[jcranmer]

There is very strong bias in clang not to emit any diagnostics once you get to the middle-end optimizations, partially because the diagnostics are now based on whims of heuristics, and partially because the diagnostics now become hard to attribute to source code (as the IR often has a loose correlation to the original source code). Indeed, once you start getting inlining and partial specialization, even figuring out if it is worth emitting a diagnostic is painfully difficult.