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The ValueObjectConstResult classes that back expression result variables play a complicated game with where the data for their values is stored. They try to make it appear as though they are still tied to the memory in the target into which their value was written when the expression is run, but they also keep a copy in the Host which they use after the value is made (expression results are "history values" so that's how we make sure they have "the value at the time of the expression".) However, that means that if you ask them to cast themselves to a value bigger than their original size, they don't have a way to get more memory for that purpose. The same thing is true of ValueObjects backed by DataExtractors, the data extractors don't know how to get more data than they were made with in general. The only place where we actually ask ValueObjects to sample outside their captured bounds is when you do ValueObject::Cast from one structure type to a bigger structure type. In https://reviews.llvm.org/D153657 I handled this by just disallowing casts from one structure value to a larger one. My reasoning at the time was that the use case for this was to support discriminator based C inheritance schemes, and you can't directly cast values in C, only pointers, so this was not a natural way to handle those types. It seemed logical that since you would have had to start with pointers in the implementation, that's how you would write your lldb introspection code as well. Famous last words... Turns out there are some heavy users of the SB API's who were relying on this working, and this is a behavior change, so this patch makes this work in the cases where it used to work before, while still disallowing the cases we don't know how to support. Note that if you had done this Cast operation before with either expression results or value objects from data extractors, lldb would not have returned the correct results, so the cases this patch outlaws are ones that actually produce invalid results. So nobody should be using Cast in these cases, or if they were, this patch will point out the bug they hadn't yet noticed.