[LLVMbugs] [Bug 20398] New: clang C++11 wrong number of template parameters using aliases with template parameter pack

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=20398

            Bug ID: 20398
           Summary: clang C++11 wrong number of template parameters using
                    aliases with template parameter pack
           Product: clang
           Version: 3.4
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++11
          Assignee: unassignedclangbugs at nondot.org
          Reporter: reagentoo at gmail.com
                CC: dgregor at apple.com, llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Bug appears when passing template parameter packs to aliases. As a result, the
alias does not accept any parameters without any warning from the compiler.

Minimal test-case:
==================
#include <string>
#include <iostream>

template <template <class...> class TT, class T, class... Rest>
using Tail = TT<Rest...>;

template <class... Ts>
struct Bar
{
    static constexpr uint size = sizeof...(Ts);
};

template <class... Ts>
struct Foo
{
    static constexpr uint size = sizeof...(Ts);

    using TailBar = Tail<Bar, Ts...>;
    static constexpr uint bar_size = TailBar::size;
};

int main(int argc, char *argv[])
{
    using ZZZ = Foo<int, int, int>;
    std::cout << "size:" << ZZZ::size << " bar_size:" << ZZZ::bar_size <<
std::endl;
}
==================
Expected output: "size:3 bar_size:2"
Resulting output: "size:3 bar_size:0"

This bug does not occur if we refuse to use an alias "Tail":
...
template <template <class...> class TT, class T, class... Rest>
struct Tail
{
    using type = TT<Rest...>;
};
...

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