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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 --- - Poor diagnostic when forgetting to repeat default template type parameter"
href="https://llvm.org/bugs/show_bug.cgi?id=25977">25977</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Poor diagnostic when forgetting to repeat default template type parameter
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>unspecified
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>C++11
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>nicolasweber@gmx.de
</td>
</tr>
<tr>
<th>CC</th>
<td>dgregor@apple.com, llvm-bugs@lists.llvm.org
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Consider this program:
template<typename T> struct DefaultTraits;
template<typename T, typename Traits = DefaultTraits<T>>
struct ScopedTypeRef {
ScopedTypeRef(T object = Traits::InvalidValue()) {}
};
template <typename NST>
struct PointerTraits {
static NST InvalidValue() { return nullptr; }
};
template <typename NST>
class scoped_nsprotocol : public ScopedTypeRef<NST, PointerTraits<NST>> {
public:
using ScopedTypeRef<NST>::ScopedTypeRef;
};
int main() {
scoped_nsprotocol<int*> array(new int);
}
clang complains:
thakis@thakis:~/Downloads$ ~/src/llvm-build/bin/clang -c
templated-inherited-ctor.cc -std=c++11
templated-inherited-ctor.cc:15:9: error: using declaration refers into
'ScopedTypeRef<int *>::', which is not a base class of
'scoped_nsprotocol<int *>'
using ScopedTypeRef<NST>::ScopedTypeRef;
^~~~~~~~~~~~~~~~~~~~
templated-inherited-ctor.cc:19:27: note: in instantiation of template class
'scoped_nsprotocol<int *>' requested here
scoped_nsprotocol<int*> array(new int);
^
templated-inherited-ctor.cc:19:27: error: no matching constructor for
initialization of 'scoped_nsprotocol<int *>'
scoped_nsprotocol<int*> array(new int);
^ ~~~~~~~
templated-inherited-ctor.cc:13:7: note: candidate constructor (the implicit
copy constructor) not viable: no known conversion from
'int *' to 'const scoped_nsprotocol<int *>' for 1st argument
class scoped_nsprotocol : public ScopedTypeRef<NST, PointerTraits<NST>> {
^
templated-inherited-ctor.cc:13:7: note: candidate constructor (the implicit
move constructor) not viable: no known conversion from
'int *' to 'scoped_nsprotocol<int *>' for 1st argument
class scoped_nsprotocol : public ScopedTypeRef<NST, PointerTraits<NST>> {
^
templated-inherited-ctor.cc:13:7: note: candidate constructor (the implicit
default constructor) not viable: requires 0 arguments,
but 1 was provided
2 errors generated.
The fix is to change
using ScopedTypeRef<NST>::ScopedTypeRef;
into
using ScopedTypeRef<NST, PointerTraits<NST>>::ScopedTypeRef;
which isn't very clear from the error. If the base class is a template with
some default args and the default args are not explicitly passed in, then we
could emit a better diagnostic in that case.</pre>
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