Qt makes the assumption that since libstdc++ and glibc have yet to take advantage of sized deallocation, they can get away with intentionally passing invalid sizes. Qt can potentially cause silent memory corruption bugs with jemalloc because it implements sized deallocation...
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C++14 added sized deallocation as an allocator optimization but this broken code was always undefined. These bugs are reported as new-delete-type-mismatch by ASan and can cause memory corruption. A review of the problem areas in core Qt code uncovered a bunch of other problems...