Diagnostics in an expansion

Decided

D-0079, settled by the language’s author: part of spec §18. Older text cites it as 18.79.

Where the code was written: in captured code at the user’s own text; in code the macro wrote at the invocation’s name, with an “in expansion of” note. A mistake in a capture copied to several places is reported once, a parameter copied to several places is unused only if every copy leaves it unused, and a warning about a macro’s own local is not shown to its user (§12.6)