A 'using' statement compiles with g++, fails compilation with clang - c++

Question

I have code of the following structure (which is of course much more complex in reality, especially "Base" is a three-liner, but I've tried to capture the gist of it):
template <class T>
class A {};
template <class T>
class B {
public:
B(){};
};
template <class T>
class C : public B<A<T>> {
public:
using Base = B<A<T>>;
using Base::B;
};
static const C<int> c{};
The code compiles fine with g++ via
g++ -c test.cpp -std=c++11
However, with clang++ I get an error message I don't really understand
clang++ -c test.cpp -std=c++11
test.cpp:14:14: error: dependent using declaration resolved to type without 'typename'
using Base::B;
Is there anything wrong with my code or is this a bug in clang?
Note: When writing using B<A<T>>::B; it compiles fine with both compilers, but this not a real solution to my problem.
Edit: clang version is 3.5.0, gcc version is 4.9.2

Answer 1

From section 12.1:
Constructors do not have names
So the usual rules for qualified lookup do not apply. Instead you have to rely on the special rule for constructor lookup (section 3.4.3.1):
In a lookup in which function names are not ignored and the nested-name-specifier nominates a class C:
— if the name specified after the nested-name-specifier, when looked up in C, is the injected-class-name of C, or
— in a using-declaration (7.3.3) that is a member-declaration, if the name specified after the nested-name-specifier is the same as the identifier or the simple-template-id’s template-name in the last component of the nested-name-specifier,
the name is instead considered to name the constructor of class C.
So you can certainly write
using Base::Base;
instead of
using Base::B;
Your original version should work under the first bullet point, but injected-class-names get complicated when templates are involved. Just go with the simpler version Base::Base, which additionally is more readable. Anyone who sees that instantly knows you are naming a constructor.

Answer 2

This case has been discussed in the C++ committee (according to Richard Smith https://llvm.org/bugs/show_bug.cgi?id=23107#c1) and things have gotten clearer now:
using Base::B
is not intended to be valid code.
The following method is the correct way to express constructor inheritance when introducing a class alias Base:
using Base::Base
However, the error messages produced by clang are misleading and will hopefully be solved as part of this bug report (https://llvm.org/bugs/show_bug.cgi?id=22242).