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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Work around CWG issue 1558 (guarantee SFINAE when using `void_t`)"
href="https://bugs.llvm.org/show_bug.cgi?id=33655">33655</a>
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<th>Summary</th>
<td>Work around CWG issue 1558 (guarantee SFINAE when using `void_t`)
</td>
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<th>Product</th>
<td>libc++
</td>
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<th>Version</th>
<td>unspecified
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Windows NT
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>All Bugs
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>vittorio.romeo@outlook.com
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org, mclow.lists@gmail.com
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<p>
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<pre>In both libstdc++ and libc++ `void_t` is currently implemented as:
template <class...> using void_t = void;
Due to a CWG issue (#1558), unused parameters in alias templates are not
guaranteed to ensure SFINAE and could be ignored:
<a href="http://open-std.org/JTC1/SC22/WG21/docs/cwg_defects.html#1558">http://open-std.org/JTC1/SC22/WG21/docs/cwg_defects.html#1558</a>
This causes code like this to surprisingly fail to compile:
struct One { using x = int; };
struct Two { using y = int; };
template <typename T, std::void_t<typename T::x>* = nullptr>
void func() {}
template <typename T, std::void_t<typename T::y>* = nullptr>
void func() {}
int main() { func<One>(); }
There's a workaround for this issue available on cppreference. I propose to use
it to implement `void_t` to guarantee SFINAE-friendliness:
template<typename... Ts> struct make_void { typedef void type;};
template<typename... Ts> using void_t = typename make_void<Ts...>::type;
From:
<a href="http://en.cppreference.com/w/cpp/types/void_t">http://en.cppreference.com/w/cpp/types/void_t</a>
Context/more information:
<a href="https://stackoverflow.com/questions/44845945">https://stackoverflow.com/questions/44845945</a></pre>
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