Question

what does standard say about such case:

template<class C>
struct A {
    A(C c = C());
};

struct C {
    C(int);
};

A<C> a(C(1));

btw, Comeau does not raise error.

Answer 1

If you don’t use the default argument (that is, you supply a value), then it is not instantiated at all.

14.7.1/2:

Unless a function template
specialization has been explicitly
instantiated or explicitly
specialized, the func- tion template
specialization is implicitly
instantiated when the specialization
is referenced in a context that
requires a function definition to
exist. Unless a call is to a function
template explicit specialization or to
a member function of an explicitly
specialized class template, a default
argument for a function template or a
member function of a class template is
implicitly instantiated when the
function is called in a context that
requires the value of the default
argument.

What a mouthful. Actually, default arguments of both functions and templates have specific instantiation rules all to themselves.