I just don't understand how "std::string of constant size" leads to the compiler inferring that a variable is undefined along a particular code path. In particular, on the not-taken code path, what is the uninitialized variable being branched on?

Edit: Is it figuring out that the pointer in the string object is uninitialized? That doesn't seem any easier than reasoning about the length value, and I don't see how it would lead to "br undef".