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<base href="http://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - missing check that deduction actually succeeded when partially ordering function templates"
href="http://llvm.org/bugs/show_bug.cgi?id=17917">17917</a>
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<th>Summary</th>
<td>missing check that deduction actually succeeded when partially ordering function templates
</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>PC
</td>
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<th>OS</th>
<td>Linux
</td>
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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>C++
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>richard-llvm@metafoo.co.uk
</td>
</tr>
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<th>CC</th>
<td>dgregor@apple.com, llvmbugs@cs.uiuc.edu
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Consider:
template<bool> struct enable_if { typedef void type; };
template <class T> class Foo {};
template <class X> constexpr bool check() { return true; }
template <class X, class Enable = void> struct Bar {};
#ifdef FUNCTION_ORDERING
template<class X> void func(Bar<X, typename enable_if<check<X>()>::type>) { }
template<class T> int func(Bar<Foo<T>>) { return 0; }
int (*p)(Bar<Foo<int>>) = func;
#else
template<typename X> struct Bar<X, typename enable_if<check<X>()>::type> {};
template<typename X> struct Bar<Foo<X>> { typedef int type; };
Bar<Foo<int>>::type bar;
#endif
Per 14.5.5.2/1, the class template partial specialization partial ordering and
the function template partial ordering above should behave exactly the same.
But they don't: for clang, gcc, and edg, the FUNCTION_ORDERING case is accepted
and the other (class ordering) case is rejected.
Morally, both cases should be rejected; this is ambiguous because the first
template has the constraint that enable_if<check<X>()>::type == void, and the
second template has the constraint that X == Foo<T> for some T. Clearly,
neither of these implies the other.
Practically, we're missing a call to something like
FinishTemplateArgumentDeduction in the FunctionTemplateDecl form of
isAtLeastAsSpecializedAs: we fail to check that the deduction produces template
arguments that make the deduced A compatible with the original A (as required
by [temp.deduct.type]p1).</pre>
</div>
</p>
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