This attribute is meant to be used in function (member or free, static or non-static, template or non-template) declarations to indicate that the function declaration is left unimplemented (without a definition) with an additional diagnostic message that can be fixed or opt-in thru an unevaluated string literal.
The interesting bit comes from its behaviour: defining any function marked with such attribute, lleads to an ill-formed program (preferably with a diagnostic message), making calls to it be always diagnosable in non-linking time, because no TU can ever define this.
The only catch so far is: redeclarations w/o this attribute would break that assumption. I thought of straight banning redeclarations but I think that’s too much. Maybe forcing redeclarations to have this attribute aswell, once the former declaration has it, could be an interesting idea to explore.
What do you all think?