Consistently treat optional-before-required as required

There was a loophole here when it came to usage with named arguments,
which was not intended. Close the loophole thoroughly by act

Consistently treat optional-before-required as required

There was a loophole here when it came to usage with named arguments,
which was not intended. Close the loophole thoroughly by actually
dropping the default value from the signature entirely. The default
is still used to make the type nullable, but not for anything else.

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