[issue44242] enum.IntFlag regression: missing values cause TypeError

[Ethan Furman]

Ethan Furman <ethan at stoneleaf.us> added the comment:

That is an intentional change.  The cause is that the masks include bits that are not named in the Flag.

The user-side fix is to add a `boundary=KEEP` option to the flag:

    class GeodIntermediateFlag(IntFlag, boundary=KEEP)

The enum library fix could be one of two things:

- automatically use the KEEP boundary when these conditions arise, and issue a DeprecationWarning; or

- lose that particular check.

I'm inclined to go with option 2, since `boundary` is designed to answer the question of what to do when Flag.A | Flag.B does not exist in Flag.

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<https://bugs.python.org/issue44242>
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