4 ms·

That is incorrect. A C++ compiler is perfectly allowed to reject any program that contains undefined behaviour. It's just not required to do so.

leni536 2y ago
It's allowed to reject programs that will evaluate an undefined operation on every possible execution. This is pretty much impossible, so no compiler even attempts this.
- mgaunard 2y ago
  It can replace any path with undefined behaviour to one that does std::abort.
  - pjmlp 2y ago
    It can, but in practice no one does it.
    - majoe 2y ago
      When you think about it, that's what -fsanitize=undefined is doing. It detects undefined behaviour at runtime and aborts the program.
      - leni536 2y ago
        It detects some, but not all UBs. AFAIK it can't detect UB due to hnsequenced access to the same variable.
        mgaunard 2y ago
        There are different sanitizers that detect different types of UB. Asan, Msan, Tsan and UBSan all detects different ones.
      - deleted 2y ago
        [deleted]
  - leni536 2y ago
    Which is very different from diagnosing at compile time.
tialaramex 2y ago
That would be easy, the problem is that Rice's Theorem tells us the compiler definitionally cannot be sure whether every possible source code represents a valid program if it has any non-trivial semantic requirements (C++, like Rust, has a whole lot of such requirements). So while "What should we do when we're sure it's not a valid program?" is easy and so it's very silly that some C++ compilers will compile that anyway, it's not enough, you also need to decide "What should we do when we're not sure?" and there the ISO document provides a firm answer, which all the compiler vendors follow - press on anyway, too bad. "Ill-formed - No Diagnostic Required" is how the ISO document describes this situation. The diagnostic isn't required because it's technically impossible to guarantee. You might insist that "Not sure" can't happen. Unfortunately the reason it's called Rice's Theorem is that a guy named Henry Rice got himself a Maths PhD proving this in 1951.

leni536 2y ago

It's allowed to reject programs that will evaluate an undefined operation on every possible execution. This is pretty much impossible, so no compiler even attempts this.

mgaunard 2y ago
It can replace any path with undefined behaviour to one that does std::abort.
- pjmlp 2y ago
  It can, but in practice no one does it.
  - majoe 2y ago
    When you think about it, that's what -fsanitize=undefined is doing. It detects undefined behaviour at runtime and aborts the program.
    - leni536 2y ago
      It detects some, but not all UBs. AFAIK it can't detect UB due to hnsequenced access to the same variable.
      - mgaunard 2y ago
        There are different sanitizers that detect different types of UB. Asan, Msan, Tsan and UBSan all detects different ones.
    - deleted 2y ago
      [deleted]
- leni536 2y ago
  Which is very different from diagnosing at compile time.

tialaramex 2y ago

That would be easy, the problem is that Rice's Theorem tells us the compiler definitionally cannot be sure whether every possible source code represents a valid program if it has any non-trivial semantic requirements (C++, like Rust, has a whole lot of such requirements). So while "What should we do when we're sure it's not a valid program?" is easy and so it's very silly that some C++ compilers will compile that anyway, it's not enough, you also need to decide "What should we do when we're not sure?" and there the ISO document provides a firm answer, which all the compiler vendors follow - press on anyway, too bad. "Ill-formed - No Diagnostic Required" is how the ISO document describes this situation. The diagnostic isn't required because it's technically impossible to guarantee. You might insist that "Not sure" can't happen. Unfortunately the reason it's called Rice's Theorem is that a guy named Henry Rice got himself a Maths PhD proving this in 1951.