Created attachment 43868 [details] Preprocessed test case This problem was initially reported to Boost here: https://github.com/boostorg/type_traits/issues/66 The test case is this: #include <boost/type_traits/is_final.hpp> #include <type_traits> #include <iostream> struct final1 final {}; template <typename T> struct final2 final {}; int main() { std::cout << "boost::is_final<final1>: " << boost::is_final<final1>::value << std::endl; std::cout << "boost::is_final<final2<int>>::value: " << boost::is_final< final2<int> >::value << std::endl; std::cout << "std::is_final<final1>::value: " << std::is_final<final1>::value << std::endl; std::cout << "std::is_final<final2<int>>::value: " << std::is_final< final2<int> >::value << std::endl; } The output with gcc 6.4 and 7.2: boost::is_final<final1>: 1 boost::is_final<final2<int>>::value: 0 std::is_final<final1>::value: 1 std::is_final<final2<int>>::value: 1 The expected output should be all 1s. I suspect the compiler intrinsic __is_final produces incorrect result because of remove_cv for some reason. boost::is_final is defined like this: template <class T> struct is_final : public integral_constant<bool, BOOST_IS_FINAL(typename remove_cv<T>::type)> {}; where BOOST_IS_FINAL is defined to __is_final. This is similar to std::is_final except for the remove_cv trait. If I remove remove_cv then the test results are as expected. remove_cv is implemented this way: template <class T> struct remove_cv{ typedef T type; }; template <class T> struct remove_cv<T const>{ typedef T type; }; template <class T> struct remove_cv<T volatile>{ typedef T type; }; template <class T> struct remove_cv<T const volatile>{ typedef T type; }; plus the similar specializations for array types, which are irrelevant to this test. Unfortunately, I could not reproduce the problem with a more reduced testcase, including when I re-implemented is_final and remove_cv locally, so it's possible that some surrounding code is at play. I'm attaching a preprocessed source a compilable test code. Compile with: g++ -I<path_to_boost> -o is_final_test is_final_test.cpp I'm using Boost develop (pre-1.67), but the problem reproduces with Boost 1.62 that comes with Kubuntu 17.10 and likely any newer versions. $ g++ -v Using built-in specs. COLLECT_GCC=g++ COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/7/lto-wrapper OFFLOAD_TARGET_NAMES=nvptx-none OFFLOAD_TARGET_DEFAULT=1 Target: x86_64-linux-gnu Configured with: ../src/configure -v --with-pkgversion='Ubuntu 7.2.0-8ubuntu3.2' --with-bugurl=file:///usr/share/doc/gcc-7/README.Bugs --enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++ --prefix=/usr --with-gcc-major-version-only --program-suffix=-7 --program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-vtable-verify --enable-libmpx --enable-plugin --enable-default-pie --with-system-zlib --with-target-system-zlib --enable-objc-gc=auto --enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-offload-targets=nvptx-none --without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu Thread model: posix gcc version 7.2.0 (Ubuntu 7.2.0-8ubuntu3.2) $ g++-6 -v Using built-in specs. COLLECT_GCC=g++-6 COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/6/lto-wrapper Target: x86_64-linux-gnu Configured with: ../src/configure -v --with-pkgversion='Ubuntu 6.4.0-8ubuntu1' --with-bugurl=file:///usr/share/doc/gcc-6/README.Bugs --enable-languages=c,ada,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-6 --program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-vtable-verify --enable-libmpx --enable-plugin --enable-default-pie --with-system-zlib --disable-browser-plugin --enable-java-awt=gtk --enable-gtk-cairo --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-6-amd64/jre --enable-java-home --with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-6-amd64 --with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-6-amd64 --with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar --with-target-system-zlib --enable-objc-gc=auto --enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu Thread model: posix gcc version 6.4.0 20171010 (Ubuntu 6.4.0-8ubuntu1)
Created attachment 43869 [details] Compilable test case
Reduced to less than 30kloc namespace boost { template <bool val> struct integral_constant { static const bool value = val; }; template <bool val> bool const integral_constant<val>::value; template <class T> struct remove_cv{ typedef T type; }; template <class T> struct is_final : integral_constant<__is_final(typename remove_cv<T>::type)> {}; } struct final1 final {}; template <typename T> struct final2 final {}; static_assert( boost::is_final<final1>::value, "" ); static_assert( boost::is_final<final2<int>>::value, "" );
Reduced a little further: template <class T> struct remove_cv{ typedef T type; }; template <class T> struct is_final { static const bool value = __is_final(typename remove_cv<T>::type); }; struct final1 final {}; template <typename T> struct final2 final {}; static_assert( is_final<final1>::value, "" ); static_assert( is_final<final2<int>>::value, "" ); This should compile but fails with: is_final_test.cpp:12:16: error: static assertion failed static_assert( is_final<final2<int>>::value, "" ); ^~~~~~~~~~~~~~~~~~~~~
I hoped this would fix it, but it didn't: --- a/gcc/cp/semantics.c +++ b/gcc/cp/semantics.c @@ -9010,6 +9010,7 @@ trait_expr_value (cp_trait_kind kind, tree type1, tree type2) return type_code1 == ENUMERAL_TYPE; case CPTK_IS_FINAL: + type1 = TYPE_MAIN_VARIANT (type1); return CLASS_TYPE_P (type1) && CLASSTYPE_FINAL (type1); case CPTK_IS_LITERAL_TYPE:
Mine. Patch available: https://gcc.gnu.org/ml/gcc-patches/2019-05/msg02183.html
Author: ville Date: Sat Jun 1 10:57:12 2019 New Revision: 271835 URL: https://gcc.gnu.org/viewcvs?rev=271835&root=gcc&view=rev Log: PR c++/85254 gcc/cp PR c++/85254 * class.c (fixup_type_variants): Handle CLASSTYPE_FINAL. testsuite/ PR c++/85254 * g++.dg/ext/is_final.C: Amend. Modified: trunk/gcc/cp/ChangeLog trunk/gcc/cp/class.c trunk/gcc/testsuite/g++.dg/ext/is_final.C
Fixed for GCC 10.
Author: ville Date: Sat Oct 26 17:32:24 2019 New Revision: 277474 URL: https://gcc.gnu.org/viewcvs?rev=277474&root=gcc&view=rev Log: PR c++/85254 Backport from mainline 2019-06-01 Ville Voutilainen <ville.voutilainen@gmail.com> PR c++/85254 * class.c (fixup_type_variants): Handle CLASSTYPE_FINAL. Modified: branches/gcc-9-branch/gcc/cp/ChangeLog branches/gcc-9-branch/gcc/cp/class.c branches/gcc-9-branch/gcc/testsuite/g++.dg/ext/is_final.C