[llvm-bugs] [Bug 34414] New: Extracting the template template of a type and placing it into a template template holder fails

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=34414

            Bug ID: 34414
           Summary: Extracting the template template of a type and placing
                    it into a template template holder fails
           Product: clang
           Version: 4.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: release blocker
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: paul at preney.ca
                CC: dgregor at apple.com, llvm-bugs at lists.llvm.org

Created attachment 19075
  --> https://bugs.llvm.org/attachment.cgi?id=19075&action=edit
C++98/11/14 code demonstrating the clang++ v4.0 issue.

The following C++ code compiles with clang++ v3.9 and earlier but not with
v4.0:

// Define a type to hold a constant value...
template <typename T, T Value>
struct value { };

// Define a type to hold a template template to a constant value...
template <template <typename T, T> class TT>
struct param_value_tt { };

// Define a type to compute the template template of a type
// that holds a constant value (e.g., value<int,2>,
// std::integral_constant<int,2>, etc.)...
template <typename T>
struct extract_param_value_tt;

template <
  template <typename T, T> class TT,
  typename U,
  U uValue
>
struct extract_param_value_tt< TT<U,uValue> >
{
  // Either typedef or using can be used here...
  typedef param_value_tt<TT> type;
};

int main()
{
  // Either typedef or using can be used here...
  typedef typename extract_param_value_tt< value<int,2> >::type holder;
}

---

With clang++ v4.0, the code fails with:

$ clang++-4.0 -Wall -Wextra -pedantic clang4-template-template.cxx
clang4-template-template.cxx:18:26: error: template template argument has
different template parameters than its
      corresponding template template parameter
  typedef param_value_tt<TT> type;
                         ^
clang4-template-template.cxx:11:26: note: template non-type parameter has a
different type 'T' in template argument
  template <typename T, T> class TT,
                         ^
clang4-template-template.cxx:4:34: note: previous non-type template parameter
with type 'T' is here
template <template <typename T, T> class TT>
                                 ^
1 error generated.

I have not tried compiling with clang++ v5 as I don't have it installed.

---

The aim of the code is to obtain the "template template" for a type and "hold"
it in a type (i.e., param_value_tt) for subsequent processing. 

In the example, if one eliminates the use of TT in extract_param_value_tt and
replaces such with specific type names (e.g., "value" and/or
"std::integral_constant") the code will compile, however, when TT is used it
fails, hence this bug report.

i.e., the following C++11 (for std::integral_constant) code shows this:

#include <type_traits>

template <typename T, T Value>
struct value { };

// NOTE: Added apply<class T, T> to apply parameters to TT...
template <template <typename T, T> class TT>
struct param_value_tt
{
  template <typename U, U uValue>
  using apply = TT<U,uValue>;
};

template <typename T>
struct extract_param_value_tt;

// Partial specializations are now specific types...
template <typename U, U uValue>
struct extract_param_value_tt< value<U,uValue> >
{
  using type = param_value_tt<value>;
};

template <typename U, U uValue>
struct extract_param_value_tt< std::integral_constant<U,uValue> >
{
  using type = param_value_tt<std::integral_constant>;
};

int main()
{
  // Code showing everything including application...
  using holder1 = typename extract_param_value_tt< value<int,2>>::type;
  using holder1_apply = typename holder1::template apply<char,'4'>;

  using holder2 = typename extract_param_value_tt<
std::integral_constant<short,2>>::type;
  using holder2_apply = typename holder2::template apply<long,5L>;
}

Paul

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