clang complains that a constexpr definition of methods[] cannot refer to
members of the incomplete Impl template parameter, and rightly so.
Making these const is sufficient for our purposes, and that enables us
to move the declaration outside of the class, where it will be
instantiated lazily (presumably at the point when |Impl| is a complete
class definition). We also need to declare the length of methods[], as
other parts of the code require knowing the length of methods[] at
compile time.
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