The standard's policy on noexcept is to only mark functions that cannot or must not fail, but not those that simply are specified not to throw exceptions. In other words, all functions that have a limited domain (pass the wrong arguments and you get undefined behavior) are not noexcept, even when they are not specified to throw.
Functions that get marked are things like swap (must not fail, because exception safety often relies on that) and numeric_limits::min (cannot fail, returns a constant of a primitive type).
The reason is that implementors might want to provide special debug versions of their libraries that throw on various undefined behavior situations, so that test frameworks can easily detect the error. For example, if you use an out-of-bound index with vector::operator[], or call front or back on an empty vector. Some implementations want to throw an exception there (which they are allowed to: since it's undefined behavior, they can do anything), but a standard-mandated noexcept on those functions makes this impossible.
与恶龙缠斗过久,自身亦成为恶龙；凝视深渊过久,深渊将回以凝视…
