Why isn’t AnyHashable implemented in pure Swift?

[xwu (Xiaodi Wu)]

@Nevin

The documentation for AnyHashable includes the sentence:

/// The `AnyHashable` type forwards equality comparisons and hashing operations
/// to an underlying hashable value, hiding its specific underlying type.

That strongly implies to me at least that the hashing operation will be forwarded to the underlying hashable value , and thus return its hash-value without modification.

@Nevin

To my other point, the documentation for the Equatable conformance of AnyHashable states:

  /// Two instances of `AnyHashable` compare as equal if and only if the
  /// underlying types have the same conformance to the `Equatable` protocol
  /// and the underlying values compare as equal.

It is evident from attempting to write (1 as Int) == (1 as Double) that the types Int and Double do not have the same conformance to Equatable , since that raises a compiler error.

Yes, this is misleading. The documentation you quote from was last touched four years ago, and the edge cases you and I have enumerated above were fixed two years later. See also the corresponding bug.

That pull request I linked to in my previous post was merged only three months ago; it corrected a glaringly out-of-date documentation example originally written when AnyHashable was first added to the standard library, but did not update the rest of the documentation.

To be fair, the type does forward the hashing operation to an underlying value (_box._canonicalBox), just not the one you're probably thinking of (_box._base). I think it would be fair to file a bug against the outdated documentation.

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Update: PR #29498