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<base href="https://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - template parameter not deducible in partial specialization of template inside template"
href="https://llvm.org/bugs/show_bug.cgi?id=31094">31094</a>
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<th>Summary</th>
<td>template parameter not deducible in partial specialization of template inside template
</td>
</tr>
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<th>Product</th>
<td>clang
</td>
</tr>
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<th>Version</th>
<td>3.9
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>normal
</td>
</tr>
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<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>
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<th>Reporter</th>
<td>akondrat@kcg.com
</td>
</tr>
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<th>CC</th>
<td>dgregor@apple.com, llvm-bugs@lists.llvm.org
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>The code below worked on clang 3.6, but warns on 3.8 and 3.9:
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;
};
};
static_assert(
is_inner_for<int>::template predicate<
outer<int>::inner<double>
>::value,
"Yay!"
);
Here is the warning:
<source>:23:12: warning: class template partial specialization contains a
template parameter that cannot be deduced; this partial specialization will
never be used
struct predicate<typename outer<T>::template inner<U>>
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:22:24: note: non-deducible template parameter 'U'
template <typename U>
^
1 warning generated.
Even though clang warns about the partial specialization not being used, the
code compiles fine and that specialization is in fact used.
GCC 6 bug that resulted in the same type of error:
<a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70141">https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70141</a>
Godbolt output: <a href="https://godbolt.org/g/pqJcQu">https://godbolt.org/g/pqJcQu</a>
Stack overflow question:
<a href="http://stackoverflow.com/questions/35875829/template-parameters-not-deducible-in-partial-specialization-in-gcc6-for-a-case">http://stackoverflow.com/questions/35875829/template-parameters-not-deducible-in-partial-specialization-in-gcc6-for-a-case</a></pre>
</div>
</p>
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