Compile-time conditionals, and where a location comes from

Decided

D-0048, settled by the language’s author: part of spec §18. Older text cites it as 18.48.

#if FLAG { … } #else #if … { … } #else { … } around declarations, members and statements, over registered flags only. Built-ins DEBUG/RELEASE/SHIPPING and OS_WINDOWS/OS_LINUX/OS_MACOS; a package’s own under [flags]; a dependency’s read as dep::FLAG and turned on by the dependent’s flags = [...] or --flag. An unknown flag is an error (DZ0309). The inactive branch is parsed but not checked. Chosen over checking both branches, which would forbid naming platform APIs absent on the host, and over free-form defines, which let a typo compile nothing. An intrinsic default argument is the caller’s: loc: SourceLocation = #location records the call site, column 0 (§12.7, §12.10)