[DHowett]

> Bounds checking and assertions shouldn’t change the type layout.

Any bounds checks and assertions that rely on storing additional data such as valid iterator ranges or mutation counters would need to change the type layout, wouldn't they?

Even if the STL were purely a header-only library (and influenced only by code generation changes for debug builds), there's still the problem of ABI compatibility across different translation units--or different libraries--which might be built with different options.

EDIT: One of your sibling comments goes into greater detail!