• You don't control your ABI. For example, you can't reserve a register for error object return like Swift does. • You don't control your stack and stack frame

• You don't control your ABI. For example, you can't reserve a register for error object return like Swift does. • You don't control your stack and stack frames. You can't implement a segmented stack, for example. • You have very limited options for unwinding. setjmp doesn't really cut it. You could leverage C++ exceptions or platform-specific things like SEH, but you can't design anything radically different. • You can't have debug information fancier than #line and maybe reusing C++ name mangling. You can't make debuggers see your higher-level constructs and data as anything other than their lowering. • You have very limited options for specifying pointer aliasing. • If you don't want to inherit UB from C, you'll have to be very clever about using alternative constructs that don't cause UB while still compile to efficient code you wanted. • If you mean standard C, not C with compiler extensions, then look at the whole list of non-standard intrinsics and built-ins for things C can't do.