https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70942
Bug ID: 70942
Summary: [c++14] Incorrect deduction of generic lambda `auto&&`
parameter
Product: gcc
Version: 6.1.0
Status: UNCONFIRMED
Severity: critical
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: vittorio.romeo at outlook dot com
Target Milestone: ---
Created attachment 38410
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=38410&action=edit
Minimal code example
g++ 6.1 was recently introduced into Arch Linux's testing repositories, and
some of my code that successfully compiled with g++ 5.3.0 does not compile
anymore. I've made a minimal example:
gcc.godbolt.org link:
https://godbolt.org/g/73RTHf
// This code compiles with g++ 5.3.0
// This does not compile with g++ 6.1
#include <type_traits>
#include <utility>
#include <tuple>
#define FWD(...) ::std::forward<decltype(__VA_ARGS__)>(__VA_ARGS__)
struct sinker
{
template <typename T>
void sink(T&)
{
}
};
template <typename T, typename TF>
void caller(T& v, TF&& f)
{
sinker s;
f(s, v);
}
template <typename T>
void interface(T& v)
{
return caller(v, [](auto& xs, auto&& xv) -> decltype(auto)
{
xs.sink(FWD(xv));
});
}
int main()
{
int x = 0;
interface(x);
}
This is the reported error:
: In instantiation of ‘get_impl(T&)::<lambda(auto:1&, auto:2&&)> [with
auto:1 = sinker; auto:2 = int; T = int]’:
:25:58: required by substitution of ‘template<class auto:1, class auto:2>
get_impl(T&)
[with T = int]::<lambda(auto:1&, auto:2&&)>::operator
decltype (((get_impl(T&) [with T = int]::<lambda(auto:1&,
auto:2&&)>)0u).operator()(static_cast<auto:1&>(<anonymous>),
static_cast<auto:2&&>(<anonymous>))) (*)(auto:1&, auto:2&&)() const
[with auto:1 = sinker; auto:2 = int]’
:19:6: required from ‘void chunk_fn_impl(T&, TF&&) [with T = int; TF =
get_impl(T&) [with T = int]::<lambda(auto:1&, auto:2&&)>]’
:25:25: required from ‘void get_impl(T&) [with T = int]’
:36:15: required from here
:27:13: error: invalid initialization of non-const reference of type ‘int&’
from an rvalue of type ‘int’
xs.sink(FWD(md));
^~
:10:10: note: initializing argument 1 of ‘void sinker::sink(T&) [with T =
int]’
void sink(T&)
^~~~
---
Changing:
return caller(v, [](auto& xs, auto&& xv) -> decltype(auto)
to:
return caller(v, [](auto& xs, auto& xv) -> decltype(auto)
allows the code to successfully compile.
---
I do not understand why this error is happening, as `xv` is being
perfectly-forwarded and the `FWD(xv)` call should produce a lvalue reference.
Note that the code worked as intended in g++ 5.3.0 and clang++ 3.7.
gcc.godbolt.org link:
https://godbolt.org/g/73RTHf
Try compiling with multiple g++ versions and changing `auto&&` to `auto&`.
Is this a g++ 6.1 bug? Or was the code incorrectly compiling with previous
versions of g++ and clang++?
---
More information:
http://stackoverflow.com/questions/37023265
