fails:Hash#[]= associates the key with the value and return the value
fails:Hash#[]= duplicates string keys using dup semantics
fails:Hash#[]= stores unequal keys that hash to the same value
fails:Hash#[]= accepts keys with private #hash method
fails:Hash#[]= duplicates and freezes string keys
fails:Hash#[]= doesn't duplicate and freeze already frozen string keys
fails:Hash#[]= raises a RuntimeError if called on a frozen instance
fails:Hash#[]= does not raise an exception if changing the value of an existing key during iteration
