Compile with "dmd -inline": import core.bitop; void main() { ulong y = 0x12345678; ulong x = bitswap(y); } /dlang/dmd/linux/bin64/../../src/druntime/import/core/bitop.d(817): Error: function `core.bitop.bitswap` cannot inline function Could also be considered a druntime issue as it uses pragma(inline,true).
Something fixed this between 2.091.1 and 2.092.1.