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

clang++ is using the correct behavior, this is described in section 4.6/9 of the standard (n1905).

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Templates 14.6/9 Name resolution

If a name does not depend on a template-parameter (as defined in 14.6.2), a declaration (or set of declarations) for that name shall be in scope at the point where the name appears in the template definition; the name is bound to the declaration (or declarations) found at that point and this binding is not affected by declarations that are visible at the point of instantiation.

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To put things in simpler terms; if the name is not dependent on a template-parameter it should be in scope where the definition is found; therefore you'll need to define A before your definition of template void fun ().