Compiler Error

[Philippe Bouchard <philippeb at corel dot com>]

The following program:
#include <list>
#include <string>
#include <iostream>
#include <algorithm>
#include <functional>

struct A
{
  int a;
  double b;
  string c;
};

ostream & operator << (ostream & out, struct A const & op)
{
  return out << "{" << op.a << ", " << op.b << ", " << op.c << "}";
}

namespace std
{
  template <typename T, typename U> struct column_equal;
  template <typename T, typename U> binder2nd< column_equal<T, U> > column_match(U T::* const, U const &);

  template <typename T, typename U> struct column_operand
  {
    friend struct column_equal<T, U>;
    friend binder2nd< column_equal<T, U> > column_match<>(U T::* const, U const &);

    U T::* column;
    U const & value;
    column_operand(U T::* a_column, U const & a_value):column(a_column), value(a_value) {}
  };

  template <typename T, typename U> struct column_equal : public binary_function<T, column_operand<T, U>, bool>
  {
    bool operator () (T const & a_line, class column_operand<T, U> const & a_operand) const { return a_line.*a_operand.column == a_operand.value; }
  };

  template <typename T, typename U> binder2nd< column_equal<T, U> > column_match(U T::* const a_column, U const & a_value)
  {
    return binder2nd< column_equal<T, U> >(column_equal<T, U>(), class column_operand<T, U>(a_column, a_value));
  }
}
 
int main()
{
  using namespace std;
 
  class list<A> table;
 
  table.push_back((class A) {1,   0,  "qwerty" });
  table.push_back((class A) {0,   10, "sdsdsd" });
  table.push_back((class A) {100, 0,  "sd" });
  table.push_back((class A) {100, 21, "Hello"});
 
  cout << * find_if(table.begin(), table.end(), column_match(& A::a, 1)) << endl;
  cout << * find_if(table.begin(), table.end(), column_match(& A::b, 21.0)) << endl;
  cout << * find_if(table.begin(), table.end(), column_match(& A::c, string("sdsdsd") )) << endl;
}

Produces:
listlookup.cpp:40: common_type called with uncommon member types (compiler error)

Under:
Reading specs from /usr/lib/gcc-lib/i386-linux/2.95.2/specs
gcc version 2.95.2 20000220 (Debian GNU/Linux)

If I remove the friends declarations in struct column_operand, everything works fine. Maybe the syntax is not right.

Thanks.

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