Requires: ForwardIterator’s value type shall be convertible to Predicate’s argument type. [first, last) shall be partitioned by pred, i.e. all elements that satisfy pred shall appear before those that do not.
The definition of undefined behaviour is (emphasis mine, page 19):
behavior for which this International Standard imposes no requirements [ Note: Undefined behavior may be expected when this International Standard omits any explicit definition of behavior or when a program uses an erroneous construct or erroneous data. Permissible undefined behavior ranges from ignoring the situation completely with unpredictable results, to behaving during translation or program execution in a documented manner characteristic of the environment (with or without the issuance of a diagnostic message), to terminating a translation or execution (with the issuance of a diagnostic message). Many erroneous program constructs do not engender undefined behavior; they are required to be diagnosed. Evaluation of a constant expression never exhibits behavior explicitly specified as undefined (8.20). — end note ]
If this is still not convincing enough unspecified behaviour talks about correct data (emphasis mine, page 19):
behavior, for a well-formed program construct and correct data, that depends on the implementation [ Note: The implementation is not required to document which behavior occurs. The range of possible behaviors is usually delineated by this International Standard. — end note ]
It might be a bit strong to have UB for this, but it's not extraordinary. If you take sorted-range algorithms for example binary_search, whenever when you don't pass in a sorted range, the behaviour is undefined according to Nico Josuttis' book on the standard library, section 11.10. This is not much different, but sadly the book says nothing explicit about this case.
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Having a look at available draft standard from 2017 this seems undefined to me.
The requirements on
partition_pointsay:The definition of undefined behaviour is (emphasis mine, page 19):
If this is still not convincing enough unspecified behaviour talks about correct data (emphasis mine, page 19):
It might be a bit strong to have UB for this, but it's not extraordinary. If you take sorted-range algorithms for example
binary_search, whenever when you don't pass in a sorted range, the behaviour is undefined according to Nico Josuttis' book on the standard library, section 11.10. This is not much different, but sadly the book says nothing explicit about this case.