Annex D (normative) Compatibility features [depr]

D.1 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.
Three-way comparisons ([expr.spaceship]) between such operands are ill-formed.
end note
enum E1 { e };
enum E2 { f };
bool b = e <= 3.7;              // deprecated
int k = f - e;                  // deprecated
auto cmp = e <=> f;             // ill-formed
end example