[Sharlin]

> return a+a+a+a; which is certainly an interpretation.

Zero is the only valid index of &a, so I presume the compiler just assumes that all the indexes in 1[&a] + 2[&a] etc must be zero. Even though they're in this case compile-time constants – the optimizer could check but why bother given that it's UB anyway. I assume modern C/C++ compilers have some flag to diagnose indexing that's known to be OOB at compile time.

[mananaysiempre]

I’m so used to sticking -Wall in my compilation flags the moment I write a build script that I didn’t realize it wasn’t there for this quick experiment. Yes, thank you, there are indeed diagnostics once you ask for them:

  test.cpp: In function ‘double solve(double, ...)’:
  test.cpp:4:20: warning: array subscript 1 is outside array bounds of ‘double [1]’ [-Warray-bounds=]
      4 |     return a + 1[&a] + 2[&a] + 3[&a];
        |                ~~~~^
  test.cpp:3:58: note: at offset 8 into object ‘a’ of size 8
      3 | extern "C" __attribute__((noinline)) double solve(double a, ...) {
        |                                                   ~~~~~~~^
  [repeat twice more for the other two accesses]