Notes taken at the C++ modules BoF
Attendees: 91

Semi-BoF on modules & header units

	* header units
		* build performance (several 100K files of tokenizing/processing), imports become very low cost
		* protection from pollution with preprocessor noise
		* protection of accidental exports
		* makes it harder to violate ODR


	* similar to PCH but language native
	* see https://gcc.gnu.org/git/?p=gcc.git;a=shortlog;h=refs/heads/devel/c%2B%2B-modules
	* header-units compiled into cached artifact, not an object file / redistributable
	* relies on ODR rather than #ifdef guards
	* ODR tricky with anonymous types (enums!)
	* ODR even applies across modules that might import locally consistent but globally conflicting type definitions
	* inline functions declared static vs. used/exported hacked around in specs
	* g++ on branch starting to detect/check above cases
	* requires building first; need enumeration facility ("what header units exist?")

Kernel vs. glibc
	* headers sometimes conflict, in annoying & irrelevant ways; glibc being relatively constrained by POSIX, kernel less so :-)
	* might need a glibc-specific namespace wrapper to fully isolate the two
	* c++ committee thinking about c++ module wrapping for glibc
	* -flto already encounters some of these conflicts
	* gcc middle-end may consider types equivalent more so than at the language level
	* Iain Sandoe, conditional code in one or other header?  Not likely due to e.g. consistency with printf formatting directives (long long => %lld).
	* DJ, why not intXX_t?  Different language level types, plus historical compatibility length-varying "long" decls.