Code doesn't compile with GCC, but does with Clang and ICC

[Nikos Chantziaras]

Why doesn't the below code compile with GCC?

  #include <iostream>

  template <typename T>
  static void fFunc()
  {
      static T var;
      (void)var; // Suppress set-but-not-used warning.
      struct g {
          static void gFunc()
          {
              std::cin >> var;
          };
      };
      g::gFunc();
  }

  int main()
  {
      fFunc<int>();
  }

It fails at link time with:

  /tmp/ccYDfCzj.o: In function `gFunc':
  tst.cpp:11: undefined reference to `var'

I'm using GCC 4.7.2 on Gentoo Linux (x86-64).  It compiles fine with 
Clang (3.1) and ICC (13.0.0).