDesc/DetDesc/DetectorElement.h:18:
In file included from /build/jenkins-build-new/workspace/nightly-builds/build/build/GAUDI/GAUDI_master/InstallArea/x86_64-slc6-clang39-opt/include/GaudiKernel/CommonMessaging.h:11:
In file included from /build/jenkins-build-new/workspace/nightly-builds/build/build/GAUDI/GAUDI_master/InstallArea/x86_64-slc6-clang39-opt/include/GaudiKernel/ISvcLocator.h:6:
In file included from /build/jenkins-build-new/workspace/nightly-builds/build/build/GAUDI/GAUDI_master/InstallArea/x86_64-slc6-clang39-opt/include/GaudiKernel/ISvcManager.h:7:
In file included from /build/jenkins-build-new/workspace/nightly-builds/build/build/GAUDI/GAUDI_master/InstallArea/x86_64-slc6-clang39-opt/include/GaudiKernel/TypeNameString.h:3:
/build/jenkins-build-new/workspace/nightly-builds/build/build/GAUDI/GAUDI_master/InstallArea/x86_64-slc6-clang39-opt/include/GaudiKernel/Property.h:350:50: error: no type named 'type' in 'std::enable_if<false, void>'; 'enable_if' cannot be used to disable this declaration
              typename = typename std::enable_if<std::is_default_constructible<ValueType>::value>::type>
                                                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/build/jenkins-build-new/workspace/nightly-builds/build/build/LHCB/LHCB_HEAD/Velo/VeloDAQ/src/VeloClusterFilter.h:42:29: note: in instantiation of template class 'Gaudi::Property<VeloClusterFilter::filter_t, Gaudi::Details::Property::NullVerifier, Gaudi::Details::Property::UpdateHandler>' requested here
  Gaudi::Property<filter_t> m_filter{ this, "FilterOption", filter_t::criterion_t::ALL };

See https://lhcb-archive.cern.ch/artifacts/nightly/lhcb-clang-test/515/summaries.x86_64-slc6-clang39-opt/LHCb/build_log.html#line_1923

Interesting... Given that filter_t is defined as

  struct filter_t final {
    enum class criterion_t { ALL = 0, LEFT, RIGHT, R, PHI, PU, OVERLAP };
    criterion_t criterion;

    filter_t( criterion_t c = criterion_t::ALL ) : criterion(c) {}
    bool operator()(LHCb::VeloChannelID id) const;
    const std::string& toString() const;
    // add support for Gaudi::Property<filter_t>
    friend std::ostream& toStream(const filter_t& crit, std::ostream& os);
    friend StatusCode parse(filter_t& result, const std::string& input );
  };

I would expect that it is default constructible. And if it isn't this should be classic SFINAE, i.e. 'Substitution Failure Is Not An Error' -- i.e. this template should just not be instantiated instead of giving an error...

If I try to reproduce it in a short example, clang 3.9.1 is happy, and confirms that filter_t is default constructible, see https://godbolt.org/g/kO4pGT So I'm open to suggestions...

An example closer to this situation (using std::enable_if to pick an overload) also compiles with clang 3.9, see https://godbolt.org/g/aLQkCx Depending on the #if clause (which flips filter_t between being default constructible and not) it either returns 1 or 0... so in the example the overload resolution is happening as expected, and SFINAE does what I think it should be doing...

Ah, I think I know why: http://stackoverflow.com/questions/30953248/why-doesnt-sfinae-enable-if-work-for-member-functions-of-a-class-template

I suspect this is the fix:

diff --git a/GaudiKernel/GaudiKernel/Property.h b/GaudiKernel/GaudiKernel/Property.h
index 9f274c9..0e44610 100644
--- a/GaudiKernel/GaudiKernel/Property.h
+++ b/GaudiKernel/GaudiKernel/Property.h
@@ -347,7 +347,7 @@ namespace Gaudi
     /// @note the use std::enable_if is required to avoid ambiguities
     template <class OWNER, class T = ValueType,
               typename = typename std::enable_if<std::is_base_of<IProperty, OWNER>::value>::type,
-              typename = typename std::enable_if<std::is_default_constructible<ValueType>::value>::type>
+              typename = typename std::enable_if<std::is_default_constructible<T>::value>::type>
     inline Property( OWNER* owner, std::string name )
         : Property( std::move( name ), ValueType{}, "" )
     {

I agree, an I just added this fix to my MR !294 (merged) (61adab82)