constraint_levelenum class constraint_level : /* unspecified */ {
none, nontrivial, nothrow, trivial
};
constraint_level defines the constraints on a type’s lifetime operations (construction, relocation, or destruction). For a given operation O:
none: No restrictions on O.nontrivial: O shall be supported.nothrow: O shall be supported and shall not throw.trivial: O shall be supported and shall be trivial.The specific semantics of each value depends on its context.