[Bug c++/59978] New: C++11 Non-Type-Template-Parameter Pack Expansion not working according to standard
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59978
Bug ID: 59978
Summary: C++11 Non-Type-Template-Parameter Pack Expansion not
working according to standard
Product: gcc
Version: 4.8.2
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: aemseemann at gmail dot com
Created attachment 31976
--> http://gcc.gnu.org/bugzilla/attachment.cgi?id=31976&action=edit
minimal-example source file from the description
In the code below "Ints..." should expand to "1, 2" and therefore correctly
match the signature for "testFunc", but g++ 4.8.2 complains about
"too few arguments to function ‘void testFunc(int, int)’"
static void testFunc(int i1, int i2) {
(void)i1;
(void)i2;
}
template void wrapper() {
testFunc(Ints...);
}
int main(int, char *[]) {
wrapper<1, 2>();
return 0;
}
Command line: g++ -std=c++11 -Wall -Wextra non-type-template-parameter-pack.cpp
Tested with g++ 4.8.2 on ArchLinux (i686):
g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/i686-pc-linux-gnu/4.8.2/lto-wrapper
Target: i686-pc-linux-gnu
Configured with: /build/gcc/src/gcc-4.8-20131219/configure --prefix=/usr
--libdir=/usr/lib --libexecdir=/usr/lib --mandir=/usr/share/man
--infodir=/usr/share/info --with-bugurl=https://bugs.archlinux.org/
--enable-languages=c,c++,ada,fortran,go,lto,objc,obj-c++ --enable-shared
--enable-threads=posix --with-system-zlib --enable-__cxa_atexit
--disable-libunwind-exceptions --enable-clocale=gnu --disable-libstdcxx-pch
--disable-libssp --enable-gnu-unique-object --enable-linker-build-id
--enable-cloog-backend=isl --disable-cloog-version-check --enable-lto
--enable-plugin --enable-install-libiberty --with-linker-hash-style=gnu
--disable-multilib --disable-werror --enable-checking=release
Thread model: posix
gcc version 4.8.2 20131219 (prerelease) (GCC)
GCC from Git-master from 2014/01/28 produces the same error message.
The code compiles fine on clang (3.3 and 3.4)
[Bug libstdc++/109947] New: std::expected monadic operations do not support move-only error types yet
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109947
Bug ID: 109947
Summary: std::expected monadic operations do not support
move-only error types yet
Product: gcc
Version: 13.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: aemseemann at gmail dot com
Target Milestone: ---
GCC13 introduce monadic operations for `std::expected`, including r-value
ref-qualified overloads, which suggests that it should be possible to use an
expected with a move-only value or error type.
However, the following [example](https://godbolt.org/z/aoWeaqoGz) does not
compile due to an attempt to use unique_ptr's the deleted copy constructor:
```cpp
#include
#include
int main()
{
using expected = std::expected>;
expected e{42};
std::move(e).and_then([](auto&&) -> expected {
return 0;
});
return 0;
}
```
The issue seems to be the use of `std::move(value())` in the &&-qualified
overloads of the monadic operations (e.g.
[here](https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/include/std/expected#L880)
which selects the `value() &` overload that in turn attempts a copy of the
error type in the [exception
path](https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/include/std/expected#L740).
When replacing the value access with `std::move(*this).value()` the example
compiles successfully.
[Bug libstdc++/109947] std::expected monadic operations do not support move-only error types yet
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109947 --- Comment #3 from Martin Seemann --- Thanks for pointing me to the LWG issue. It makes sense that the error type must be copyable for the `value()` overloads due to potentially throwing a `bad_expected_access` with the embedded error embedded. However, the monadic operations will never throw this exception. Consequently, the standard draft for the monadic operations (https://eel.is/c++draft/expected.object.monadic) does not contain any "Throws:" clause nor is copyability of the error type included in the "Constraints:" clause. So it comes down to how to interpret the "Effects:" clause: Does "Equivalent to " mean that all restrictions of `value()` apply transitively or is it merely an implementation hint? (Strangely enough, in the "Effects:" clause of `value_or()&&` the expression `std::move(**this)` is used instead of `std::move(value())`. Maybe this is an oversight/inconsistency of the standard.)
[Bug libstdc++/109947] std::expected monadic operations do not support move-only error types yet
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109947 Martin Seemann changed: What|Removed |Added Status|UNCONFIRMED |RESOLVED Resolution|--- |INVALID --- Comment #5 from Martin Seemann --- Thanks for the clarification! Now I am convinced that it is not a bug in libstdc++ (although I still doubt that the side-effects were intended when the committee formulated the "Effects" for monadic operations, but that's not relevant here). Marking as resolved and sorry for the noise.
[Bug libstdc++/109947] std::expected monadic operations do not support move-only error types yet
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109947 --- Comment #9 from Martin Seemann --- That's great news, looking forward to the next point release. Thank you very much for taking this to the committee and getting the process in motion so quickly!
