https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80451
Bug ID: 80451
Summary: return implicit type conversion to std::optional does
not compile
Product: gcc
Version: 6.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: krisk0.2017.02.27 at protonmail dot com
Target Milestone: ---
Host: x86_64-pc-linux-gnu
Target: x86_64-pc-linux-gnu
Build: x86_64-pc-linux-gnu
Created attachment 41213
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=41213&action=edit
preprocessed source
* Short story:
under certain conditions, `return v;` statement does not compile.
* the exact version of GCC;
gcc-6.3.0 (Gentoo 6.3.0 p1.0) 6.3.0
* the system type;
Gentoo on x86_64
* the options given when GCC was configured/built;
COLLECT_GCC=gcc-6.3.0
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-pc-linux-gnu/6.3.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /tmp/portage/sys-devel/gcc-6.3.0/work/gcc-6.3.0/configure
--host=x86_64-pc-linux-gnu --build=x86_64-pc-linux-gnu --prefix=/usr
--bindir=/usr/x86_64-pc-linux-gnu/gcc-bin/6.3.0
--includedir=/usr/lib/gcc/x86_64-pc-linux-gnu/6.3.0/include
--datadir=/usr/share/gcc-data/x86_64-pc-linux-gnu/6.3.0
--mandir=/usr/share/gcc-data/x86_64-pc-linux-gnu/6.3.0/man
--infodir=/usr/share/gcc-data/x86_64-pc-linux-gnu/6.3.0/info
--with-gxx-include-dir=/usr/lib/gcc/x86_64-pc-linux-gnu/6.3.0/include/g++-v6
--with-python-dir=/share/gcc-data/x86_64-pc-linux-gnu/6.3.0/python
--enable-languages=c,c++ --enable-obsolete --enable-secureplt --disable-werror
--with-system-zlib --disable-nls --enable-checking=release
--with-bugurl=https://bugs.gentoo.org/ --with-pkgversion='Gentoo 6.3.0 p1.0'
--disable-esp --enable-libstdcxx-time --enable-shared --enable-threads=posix
--enable-__cxa_atexit --enable-clocale=gnu --enable-multilib
--with-multilib-list=m32,m64 --disable-altivec --disable-fixed-point
--enable-targets=all --disable-libgcj --enable-libgomp --disable-libmudflap
--disable-libssp --disable-libcilkrts --disable-libmpx --enable-vtable-verify
--enable-libvtv --disable-libquadmath --enable-lto --without-isl
--enable-libsanitizer --disable-default-pie --disable-default-ssp
Thread model: posix
gcc version 6.3.0 (Gentoo 6.3.0 p1.0)
* the complete command line that triggers the bug;
gcc-6.3.0 -xc++ -O2 -std=gnu++14 return-bug.c
* the compiler output (error messages, warnings, etc.);
return-bug.cc: In function
'std::experimental::fundamentals_v1::optional<Bread_and_butter>
meal(std::__cxx11::string)':
return-bug.cc:39:12: error: could not convert 'b' from 'Bread_and_butter' to
'std::experimental::fundamentals_v1::optional<Bread_and_butter>'
return b;
^
* the preprocessed file (*.i*) that triggers the bug
see attachment
* Why the code should compile
1. According to C++ standard ยง6.6.3 `The value of the expression is implicitly
converted to the return type of the function in which it appears.`
2. According to
http://en.cppreference.com/w/cpp/experimental/optional/optional, non-explicit
constructor from T& to std::experimental::optional<T> should exist.
3. The code compiles under gcc 5.4
