Don't use deprecated form of user-defined literal operator (#408)

* Don't use deprecated form of user-defined literal operator
* add clang warning regarding deprecated literal operator

---------

Co-authored-by: Hans Dembinski <hans.dembinski@gmail.com>

.github/workflows/slow.yml

@@ -85,4 +85,7 @@ jobs:
     - name: Test cxxstd=17 ubsan asan
       run: |
         cd libs/histogram
-        ../../b2 $B2_OPTS toolset=clang cxxstd=17 variant=histogram_ubasan test//all
+
+        # workaround: -Wdeprecated-literal-operator only works with clang and not with appleclang
+        # Apparently, build.jam cannot make the distinction, so we have to specify it here
+        ../../b2 $B2_OPTS toolset=clang cxxstd=17 cxxflags="-Wdeprecated-literal-operator" variant=histogram_ubasan test//all

build.jam

@@ -22,7 +22,6 @@ constant boost_dependencies :
 project /boost/histogram
     : requirements
         <toolset>clang:<cxxflags>"-Wsign-compare -Wstrict-aliasing -fstrict-aliasing -Wvexing-parse -Wfloat-conversion"
-        <toolset>darwin:<cxxflags>"-Wsign-compare -Wstrict-aliasing -fstrict-aliasing -Wvexing-parse -Wfloat-conversion"
         <toolset>gcc:<cxxflags>"-Wsign-compare -Wstrict-aliasing -fstrict-aliasing -Wfloat-conversion"
         <toolset>msvc:<cxxflags>"/bigobj"
         <toolset>intel-win:<cxxflags>"/bigobj"

include/boost/histogram/literals.hpp

@@ -23,7 +23,7 @@ constexpr unsigned parse_number(unsigned n, char f, Rest... rest) {
 namespace literals {
 /// Suffix operator to generate literal compile-time numbers, 0_c, 12_c, etc.
 template <char... digits>
-auto operator"" _c() {
+auto operator ""_c() {
   return std::integral_constant<unsigned, detail::parse_number(0, digits...)>();
 }
 } // namespace literals