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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - cosntexpr constructor breaks out expression SFINAE"
href="https://bugs.llvm.org/show_bug.cgi?id=42330">42330</a>
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<th>Summary</th>
<td>cosntexpr constructor breaks out expression SFINAE
</td>
</tr>
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<th>Product</th>
<td>clang
</td>
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<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Windows NT
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>C++
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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</tr>
<tr>
<th>Reporter</th>
<td>lichray@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>blitzrakete@gmail.com, dgregor@apple.com, erik.pilkington@gmail.com, llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk
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<pre>Created <span class=""><a href="attachment.cgi?id=22124" name="attach_22124" title="reproduce">attachment 22124</a> <a href="attachment.cgi?id=22124&action=edit" title="reproduce">[details]</a></span>
reproduce
Clang does not compile the following program:
#include <type_traits>
struct A {};
struct B {};
template <class T>
struct MyPtr {
MyPtr() = default;
template <class U>
constexpr
MyPtr(MyPtr<U> v) : ptr_(v.ptr_) {}
T* ptr_;
};
template <class T>
struct __identity { using type = T; };
template <class... _Types>
struct __overload;
template <>
struct __overload<> { void operator()() const; };
template <class _Tp, class... _Types>
struct __overload<_Tp, _Types...> : __overload<_Types...> {
using __overload<_Types...>::operator();
static auto __test(_Tp (&&)[1]) -> __identity<_Tp>;
template <class _Up>
auto operator()(_Tp, _Up&& __t) const
-> decltype(__test({ std::forward<_Up>(__t) }));
};
template <class _Tp, class... _Types>
using __best_match_t =
typename std::invoke_result_t<__overload<_Types...>, _Tp, _Tp>::type;
void test() {
using t = __best_match_t<MyPtr<A>, MyPtr<A>, MyPtr<B>>;
}
<source>:11:23: error: cannot initialize a member subobject of type 'B *' with
an lvalue of type 'A *'
MyPtr(MyPtr<U> v) : ptr_(v.ptr_) {}
^ ~~~~~~
<source>:32:26: note: in instantiation of function template specialization
'MyPtr<B>::MyPtr<A>' requested here
-> decltype(__test({ std::forward<_Up>(__t) }));
^
[...]/include/c++/v1/type_traits:4425:23: note: while substituting deduced
template arguments into function template 'operator()' [with _Up = MyPtr<A>]
Essentially it is saying that a constexpr constructor is called and failed
during substitution, but that shouldn't happen. What should happen though is
MyPtr<A> being considered convertible to MyPtr<B>, but it's a worse match.
MSVC gives the right answer.
Commenting out the `constexpr` on line 10 fixes this issue.</pre>
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