There is a simple workaround: a static variable in a class template:

template <typename T>
struct A {
    static int var;
};
// A<int>::var can be referenced without the initializer being defined
at this point

template <> int A<int>::var = 7;

On Mon, 2025-01-13 at 20:07 +0200, Avi Kivity wrote:
> It's possible to declare-but-not-define a full specialization of a
> function or variable template:
>
> template <typename T> void function_template();
> extern template void function_template<int>(); // Some other TU must
> define, but any TU can reference
>
> template <typename T> class class_template;
> template <> class class_template<int>; // Incomplete type, but can
> still be used
>
> However, it's not possible [1] to declare-but-not-define a variable
> template full specialization.
>
> template <typename T> extern int variable_template;
> template <> extern int variable_template<int>; // rejected
>
> Such a declaration-but-not-definition would allow the initializer for
> the variable template to be hidden from its users, for the purpose of
> reducing compile-time dependencies.
>
> I wasn't able to fully determine if the defect is in the Standard, in
> existing implementations, or in the user interpreting the Standard
> and
> existing implementations.