https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98394
Bug ID: 98394
Summary: Concept with auto... template parameter pack in
if-statement causes compilation error
Product: gcc
Version: 10.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: ngmr80 at web dot de
Target Milestone: ---
Created attachment 49807
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=49807&action=edit
Preprocessed source
In C++20, when using a concept 'C' with an auto... template parameter pack in
an if-statement, the compiler reports the error "'C' does not constrain a
type.". The same concept, however, _is_ usable in other contexts.
Example:
template<auto... T_values>
concept C = true;
int main()
{
auto b = C<1, 2>; // ok
if (C<1, 2>); // error
}
--
Output of g++-10 -v -save-temps -std=c++20 main.cpp
Using built-in specs.
COLLECT_GCC=g++-10
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/10/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa:hsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu
10.1.0-2ubuntu1~18.04' --with-bugurl=file:///usr/share/doc/gcc-10/README.Bugs
--enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++,m2 --prefix=/usr
--with-gcc-major-version-only --program-suffix=-10
--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-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 --enable-libphobos-checking=release
--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=/build/gcc-10-eDoCEC/gcc-10-10.1.0/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-10-eDoCEC/gcc-10-10.1.0/debian/tmp-gcn/usr,hsa
--without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 10.1.0 (Ubuntu 10.1.0-2ubuntu1~18.04)
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-std=c++2a' '-shared-libgcc'
'-mtune=generic' '-march=x86-64'
/usr/lib/gcc/x86_64-linux-gnu/10/cc1plus -E -quiet -v -imultiarch
x86_64-linux-gnu -D_GNU_SOURCE main.cpp -mtune=generic -march=x86-64 -std=c++2a
-fpch-preprocess -fasynchronous-unwind-tables -fstack-protector-strong -Wformat
-Wformat-security -o main.ii
ignoring duplicate directory "/usr/include/x86_64-linux-gnu/c++/10"
ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/10/include-fixed"
ignoring nonexistent directory
"/usr/lib/gcc/x86_64-linux-gnu/10/../../../../x86_64-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
/usr/include/c++/10
/usr/include/x86_64-linux-gnu/c++/10
/usr/include/c++/10/backward
/usr/lib/gcc/x86_64-linux-gnu/10/include
/usr/local/include
/usr/include/x86_64-linux-gnu
/usr/include
End of search list.
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-std=c++2a' '-shared-libgcc'
'-mtune=generic' '-march=x86-64'
/usr/lib/gcc/x86_64-linux-gnu/10/cc1plus -fpreprocessed main.ii -quiet
-dumpbase main.cpp -mtune=generic -march=x86-64 -auxbase main -std=c++2a
-version -fasynchronous-unwind-tables -fstack-protector-strong -Wformat
-Wformat-security -o main.s
GNU C++17 (Ubuntu 10.1.0-2ubuntu1~18.04) version 10.1.0 (x86_64-linux-gnu)
compiled by GNU C version 10.1.0, GMP version 6.1.2, MPFR version
4.0.1, MPC version 1.1.0, isl version isl-0.19-GMP
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
GNU C++17 (Ubuntu 10.1.0-2ubuntu1~18.04) version 10.1.0 (x86_64-linux-gnu)
compiled by GNU C version 10.1.0, GMP version 6.1.2, MPFR version
4.0.1, MPC version 1.1.0, isl version isl-0.19-GMP
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 435c9a72f92ce04eba08a47828fa63c4
main.cpp: In function ‘int main()’:
main.cpp:7:9: error: ‘C’ does not constrain a type
7 | if (C<1, 2>); // error
| ^~~~~~~
main.cpp:2:9: note: concept defined here
2 | concept C = true;
| ^
