Example of C++11 template parameter misbind?

Question

In 6.8.3 of the C++11 standard it says:

If, during parsing, a name in a template parameter is bound differently than it would be bound during a trial parse, the program is ill-formed.

What is an example of a program that is ill-formed as a result of this requirement?

Answer 1

#include <iostream>
#include <typeinfo>

typedef const int cint;

template <int a> struct x
{
  static cint b = 0;
};

template <> struct x<42>
{
  typedef cint b;
};

cint w = 17;

int main ()
{
  cint (w)(42), (z)(x<w>::b);

  std::cout << typeid(z).name() << std::endl;
}

The first declaration in main() needs to be disambiguated, so a trial parse is performed. During this parse, the local w is unknown, since the parse is purely syntactic (things are only parsed, no semantic actions are performed). Consequently, w is a global constant, its value is 17, x<w>::b is a value, and z is a variable.

During the real parse, semantic actions take place. Thus the name w is bound to the freshly declared local constant, its value is 42, x<w>::b becomes a type, and z is a function declaration.