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### Type Ordering <a id="compare.type">[[compare.type]]</a>
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There is an *implementation-defined* total ordering of all types. For
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any (possibly incomplete) types `X` and `Y`, the expression
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`TYPE-ORDER(X, Y)` is a constant expression [[expr.const]] of type
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`strong_ordering` [[cmp.strongord]]. Its value is
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`strong_ordering::less` if `X` precedes `Y` in this
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implementation-defined total order, `strong_ordering::greater` if `Y`
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precedes `X`, and `strong_ordering::equal` if they are the same type.
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[*Note 1*: `int`, `const int` and `int&` are different
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types. — *end note*]
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[*Note 2*: This ordering need not be consistent with the one induced by
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`type_info::before`. — *end note*]
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[*Note 3*: The ordering of TU-local types from different translation
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units is not observable, because the necessary specialization of
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`type_order` is impossible to name. — *end note*]
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``` cpp
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template<class T, class U>
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struct type_order;
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```
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The name `type_order` denotes a *Cpp17BinaryTypeTrait*[[meta.rqmts]]
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with a base characteristic of
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`integral_constant<strong_ordering, `*`TYPE-ORDER`*`(T, U)>`.
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*Recommended practice:* The order should be lexicographical on
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parameter-type-lists and template argument lists.
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