https://issues.dlang.org/show_bug.cgi?id=14269
--- Comment #34 from Walter Bright <bugzi...@digitalmars.com> --- Here's the change that did it: https://github.com/D-Programming-Language/dmd/pull/4177/files Essentially, it's the removal of the toBasetype() call. The problem with deprecating rather than erroring is this function is not checking for errors, it does match levels, which can cause various changes upstream. --