Function body macros

[Douglas_Gregor]

@wadetregaskis

Tangentially, there are similar use-cases which require being able to modify compile-time constants and/or execute at module load time, e.g. a @Registered macro which inserts a tagged function into a list of handlers for a server (a list that's either an implicitly-referenced global or passed as an argument to the macro).

This is the reason why SE-0385 was returned for revision.

Doug