So I was recently trying to optimise a string substitution function by reserving capacity. however this was being used at compile time (mixin) and the body of arr.reserve(n) was not available causing a bug that was easy to observe but very difficult to determine why. As a result code was taking a wrong static if branch. I know __traits(compiles, ... ) is a blunt tool but it feels very wrong to have a function that works and complies at runtime, fail to compile at compile time and not receive any warning.