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Question: when would you ever need to explicitly declare something as internal in Swift?

Seems like internal is always the default. So, under what conditions would you need to make it explicit?

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One scenario I can think of from the top of my head is when working with public properties. Suppose you wanted to declare a public property, but want to disallow anyone outside the module (but not the containing file) from calling its setter. In that scenario, you would have to use internal. For example:

public internal(set) var value: Int

Some people also like to use it in scenarios like:

public struct A {
  internal struct B {}
}

to make it clear that B is internal otherwise someone not familiar with the access control rules might assume its public (since its nested inside a public struct, I can see why a reasonable person might think that!). It's really a stylistic choice though in my opinion.

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I've also been told that

The use of explicit access levels is a common style choice that's widely used in first-party Swift code and recommended by at least a few style guides.

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Also, you can specify the default access control level for all members in an extension and then override that.

public extension MyType {
  // This one's public
  func myPublicMethod() {}

  // This one's explicitly internal
  internal func myInternalMethod() {}
}
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