I did however not know that GCC doesn't warn on NULL. Interesting compromise, even though I don't quite see the point of it.
Clang does warn on NULL, however.
I do see the point of it now. Under GCC the warning is pragmatically intended to catch actual mistakes such as mistyping while( *p != 0 ) as while( p != 0 ) so the warning only triggers on the actual 0, and is enabled under C++03 as well. Clang, and the various static analysis tools such as the MS analyzer, enforce a stylistic preference for nullptr, so they warn on both 0 and NULL, but Clang doesn't warn in C++03 mode. Either way, I'm not looking forward to adding a bunch of lines to suppress the various warnings, instead of just telling the PR author to use BOOST_NULLPTR instead of nullptr.