16 Language support library [language.support]

16.11 Comparisons [cmp]

16.11.3 Class template common_­comparison_­category [cmp.common]

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The type common_­comparison_­category provides an alias for the strongest comparison category to which all of the template arguments can be converted.
[ Note
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A comparison category type is stronger than another if they are distinct types and an instance of the former can be converted to an instance of the latter.
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template<class... Ts> struct common_comparison_category { using type = see below; };
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Remarks: The member typedef-name type denotes the common comparison type ([class.spaceship]) of Ts..., the expanded parameter pack.
[ Note
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This is well-defined even if the expansion is empty or includes a type that is not a comparison category type.
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 ]