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List:       gcc-bugs
Subject:    [Bug c++/52440] New: [C++11] Wrong template argument deduction/substitution failures
From:       "ai.azuma at gmail dot com" <gcc-bugzilla () gcc ! gnu ! org>
Date:       2012-02-29 20:28:57
Message-ID: bug-52440-4 () http ! gcc ! gnu ! org/bugzilla/
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52440

             Bug #: 52440
           Summary: [C++11] Wrong template argument deduction/substitution
                    failures
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: ai.azuma@gmail.com


Created attachment 26790
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=26790
Output of -v option and preprocessed file

I can't find anything to prevent the following code from compiling;

/////////////////////////////////////////////////////////
template<bool>
struct V
{
  typedef void type;
};

template<typename T>
struct X
{
  template<typename>
  static constexpr bool always_true()
  {
    return true;
  };

  template<typename U,
           typename = typename V<always_true<U>()>::type>
  X(U &&) {}    // Line 18
};

int main()
{
  X<int> x(42); // Line 23
}
/////////////////////////////////////////////////////////

However, GCC complains about deduction/substitution failure;

main.cpp: In function 'int main()':
main.cpp:23:14: error: no matching function for call to 'X<int>::X(int)'
main.cpp:23:14: note: candidates are:
main.cpp:18:3: note: template<class U, class> X::X(U&&)
main.cpp:18:3: note:   template argument deduction/substitution failed:
main.cpp:8:8: note: constexpr X<int>::X(const X<int>&)
main.cpp:8:8: note:   no known conversion for argument 1 from 'int' to 'const
X<int>&'
main.cpp:8:8: note: constexpr X<int>::X(X<int>&&)
main.cpp:8:8: note:   no known conversion for argument 1 from 'int' to
'X<int>&&'

The constructor template in line 18 should always succeed in template argument
deduction/substitution and be used for the invocation in Line 23.

My guess is that this is a bug of GCC.
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