https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95412
Bug ID: 95412
Summary: Constexpr variables captured in templated lambdas
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: maurimo at fb dot com
Target Milestone: ---
I am getting (9.0.1 and 10.0.1, exact versions at the end) a curious issue with
constexpr variable captured in a templated lambda, here is a concise test case:
template<typename F>
void runLambda(F&& f) {
f.template operator()<3>();
f.template operator()<7>();
}
int main(int argc, char* argv[]) {
int retv = 0;
constexpr bool q = false;
runLambda([&]<int j>(){ // capturing with [&, q] always works
constexpr bool z = j != 8;
/*if constexpr(q) // <- this equivalent formulation works! ¯\_(ツ)_/¯
if constexpr(z)
retv += i*j;*/
if constexpr(q && z)
retv += j;
});
return retv;
}
Error (that should not happen):
test.cpp: In lambda function:
test.cpp:18:24: error: lambda capture of ‘q’ is not a constant expression
18 | if constexpr(q && z)
|
Compiled with:
g++-latest -std=c++2a simple_example/test.cpp -o test
Tried with both gcc 9.0.1:
Using built-in specs.
COLLECT_GCC=g++-9
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/9/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu
9-20190428-1ubuntu1~18.04.york0'
--with-bugurl=file:///usr/share/doc/gcc-9/README.Bugs
--enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++ --prefix=/usr
--with-gcc-major-version-only --program-suffix=-9
--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 --enable-bootstrap --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes
--with-default-libstdcxx-abi=new --enable-gnu-unique-object
--disable-vtable-verify --enable-plugin --enable-default-pie --with-system-zlib
--with-target-system-zlib=auto --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 9.0.1 20190428 (prerelease) [gcc-9-branch revision 270630] (Ubuntu
9-20190428-1ubuntu1~18.04.york0)
and gcc 10.0.1:
Using built-in specs.
COLLECT_GCC=g++-latest
COLLECT_LTO_WRAPPER=/opt/gcc-latest/libexec/gcc/x86_64-pc-linux-gnu/10.0.1/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../configure --prefix=/opt/gcc-latest --enable-languages=c,c++
--enable-libstdcxx-debug --disable-bootstrap --disable-multilib
--disable-libvtv --with-system-zlib --without-isl --enable-multiarch
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 10.0.1 20200412 (experimental) (GCC)
on an Ubuntu 18.04 (x86-64).
