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* [Bug c++/98394] New: Concept with auto... template parameter pack in if-statement causes compilation error
@ 2020-12-19 18:20 ngmr80 at web dot de
  2021-11-09 14:11 ` [Bug c++/98394] " cvs-commit at gcc dot gnu.org
                   ` (3 more replies)
  0 siblings, 4 replies; 5+ messages in thread
From: ngmr80 at web dot de @ 2020-12-19 18:20 UTC (permalink / raw)
  To: gcc-bugs

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;
      |         ^

^ permalink raw reply	[flat|nested] 5+ messages in thread
end of thread, other threads:[~2021-12-15 19:56 UTC | newest]

Thread overview: 5+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2020-12-19 18:20 [Bug c++/98394] New: Concept with auto... template parameter pack in if-statement causes compilation error ngmr80 at web dot de
2021-11-09 14:11 ` [Bug c++/98394] " cvs-commit at gcc dot gnu.org
2021-11-09 14:12 ` ppalka at gcc dot gnu.org
2021-12-15 19:55 ` cvs-commit at gcc dot gnu.org
2021-12-15 19:56 ` ppalka at gcc dot gnu.org

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