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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 - CTAD fails to find some defaulted template parameters after first declaration of template"
href="https://bugs.llvm.org/show_bug.cgi?id=41549">41549</a>
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<th>Summary</th>
<td>CTAD fails to find some defaulted template parameters after first declaration of template
</td>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>All
</td>
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<th>OS</th>
<td>All
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>-New Bugs
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>arthur.j.odwyer@gmail.com
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<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org, neeilans@live.com, richard-llvm@metafoo.co.uk
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<div>
<pre>Reduced from a test case involving class template argument deduction on libc++
unordered_multimap.
// <a href="https://godbolt.org/z/X3OqH0">https://godbolt.org/z/X3OqH0</a>
#if FAIL
template <class H, class P> struct umm;
#endif
template <class H = int, class P = int>
struct umm {
explicit umm(H h = 0, P p = 0);
};
void t() {
umm m(1);
}
====
$ clang++ -std=c++17 test.cc
(OK)
$ clang++ -std=c++17 test.cc -DFAIL
<source>:11:9: error: no viable constructor or deduction guide for deduction of
template arguments of 'umm'
umm m(1);
^
<source>:7:14: note: candidate template ignored: couldn't infer template
argument 'P'
explicit umm(H h = 0, P p = 0);
^
<source>:2:36: note: candidate template ignored: could not match 'umm<H, P>'
against 'int'
template <class H, class P> struct umm;
^
1 error generated.
====
GCC, ICC, and MSVC all agree that this code should be considered well-formed:
CTAD should happily take the defaulted value of `P` from the visible
declaration of the primary template, even though that declaration is not the
first declaration in the source file.</pre>
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