Can we get this reopened? As David MacIver points out, this seems entirely a wart in tuple's constructor (compared to all the other builtin types), whereas 10977 is worrying about how 3rd party code using the C API can corrupt subclasses of builtin types (a much larger scope, and much less likely to be resolved in a good way).
Does it make sense to require python code wishing to case a tuple or tuple subclass do so using tuple(list(o)), or should tuple(o) work as expected? The primary use is of course converting a mutable sequence to an immutable representation to use as a dict key.