Why were parameter packs in generic types only made available in Somoma?

[xwu (Xiaodi Wu)]

This is discussed in the relevant Swift Evolution document:

@Slava_Pestov

ABI compatibility

Variadic type aliases are not part of the binary interface of a module and do not require runtime support.

All other variadic types make use of new entry points and other behaviors being added to the Swift runtime. Since the runtime support requires extensive changes to the type metadata logic, backward deployment to older runtimes is not supported.

Replacing a non-variadic generic type with a variadic generic type is not binary-compatible in either direction. When adopting variadic generic types, binary-stable frameworks must introduce them as wholly-new symbols.