What is a "copy"?

vanvoorden · August 28, 2025, 11:07pm

I believe I also found references in standard library to "distinguishability":

stdlib/public/core/SetAlgebra.swift

swift-6.1.2-RELEASE


      
          /// - Note: if this set and `other` contain elements that are equal but
          ///   distinguishable (e.g. via `===`), which of these elements is present
          ///   in the result is unspecified.
          __consuming func union(_ other: __owned Self) -> Self

So if equality implies substitutability by definition we then have elements that can be substitutable and distinguishable.

The === operator is "informally" also referred to as the "identical-to" operator:

github.com/swiftlang/swift

stdlib/public/core/Equatable.swift

swift-6.1.2-RELEASE


      
          /// The identical-to operator (`===`) returns `false` when comparing two
          /// references to different object instances, even if the two instances have
          /// the same value.
          ///
          ///     let c = IntegerRef(10)
          ///     print(a == c)
          ///     // Prints "true"
          ///     print(a === c)
          ///     // Prints "false"
          ///

Which helps tie distinguishability back to identity.

So far I see "identical-indistinguishable" mostly implying "interchangeable-substitutable"… but I don't usually see the other direction as holding.