Comment (by Roland Haas):

Concerning your questing about mutable: well I guess changing iter_memo was never quite correct since it would have always produced observable differences (since the old iterators would become invalid upon clear()). With C++11 and thread safety the concepts seem to have changed https://softwareengineering.stackexchange.com/questions/379516/is-the-meaning-of-const-still-thread-safe-in-c11 though I try to stay away from these things myself normally :-)

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2426/g-10-possibly-miscompiles-parts-of-carpet