SE-0415: Function Body Macros

[Douglas_Gregor]

@stephencelis

@Douglas_Gregor

Define composition of body macros in this proposal such that later body macros see the function bodies produced by earlier body macros. This deviates slightly from the way other macros work, in that macros generally don't see the outputs of other macros, but is not terribly complicated.

If this route were taken would the limitation of existing macros be re-evaluated/lifted?

I'm not enthusiastic about doing that. Macros are intended not to step on each other---you can expand one macro and see what it does, without having to carefully follow the order of expansion to get to the result. Body macros today don't compose today because they're following that same philosophy.

@stephencelis

I was recently working on a member attribute macro and wanted to annotate members with a "marker" of sorts so that a peer/accessor macro could witness this annotation and change its expansion accordingly.

Since the member-attribute macro would be present on an enclosing declaration, it should be possible to address this by looking at the parent context when the API becomes available. It means duplicating some logic, but it also means you can reason about the inner macro expansion without having performed the outer macro expansion.

Doug