Annex D (normative) Compatibility features [depr]

D.2 Arithmetic conversion on enumerations [depr.arith.conv.enum]

The ability to apply the usual arithmetic conversions ([expr.arith.conv]) on operands where one is of enumeration type and the other is of a different enumeration type or a floating-point type is deprecated.
[Note 1: 
Three-way comparisons ([expr.spaceship]) between such operands are ill-formed.
— end note]
[Example 1: enum E1 { e }; enum E2 { f }; bool b = e <= 3.7; // deprecated int k = f - e; // deprecated auto cmp = e <=> f; // error — end example]