In the documentation, it is said that "This diagnostic is enabled by default.": https://clang.llvm.org/docs/DiagnosticsReference.html#wlarge-by-value-copy However, this is an incorrect statement as it is only the case with a value is provided. https://github.com/llvm-mirror/clang/blob/master/lib/Sema/SemaDecl.cpp#L12081 I could not see how to update the source to provide this info so that clang-tblgen uses it.