repro.cpp

// g++ -std=c++11 test.cpp
template <class Tp>
struct  MyType {
   __attribute__((visibility("hidden"), always_inline)) ~MyType();
};

// Moving the definition above this line will allow the compilation to succeed. Why?
extern template __attribute__((visibility("default"))) MyType<int>::~MyType();

// DEFINITION
template <class Tp> inline MyType<Tp>::~MyType() {}
  
// Other is a class that requires the definition of MyType<size_t>::~MyType()
struct GeneratedDtor { MyType<int> t; };
GeneratedDtor o;