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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 - class template partial specialization contains a template parameter that cannot be deduced"
href="https://bugs.llvm.org/show_bug.cgi?id=38268">38268</a>
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<th>Summary</th>
<td>class template partial specialization contains a template parameter that cannot be deduced
</td>
</tr>
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<th>Product</th>
<td>clang
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Linux
</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>
</tr>
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<th>Component</th>
<td>C++
</td>
</tr>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>zhonghao@pku.org.cn
</td>
</tr>
<tr>
<th>CC</th>
<td>dgregor@apple.com, llvm-bugs@lists.llvm.org
</td>
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<p>
<div>
<pre>The code is as follow:
template <typename T>
struct outer
{
template <typename U>
struct inner
{
};
};
template <typename T>
struct is_inner_for
{
template <typename Whatever>
struct predicate
{
static constexpr bool value = false;
};
template <typename U>
struct predicate<typename outer<T>::template inner<U>>
{
static constexpr bool value = true;
};
};
clang++ rejects the code:
code0.cpp:22:9: error: class template partial specialization contains a
template parameter that cannot be deduced; this partial specialization
will never be used [-Wunusable-partial-specialization]
struct predicate<typename outer<T>::template inner<U>>
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
code0.cpp:21:21: note: non-deducible template parameter 'U'
template <typename U>
^
1 error generated.
g++ 4.8, 5.2 accept the above code. It seems to be difficult to determine
whether the code is legal or not. I found that g++ 6 rejects the code with a
similar message:
code0.cpp:22:9: error: template parameters not deducible in partial
specialization:
struct predicate<typename outer<T>::template inner<U>> :
std::true_type
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
code0.cpp:22:9: note: 'U'
code0.cpp:26:1: error: static assertion failed: Yay!
static_assert(
^~~~~~~~~~~~~
However, g++ 9.0 again accepts the code.</pre>
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