template<typename T>
class Test {
public:
  T &operator[] (size_t i) {
    return t;
  }
  T t;
};

void foo() {
  Test<int> t;
  t[2];
}

==========

$ gcc error.cpp
error.cpp:6: error: declaration of ‘operator[]’ as non-function
error.cpp:6: error: expected ‘;’ before ‘(’ token
error.cpp:9: error: expected ‘;’ before ‘T’
error.cpp: In function ‘void foo()’:
error.cpp:14: error: no match for ‘operator[]’ in ‘t[2]’

$ clang error.cpp
error.cpp:6:18: error: unknown type name 'size_t'
  T &operator[] (size_t i) {
                 ^
1 diagnostic generated.