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Guard against passing endIndex to foreignScalarAlign when back-deploying to 5.0 stdlibs #27779

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Oct 18, 2019
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Guard against passing endIndex to foreignScalarAlign when back-deploying to 5.0 stdlibs #27779

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7 changes: 7 additions & 0 deletions stdlib/public/core/UnicodeHelpers.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -186,6 +186,13 @@ extension _StringGuts {
return String.Index(_encodedOffset: idx._encodedOffset)._scalarAligned
}
if _slowPath(self.isForeign) {
// In 5.1 this check was added to foreignScalarAlign, but when this is
// emitted into a client that then runs against a 5.0 stdlib, it calls
// a version of foreignScalarAlign that doesn't check for this, which
// ends up asking CFString for its endIndex'th character, which throws
// an exception. So we duplicate the check here for back deployment.
guard idx._encodedOffset != self.count else { return idx._scalarAligned }

let foreignIdx = foreignScalarAlign(idx)
_internalInvariant_5_1(foreignIdx._isScalarAligned)
return foreignIdx
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