This is the definition of layout-compatible for enumerations, and is referenced by the definition of layout-compatible in general in [basic.types]/11:

"Two types cv1 T1 and cv2 T2 are layout-compatible types if T1 and T2 are the same type, layout-compatible enumerations (7.2), or layout-compatible standard-layout class types (9.2)."

How about this instead:

Two enumeration types are \defn{layout-compatible enumerations} if they have the same

It looks like a similar change is warranted in [class.mem]. We probably also want the index entry to be "layout-compatible!enumerations" rather than "layout-compatible enumerations".

I thought about that approach, but since we don't appear to use "layout-compatible enumeration" (or "layout-compatible class") anywhere else in the standard aside from this instance, it seemed more sensible to remove the definition from dcl.enum instead of adding one-time use definitions.

Well, [dcl.enum] is where we define what "layout-compatible" means for enumeration types. The wording in [basic.types] is really just saying that the definition is split across two other clauses. It makes much more sense for the index entries to point to the useful definition, in my opinion.

I've come around to your logic, and I think I've implemented what you were suggesting (though I freely admit that index entries are new to me, so I may not have done the entries properly).

Looks good! @zygoloid: PTAL.