public struct Foo<T> {} extension Foo where T == Float { private typealias A = Double } extension Foo where T == Int { private typealias A = Int64 }

private だと別ファイルと同ファイルで挙動がちがう。これは SE-0169 の副作用っぽいです。 https://github.com/apple/swift-evolution/blob/master/proposals/0169-improve-interaction-between-private-declarations-and-extensions.md

apple/swift-evolution

swift-evolution - This maintains proposals for changes and user-visible enhancements to the Swift Programming Language.