Decided to keep the strong_ordering semantics, as user code is very much guaranteed to rely on it. Update the docs. [ChangeLog][Potentially Source-Incompatible Changes] Made several operators (relational, QDebug streaming, etc) hidden friends. This may break code that calls these operators a) with types that require implicit conversion on the arguments, or b) using member-function syntax (lhs.operator<(rhs)). The backwards-compatible fix for (a) is to make at least one of the argument's implicit conversions explicit. For (b), use infix notation (lhs < rhs). [ChangeLog][EDITORIAL] Replace all "Potentially Source-Incompatible Changes" items that mention making operators hidden friends with the catch-all one from the QPersistentModelIndex change. Fixes: QTBUG-117660 Change-Id: I8175af5e2cb3c77e7486e972630b9410b3a0896c Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>