Forward declaration of class used in template function is not compiled by clang++

Question

There is this code:

class A;

template 
void fun() {
   A a;
}

class A { 
public: 
   A() {  } 
};

int main() { 
   fun(); 
   return         


        

Answer 1

Comeau's compiler does not like it either:

"ComeauTest.c", line 5: error: incomplete type is not allowed
     A a;
       ^

However my attempts at finding chapter and verse in the C++ standard were fruitless. It seems hidden in between the lines and interactions of "point of instantiation", "name resolution". Paragraphs 14.6/8 and 14.6/9 of the 2003 standard seem relevant.