[AlotOfReading]

There's a line in the standards that basically says a conforming program is anything acceptable by a conforming implementation. In theory you could have an implementation that gives semantics to UB like Fil-C or CCured do. No mainstream implementation does that for memory unsafety due to the performance overhead, and conforming implementations are required to document those extensions. I don't think there's a sane argument for an implementation to intentionally choose the behavior in the example I provided and Clang certainly doesn't, so it's non-conformant regardless.

[jkrejcha]

> No mainstream implementation does that for memory unsafety due to the performance overhead

It depends on what is considered memory safety here (especially when some of them are arguably unforced errors in the standards), but many implementations do in fact have options for this ("no delete null pointer checks" for example is an example of one such option, for example, which is used extensively by the Linux kernel for example).

The performance impact tends to be much more negligible outside of, sometimes contrived, benchmarks, especially when compared to algorithmic efficiencies or the like.

[tialaramex]

> There's a line in the standards that basically says a conforming program is anything acceptable by a conforming implementation.

Perhaps it "basically" says that, but it certainly doesn't appear to literally say any such thing, so you're going to need to specify where you believe you saw this so that I can have any idea what it actually says.

[AlotOfReading]

C standard N3096, Section 4:

    A conforming program is one that is acceptable to a conforming implementation.

That definition goes all the way back to C89. The C++ standard drops it for the term "well-formed program", but adds enough clarifications in 1.4 to mean essentially the same thing.

[tialaramex]

Ah, no. Most C++ programs that compile are not well-formed programs. This functions as an escape hatch for Rice's Theorem. You see, C++ even more so than C has semantic requirements - but Rice says all non-trivial semantic requiremnts are Undecidable. So, if you want what C++ says it wants it appears that compilers would be entirely impossible and that's awkward. To "fix" that C++ says it's fine if the compiler will compile your program even though it is not well-formed, the program doesn't have any meaning of course, only well-formed programs have meaning, but it did compile, so as a programmer you're happy...

C++ has a recurring phrase in its standard document "Ill-formed No Diagnostic Required" or IFNDR which carries this intent. The compiler can't tell you made a mistake, but you didn't actually write a valid C++ program so -shrug-

Because there's no way to tell for sure without exhaustive human examination we don't know for sure how many C++ programs aren't actually well-formed but experts who've thought about it tend to think the answer for large C++ software projects is all or most of them.