Yes, this concept has been proposed in various forms. What you're describing is essentially a "variadic friends" feature that would allow declaring multiple friend classes or functions through a template parameter pack.
While the exact syntax you proposed (friends... Ts;) isn't part of the C++ standard, there have been discussions and papers about similar functionality. The general idea would be something like:
template<class... Ts>
class MyClass {
template<class... Ts> friend class; // Or some similar syntax
// Alternatively: friend Ts...;
};
This kind of feature would be useful in template metaprogramming scenarios where you want to grant friendship to a set of related types generated from a template.
There are a few challenges with such a proposal:

1. Syntax design that aligns with C++'s existing grammar

1. Semantic questions about how the friendship would propagate across template instantiations

1. Questions about whether friendship should be granted to all instantiations of a template or only specific ones

The C++ committee generally approaches friendship features carefully since they impact encapsulation. I don't believe this specific feature has been standardized yet, though similar ideas have been discussed in committee papers.
Alternative approaches currently used include:

• Using a base class with protected members

• Using explicit friend declarations for each needed type

• Using a proxy pattern to grant controlled access

Would you like more information about current workarounds for this use case or the status of any similar proposals?