Question

There is this code:

class A;

template <class T>
void fun() {
   A a;
}

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

int main() { 
   fun<int>(); 
   return 0;
}

g++ 4.5 and g++ 4.7 compiles this without error. But clang++ 3.2 (trunk) gives this error:

main.cpp:5:6: error: variable has incomplete type 'A'
   A a;
     ^
main.cpp:1:7: note: forward declaration of 'A'
class A;
      ^

Which compiler is right then according to C++ standard?

Answer 1

Which compiler is right then according to C++ standard?

Both are correct. This is an ill-formed program. Emphasis mine:

N3290 14.6¶9
If a type used in a non-dependent name is incomplete at the point at which a template is defined but is complete at the point at which an instantiation is done, and if the completeness of that type affects whether or not the program is well-formed or affects the semantics of the program, the program is ill-formed; no diagnostic is required.

That clang++ and other compilers do issue a diagnostic here is a nice-to-have added feature, but a diagnosis is not mandatory. That clause “the program is ill-formed; no diagnostic is required” gives a compiler developer free reign to do just about anything in such circumstances and still be compliant.