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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Unhelpful diagnostic "qualified reference to 'A' is a constructor name rather than a type in this context""
href="https://bugs.llvm.org/show_bug.cgi?id=43733">43733</a>
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<th>Summary</th>
<td>Unhelpful diagnostic "qualified reference to 'A' is a constructor name rather than a type in this context"
</td>
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<th>Product</th>
<td>clang
</td>
</tr>
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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
</td>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
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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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</tr>
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<th>Reporter</th>
<td>arthur.j.odwyer@gmail.com
</td>
</tr>
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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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<pre><a href="https://godbolt.org/z/Wy8vGG">https://godbolt.org/z/Wy8vGG</a>
A newbie (not me originally) wrote this:
template<class K>
bool foo(const K& a, const K& b) {
return std::less<K>::less()(a, b);
}
int i = foo(1,2);
Compiler complains:
<source>:5:16: error: missing 'typename' prior to
dependent type name 'std::less<int>::less'
return std::less<K>::less()(a, b);
^~~~~~~~~~~~~~~~~~
Easy peasy. Seen this error message before. Explain to newbie that they need
`typename` here (and why). Add `typename` and recompile. Compiler *still
complains!*
<source>:5:39: warning: ISO C++ specifies that
qualified reference to 'less' is a constructor name
rather than a type in this context,
despite preceding 'typename' keyword [-Winjected-class-name]
return typename std::less<K>::less()(a, b);
^
Now this is confusing. Complains with typename, complains without typename. I
had to go to Slack and ask what was supposed to be happening here.
What the compiler should say in this second case is not just a dry note about
the standardese names for things, but specifically something that would help
the user find and fix their bug. For example:
hypothetical error: qualified reference to 'less' names
a constructor, not a type,
despite preceding 'typename' keyword [-Winjected-class-name]
return typename std::less<K>::less()(a, b);
^~~~
hypothetical note: to construct an object of this type,
remove the explicit constructor name
return typename std::less<K>::less()(a, b);
^~~~~~~~~ ~~~~~~
And in the preceding case, where adding `typename` was precisely the wrong
"fix," Clang really should not have suggested it.
hypothetical error: qualified reference to 'less' names
a constructor [-Winjected-class-name]
return std::less<K>::less()(a, b);
^~~~~~~~~~~~~~~~~~
hypothetical note: to construct an object of this type,
remove the explicit constructor name
return std::less<K>::less()(a, b);
^~~~~~</pre>
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