https://bugs.llvm.org/show_bug.cgi?id=43733
Bug ID: 43733
Summary: Unhelpful diagnostic "qualified reference to 'A' is a
constructor name rather than a type in this context"
Product: clang
Version: trunk
Hardware: All
OS: All
Status: NEW
Severity: enhancement
Priority: P
Component: -New Bugs
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected],
[email protected], [email protected]
https://godbolt.org/z/Wy8vGG
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);
^~~~~~
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