My argument is that what's happening here in C++ is not merely Undefined Behaviour. UB is a runtime thing, at runtime your program tried to do this operation w

My argument is that what's happening here in C++ is not merely Undefined Behaviour. UB is a runtime thing, at runtime your program tried to do this operation we said mustn't ever happen, so all bets are off. C++ also has this idea of programs which are Ill-Formed, No Diagnostic Required, meaning that this was never a C++ program, the ISO document doesn't tell you what this program will do, and a compiler isn't expected to tell you that you didn't write a C++ program in these cases either, so too bad but it's not our fault. IFNDR is there because these are non-trivial semantic properties, so by Rice's Theorem they're Undecidable, the compiler cannot be sure if every arbitrary program meets the semantic requirement. The ISO C++ document would need to document a check which can be performed, allowing compilers to accept only programs which pass that check, or something similar giving enough rope for them to avoid cases where your program compiles in Clang but not MSVC or vice versa, that sounds awkward to write, instead it just YOLOs the whole problem with IFNDR -- if any program unknowingly does not meet these requirements it was silently never a C++ program anyway, not our fault it doesn't work. As it has grown over the decades, C++ has added more, and more, and more IFNDR to the text, because it sure is easier to wave away such programs as "That isn't C++" than to either insist compiler vendors write more diagnostics (Boring! Expensive!) or define the language more carefully so as to make the wrong programs impossible to write. They do not have a comprehensive list of IFNDR (nor UB) in their language today. My belief (and some well known C++ people agree) is that the vast majority of real world C++ programs today are Ill-Formed by these definitions and so do not have and never had any meaning by the text of the standard. I think this is substantially worse than the situation in C.