https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86970
Bug ID: 86970
Summary: Rejected constexpr expression involving lambdas and
inheritance, "use of this in a constant expression"
Product: gcc
Version: 8.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: jbassett271 at gmail dot com
Target Milestone: ---
Created attachment 44549
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=44549&action=edit
The .ii file from -save-temps
The following code fails to compile under g++-8:
Command: g++-8 -Wall -Wextra -std=c++17 min.cpp
Compiler Explorer link: https://godbolt.org/g/iByFDC
#include <tuple>
#include <utility>
namespace ns {
template <typename A>
class Foo : private A
{
public:
template <typename FA>
explicit constexpr Foo(FA&& a)
: A{std::forward<FA>(a)}
{}
};
template <typename A>
Foo(A)->Foo<A>;
template <typename T>
constexpr auto frobnicate(T&& val)
{
return [val = std::forward<T>(val)] {};
}
template <typename A, typename B>
class Bar
{
A a;
std::tuple<B> b;
public:
template <typename FA, typename FB>
explicit constexpr Bar(FA&& a, FB&& b)
: a{a}
, b{b}
{}
};
template <typename A, typename B>
Bar(A, B)->Bar<A, B>;
constexpr auto Baz = ns::Foo{ns::frobnicate(ns::Bar{[] {}, [](int) {}})};
}
Compiler diagnostics:
min.cpp:41:76: error: ‘constexpr ns::Foo<A>::Foo(FA&&) [with FA =
ns::frobnicate(T&&) [with T = ns::Bar<ns::<lambda()>, ns::<lambda(int)>
>]::<lambda()>; A = ns::frobnicate(T&&) [with T = ns::Bar<ns::<lambda()>,
ns::<lambda(int)> >]::<lambda()>]’ called in a constant expression
constexpr auto Baz = ns::Foo{ns::frobnicate(ns::Bar{[] {}, [](int) {}})};
^
min.cpp:10:28: note: ‘constexpr ns::Foo<A>::Foo(FA&&) [with FA =
ns::frobnicate(T&&) [with T = ns::Bar<ns::<lambda()>, ns::<lambda(int)>
>]::<lambda()>; A = ns::frobnicate(T&&) [with T = ns::Bar<ns::<lambda()>,
ns::<lambda(int)> >]::<lambda()>]’ is not usable as a ‘constexpr’ function
because:
explicit constexpr Foo(FA&& a)
^~~
min.cpp:11:36: error: call to non-‘constexpr’ function ‘ns::frobnicate(T&&)
[with T = ns::Bar<ns::<lambda()>, ns::<lambda(int)>
>]::<lambda()>::<lambda>(ns::frobnicate(T&&) [with T = ns::Bar<ns::<lambda()>,
ns::<lambda(int)> >]::<lambda()>&&)’
: A{std::forward<FA>(a)}
^
min.cpp:21:46: note: ‘ns::frobnicate(T&&) [with T = ns::Bar<ns::<lambda()>,
ns::<lambda(int)> >]::<lambda()>::<lambda>(ns::frobnicate(T&&) [with T =
ns::Bar<ns::<lambda()>, ns::<lambda(int)> >]::<lambda()>&&)’ is not usable as a
‘constexpr’ function because:
return [val = std::forward<T>(val)] {};
^
min.cpp:21:46: error: use of ‘this’ in a constant expression
System info (from running `g++-8 -v`):
Using built-in specs.
COLLECT_GCC=g++-8
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/8/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu
8.1.0-5ubuntu1~16.04' --with-bugurl=file:///usr/share/doc/gcc-8/README.Bugs
--enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++ --prefix=/usr
--with-gcc-major-version-only --program-suffix=-8
--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 --with-sysroot=/ --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes
--with-default-libstdcxx-abi=new --enable-gnu-unique-object
--disable-vtable-verify --enable-libmpx --enable-plugin --with-system-zlib
--with-target-system-zlib --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 8.1.0 (Ubuntu 8.1.0-5ubuntu1~16.04)
If this is not a bug in GCC, but the code is invalid, perhaps the diagnostic
could be improved. The diagnostics end in a lambda, saying "use of this in a
constant expression", which is confusing, considering that the lambda is not in
a context where `this` is valid and `this` does not appear to be used there.
