[5.8][stdlib] String: Fix forward implementation of grapheme breaking rule 11 #63047
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Rule GB11 in Unicode Annex 29 is: GB11: Extended_Pictographic Extend* ZWJ × Extended_Pictographic However, our forward grapheme breaking state machine implements it as: GB11: Extended_Pictographic Extend* ZWJ+ × Extended_Pictographic We implement the correct rules when going backward, which can cause String values to have different counts whether we’re going forward or back. The rule as implemented would be fine (Unicode doesn’t care much about the placement of grapheme breaks in invalid sequences), but the directional inconsistency messes with String’s Collection conformance. rdar://104279671 (cherry picked from commit a3e517e)
lorentey
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xedin,
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artemcm
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lorentey
requested review from
Azoy and
stephentyrone
and removed request for
xedin,
slavapestov,
hyp,
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egorzhdan,
artemcm,
hborla,
zoecarver and
eeckstein
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Azoy
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Jan 16, 2023
stephentyrone
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(Cherry picked from #63043.)
Rule GB11 in Unicode Annex 29 is:
However, our forward grapheme breaking state machine implements it as:
We implement the correct rules when going backward, which can cause String values to have different counts whether we’re going forward or back.
The rule as implemented would be fine (Unicode doesn’t care much about the placement of grapheme breaks in invalid sequences), but the directional inconsistency messes with String’s Collection conformance.
rdar://104279671