[Merged by Bors] - fix(algebra/ordered_monoid): Ensure that ordered_cancel_comm_monoid can provide a cancel_comm_monoid instance
#5713
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… can provide a `cancel_comm_monoid` instance
eric-wieser
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Jan 12, 2021
src/algebra/group/defs.lean
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| @@ -285,7 +285,7 @@ class add_cancel_comm_monoid (M : Type u) | |||
| extends add_left_cancel_comm_monoid M, add_right_cancel_comm_monoid M | |||
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| /-- Commutative version of cancel_monoid. -/ | |||
| @[protect_proj, ancestor right_cancel_comm_monoid left_cancel_comm_monoid, to_additive add_cancel_comm_monoid] | |||
| @[protect_proj, ancestor left_cancel_comm_monoid right_cancel_comm_monoid, to_additive add_cancel_comm_monoid] | |||
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Turns out that by getting this backwards, to_additive would map to_left_cancel_comm_monoid to to_add_right_cancel_comm_monoid! The order matters.
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Perhaps we should add a warning about this to the to_additive doc entry?
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Sounds like a job for #5338
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Yes, fair enough.
Thanks!
bors r+
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… can provide a `cancel_comm_monoid` instance (#5713)
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… can provide a `cancel_comm_monoid` instance (#5713)
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