https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92270
Bug ID: 92270
Summary: Non-const ref treated as const-ref in the compiler.
Product: gcc
Version: 7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: jostaberry.jam at gmail dot com
Target Milestone: ---
GCC rejects this valid code, but it can be compiled without "i = 0;".
And as expected, Overload #1 is selected.
=================== CODE ===================
#include <iostream>
#include <type_traits>
template <class T>
struct test {
T value;
template <class F>
std::enable_if_t<std::is_invocable_v<F&&, T&>>
fn(F&& op) & { op(value); } // #1
template <class F>
std::enable_if_t<std::is_invocable_v<F&&, T const&>>
fn(F&& op) const& { op(value); } // #2
};
template <class T> test(T) -> test<T>;
int main() {
test instance{1};
instance.fn([](auto& i){
// error: cannot assign to variable 'i'
// with const-qualified type 'const int &'
i = 0;
});
}
================= END CODE =================
============================================
source>:24:11: error: cannot assign to variable 'i' with const-qualified type
'const int &'
i = 0;
~ ^
/opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/type_traits:2283:7:
note: in instantiation of function template specialization 'main()::(anonymous
class)::operator()<const int>' requested here
std::declval<_Fn>()(std::declval<_Args>()...)
^
/opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/type_traits:2294:24:
note: while substituting deduced template arguments into function template
'_S_test' [with _Fn = (lambda at <source>:21:17) &&, _Args = (no value)]
typedef decltype(_S_test<_Functor, _ArgTypes...>(0)) type;
^
/opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/type_traits:2300:14:
note: in instantiation of template class 'std::__result_of_impl<false, false,
(lambda at <source>:21:17) &&, const int &>' requested here
: public __result_of_impl<
^
/opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/type_traits:2612:65:
note: in instantiation of template class 'std::__invoke_result<(lambda at
<source>:21:17) &&, const int &>' requested here
struct __is_invocable_impl<_Result, _Ret, __void_t<typename _Result::type>>
^
/opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/type_traits:2696:7:
note: during template argument deduction for class template partial
specialization '__is_invocable_impl<_Result, _Ret, __void_t<typename
_Result::type> >' [with _Result = std::__invoke_result<(lambda at
<source>:21:17) &&, const int &>, _Ret = void]
: __is_invocable_impl<__invoke_result<_Fn, _ArgTypes...>, void>::type
^
/opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/type_traits:2696:7:
note: in instantiation of template class
'std::__is_invocable_impl<std::__invoke_result<(lambda at <source>:21:17) &&,
const int &>, void, void>' requested here
/opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/type_traits:2732:44:
note: in instantiation of template class 'std::is_invocable<(lambda at
<source>:21:17) &&, const int &>' requested here
inline constexpr bool is_invocable_v = is_invocable<_Fn, _Args...>::value;
^
<source>:13:27: note: in instantiation of variable template specialization
'std::is_invocable_v<(lambda at <source>:21:17) &&, const int &>' requested
here
std::enable_if_t<std::is_invocable_v<F&&, T const&>>
^
<source>:21:14: note: while substituting deduced template arguments into
function template 'fn' [with F = (lambda at <source>:21:17)]
instance.fn([](auto& i){
^
<source>:21:26: note: variable 'i' declared const here
instance.fn([](auto& i){
~~~~~~^
1 error generated.
ASM generation compiler returned: 1
<source>:24:11: error: cannot assign to variable 'i' with const-qualified type
'const int &'
i = 0;
~ ^
/opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/type_traits:2283:7:
note: in instantiation of function template specialization 'main()::(anonymous
class)::operator()<const int>' requested here
std::declval<_Fn>()(std::declval<_Args>()...)
^
/opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/type_traits:2294:24:
note: while substituting deduced template arguments into function template
'_S_test' [with _Fn = (lambda at <source>:21:17) &&, _Args = (no value)]
typedef decltype(_S_test<_Functor, _ArgTypes...>(0)) type;
^
/opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/type_traits:2300:14:
note: in instantiation of template class 'std::__result_of_impl<false, false,
(lambda at <source>:21:17) &&, const int &>' requested here
: public __result_of_impl<
^
/opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/type_traits:2612:65:
note: in instantiation of template class 'std::__invoke_result<(lambda at
<source>:21:17) &&, const int &>' requested here
struct __is_invocable_impl<_Result, _Ret, __void_t<typename _Result::type>>
^
/opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/type_traits:2696:7:
note: during template argument deduction for class template partial
specialization '__is_invocable_impl<_Result, _Ret, __void_t<typename
_Result::type> >' [with _Result = std::__invoke_result<(lambda at
<source>:21:17) &&, const int &>, _Ret = void]
: __is_invocable_impl<__invoke_result<_Fn, _ArgTypes...>, void>::type
^
/opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/type_traits:2696:7:
note: in instantiation of template class
'std::__is_invocable_impl<std::__invoke_result<(lambda at <source>:21:17) &&,
const int &>, void, void>' requested here
/opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/type_traits:2732:44:
note: in instantiation of template class 'std::is_invocable<(lambda at
<source>:21:17) &&, const int &>' requested here
inline constexpr bool is_invocable_v = is_invocable<_Fn, _Args...>::value;
^
<source>:13:27: note: in instantiation of variable template specialization
'std::is_invocable_v<(lambda at <source>:21:17) &&, const int &>' requested
here
std::enable_if_t<std::is_invocable_v<F&&, T const&>>
^
<source>:21:14: note: while substituting deduced template arguments into
function template 'fn' [with F = (lambda at <source>:21:17)]
instance.fn([](auto& i){
^
<source>:21:26: note: variable 'i' declared const here
instance.fn([](auto& i){
~~~~~~^
1 error generated.
Execution build compiler returned: 1
============================================
https://godbolt.org/z/JvWq8i
