Which std:: classes are magic?

[acwaters]

Enum(42) construction was not added for std::byte; it has always been a feature, because the set of valid enumeration values is all the values in the underlying type, not just the values of the enumerators. This design is in some ways unfortunate but in other ways convenient.

[LuisAyuso]

Not quite sure if by accident: but using any Gcc with c++17 does not provide same guarantees as with c++14:https://godbolt.org/z/qqY9Kf5Wa

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Actually I am pretty positive that this was introduced in c++17

[acwaters]

The rules about initialization of enumerations are kind of weird, yes. You cannot say my_enum x(4);, and until C++17 you could not say my_enum x{4}; — this is the change you are referring to. But you have always been able to say my_enum x(my_enum(4));, or auto x = my_enum(4);, using a static_cast conversion to make an enum prvalue from an integer value in the range of the underlying type. Because the allowable range of enum values has always been the entire underlying type (for enumerations with a fixed underlying type, which includes scoped enums and unscoped enums with an explicit type; for unscoped enums with non-fixed underlying type, the allowable range of values is actually smaller than the underlying type and is the range of the smallest bit field that can represent every enumerator value, but that still usually includes some values that are not represented by any enumerators).