Let's say I have some templated class depending on type T. T could be almost anything: int, int*, pair <int, int> or struct lol; it cannot be void, a reference or anything cv-qualified though. For some optimization I need to know if I can subclass T. So, I'd need some trait type is_subclassable, determined as a logical combination of basic traits or through some SFINAE tricks.

In the original example, int and int* are not subclassable, while pair <int, int> and struct lol are.

EDIT: As litb pointed out below, unions are also not subclassable and T can be a union type as well.

How do I write the trait type I need?

c++ template-meta-programming type-traits
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