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flatten_iterator::operator++ increments to the next non-empty inner range and unconditionally dereferences the outer iterator to do so (even when the outer iterator is past its end). Correctness-wise, this never uses the result of the dereference, so it is usually fine. However, this actually causes a segfault when the dereferenced memory address is not mapped, which I found occurs in PBBS backForwardBFS. The fix is to give flatten_iterator the end of its outer range and stop there. Both construction sites (flatten, and block_delayed_sequence's whole-sequence iterator) have that end available, and flatten_iterator is used nowhere else. After the last element the iterator now parks at the outer end, which the enclosing forward_delayed_sequence never dereferences because its count has reached zero.
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flatten_iterator::operator++increments to the next non-empty inner range and unconditionally dereferences the outer iterator to do so (even when the outer iterator is past its end). Correctness-wise, this never uses the result of the dereference, so it is usually fine. However, this actually causes a segfault when the dereferenced memory address is not mapped, which I found occurs in PBBS backForwardBFS.The fix is to give
flatten_iteratorthe end of its outer range and stop there. Both construction sites (flatten, andblock_delayed_sequence's whole-sequence iterator) have that end available. After the last element the iterator now stops at the outer end, which the enclosingforward_delayed_sequencenever dereferences because its count has reached zero.