On 08/19/2018 03:25 PM, Nick Desaulniers wrote: > + gbiv who wrote this cool paste (showing alternatives to > _Static_assert, which is supported by both compilers in -std=gnu89, > but not until gcc 4.6): https://godbolt.org/g/DuLsxu > > I can't help but think that BUILD_BUG_ON_MSG should use > _Static_assert, then have fallbacks for gcc < 4.6. Unfortunately _Static_assert is a woefully inadequate replacement because it requires a C constant expression. Example: int a = 1; _Static_assert(a == 1, "a != 1"); results in "error: expression in static assertion is not constant." Language standards tend to shy away from defining implementation details like optimizations, but we need to have completed a good data flow analysis and constant propagation in order to do BUILD_BUG_ON_MSG, et. al.; this is why they only work when optimizations are enabled. As the optimizer improves, new expressions can be used with BUILD_BUG_ON*. I did an analysis of this back in 2012 of how various types of variables could be resolved to constants at compile-time and how that evolved from gcc 3.4 to 4.7: https://drive.google.com/open?id=1cQRAAOzjFy6Aw7CDc4QauHvd_spVkd5a This changed again when -findirect-inline was added -- i.e., BUILD_BUG_ON could be used on parameters of inline functions even when called by pointer, although the caller needed __flatten in some cases -- a bit messy. Daniel