While working on getting Boost & its friends work under Clang, I've stumbled upon the code that looks like the following:
decltype(x, y) z = w; The "x, y" is a perfectly valid expression, and should work. But it's not, because decltype is actually defined as a macros in libcxx: #if !__is_identifier(__decltype) || _GNUC_VER >= 406 # define decltype(__x) __decltype(__x) #else # define decltype(__x) __typeof__(__x) #endif This is easily being fixed by variadic macros, which are supported by GCC 3.0+ and all Clang versions. So I'm proposing the following change, which likely worths getting upstream as well. -- WBR, Vadim Zhukov Index: __config =================================================================== RCS file: /cvs/src/lib/libcxx/include/__config,v retrieving revision 1.3 diff -u -p -r1.3 __config --- __config 19 Sep 2016 22:17:22 -0000 1.3 +++ __config 28 May 2017 14:57:43 -0000 @@ -669,10 +669,11 @@ template <unsigned> struct __static_asse #ifdef _LIBCPP_HAS_NO_DECLTYPE // GCC 4.6 provides __decltype in all standard modes. +// variadic macros are required since decltype(x, y) is valid #if !__is_identifier(__decltype) || _GNUC_VER >= 406 -# define decltype(__x) __decltype(__x) +# define decltype(...) __decltype(__VA_ARGS__) #else -# define decltype(__x) __typeof__(__x) +# define decltype(...) __typeof__(__VA_ARGS__) #endif #endif
