cet-is » fhicl » fhicl-cpp

Attached are a sequence of patches to resolve compilation warnings (and hence errors with cetbuildtools CAUTIOUS diagnostics) when using Clang to compile fhicl-cpp. I have separated by file to assist in evaluating the fixes as I realise Clang is not directly supported - though I believe the fixes to be genuine issues with C++ Standard and/or code cruft.

Note that some additional warnings were found, but I feel are due to Clang being overzealous:

• Use of source:fhiclcpp/type_traits.h triggers Clang's "mismatched-tags" warning due to fhicl-cpp objects being forward declared as structs when the physical declaration will be a class. I believe Clang adds this warning to their -Wall diagnostic (hence it appearing with CAUTIOUS diagnostics) for back compatibility with older versions of MSVC that mangled structs and classes differently (see, for example https://bugzilla.mozilla.org/show_bug.cgi?id=780474). I've handled this by adding -Wno-mismatched-tags to cetbuildtools' flags when using Clang, which matches GCC's default behaviour and shouldn't affect anything as long as the Itanium ABI mangling scheme is used.
• Many -Wmissing-brace warnings are emitted for innocuous code such as source:fhiclcpp/types/Sequence.h#L86
  • I think this is running into these bugs: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=25137 and https://llvm.org/bugs/show_bug.cgi?id=21629 which upstream Clang hasn't fixed yet.
  • As with the previous warning, I've handled this by adding -Wno-missing-braces to the cetbuildtools set flags when using Clang to match the GCC behaviour.

So these additional "-Wno-" options are really things to be added in cetbuildtools as and when it supports Clang, so I just note the above issues here for information.