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<head>
<base href="https://bugs.llvm.org/">
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<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Non-const ref treated as const-ref in the compiler."
href="https://bugs.llvm.org/show_bug.cgi?id=43842">43842</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Non-const ref treated as const-ref in the compiler.
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>7.0
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>C++17
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>jostaberry.jam@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>blitzrakete@gmail.com, erik.pilkington@gmail.com, llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk
</td>
</tr></table>
<p>
<div>
<pre>Clang 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
============================================
<a href="https://godbolt.org/z/qR70hS">https://godbolt.org/z/qR70hS</a></pre>
</div>
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