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<base href="https://llvm.org/bugs/" />
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<body><table border="1" cellspacing="0" cellpadding="8">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - SFINAE issue with constexpr operator"
href="https://llvm.org/bugs/show_bug.cgi?id=30763">30763</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>SFINAE issue with constexpr operator
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>3.9
</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++
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>officesamurai@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>dgregor@apple.com, llvm-bugs@lists.llvm.org
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=17476" name="attach_17476" title="test code (also in the text)">attachment 17476</a> <a href="attachment.cgi?id=17476&action=edit" title="test code (also in the text)">[details]</a></span>
test code (also in the text)
The following code fails to compile:
#include <type_traits>
#include <utility>
template <typename TT1, typename TT2>
struct Helper
{
template <typename T1, typename T2>
static auto test(int) -> decltype(std::declval<T1>() == std::declval<T2>(),
std::true_type());
template <typename T1, typename T2>
static auto test(...) -> std::false_type;
using Result = decltype(test<TT1, TT2>(0));
};
template <typename TT1, typename TT2>
constexpr bool isEqualityComparable()
{
using Result = typename Helper<TT1, TT2>::Result;
return Result::value;
}
struct S
{
};
template <typename T>
struct Wrapper
{
T t;
};
// Removing constexpr here will make the code compilable
template <typename T1, typename T2>
constexpr bool operator==(const Wrapper<T1>& w1, const Wrapper<T2>& w2)
{
return w1.t == w2.t;
}
using W = Wrapper<S>;
int main()
{
isEqualityComparable<W, W>();
}
The result is:
$ clang++-3.9 error_test.cpp -std=c++11
error_test.cpp:37:17: error: invalid operands to binary expression ('const S'
and 'const S')
return w1.t == w2.t;
~~~~ ^ ~~~~
error_test.cpp:8:58: note: in instantiation of function template specialization
'operator==<S, S>' requested here
static auto test(int) -> decltype(std::declval<T1>() == std::declval<T2>(),
std::true_type());
^
error_test.cpp:13:29: note: while substituting explicitly-specified template
arguments into function template 'test'
using Result = decltype(test<TT1, TT2>(0));
^
error_test.cpp:19:29: note: in instantiation of template class
'Helper<Wrapper<S>, Wrapper<S> >' requested here
using Result = typename Helper<TT1, TT2>::Result;
^
error_test.cpp:44:5: note: in instantiation of function template specialization
'isEqualityComparable<Wrapper<S>, Wrapper<S> >' requested here
isEqualityComparable<W, W>();
^
error_test.cpp:35:16: note: candidate template ignored: could not match
'Wrapper<type-parameter-0-0>' against 'const S'
constexpr bool operator==(const Wrapper<T1>& w1, const Wrapper<T2>& w2)
Also:
1) -std=c++14 gives the same result,
2) removing constexpr from operator== fixes the problem,
3) GCC 4.9.3 produces a similar error while GCC 5.2.0 compiles it without
errors.</pre>
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