Failure to match a set element

[stevengj]

@canthonissen

Mutating set elements (or dictionary key) screws up the indexing mechanics and is very likely to cause the membership check on that element to fail.

Right. See also

• Modifying object dict keys
• Unable to access value in dict via known keys
• Dictionary with multiple identical keys
• Potential confusing behaviour with mutable-key Dict · Issue #33176 · JuliaLang/julia · GitHub

An alternative is to use IdDict, which hashes based on object identity. A set is simply based on a dictionary whose values are nothing. (In fact, there is an undocumented Base.IdSet that implements this, but doesn’t cover the whole Set API.)