On Sat, May 8, 2021 at 3:04 PM JF Bastien via SG12 <sg12@lists.isocpp.org> wrote:
On Sat, May 8, 2021 at 11:52 AM Nevin Liber via SG12 <sg12@lists.isocpp.org> wrote:
On Thu, May 6, 2021 at 6:47 PM JF Bastien via SG12 <sg12@lists.isocpp.org> wrote:
Compilers diagnose when functions can't be proved to return, and I wouldn't work on a codebase without this diagnostic enabled as an error. Is there a valid reason to keep this UB around?

How does one write an assert-type macro which, when it is disabled, still prevents this type of warning/error?  Because people do write:

my_assert(false);

to mean abort in debug mode, take my chances in release mode.

I don’t think I understand what you’re asking... but it sounds like 

#ifdef _DEBUG
#define YOLO() abort()
#else
#define YOLO() std::unreachable() // or __builtin_unreachable()

That looks right to me.

JF,
(1) You originally mentioned "[[noreturn]]" in the same breath with "unreachable," which is confusing to me. Suppose I write a function `f1` declared `[[noreturn]]`, which "seemingly falls off the end"; say, `void g(); [[noreturn]] int f1() { g(); }` — Does f's code clearly indicate my intent that "I know g() is non-returning, so please don't warn here," or does f's code suggest to the compiler that I've written a bug?
Would you make it so that a conforming compiler required to diagnose this code as ill-formed?  Or required not to diagnose it?

(2) How do we standardize the idea that the compiler should be able to do control flow analysis here? I mean, if we have, like, `int f2() { while (true) { if (rand()) return 42; } }`, is a conforming compiler supposed to be smart enough to see the end of the function is unreachable?  Or what if it's, like,
    int f3(bool b) {
        if (b) { return 42; }
        if (!b) { abort(); }  // or std::unreachable(), your choice
    }
Would you make it so that a conforming compiler required to diagnose this code as ill-formed?  Or required not to diagnose it?

(3) Alternatively, are you proposing that a conforming compiler should be forced to insert a call to `std::terminate()` at the bottom of any function it can't prove isn't reached? (That would be nicely consistent with the big oops C++ made with `noexcept`.)

Basically, "needs a paper, needs Tony Tables."

my $.02,
–Arthur