sequential_or: Fixed random order execution of underlying parsers

[Kojoley]

After two hours of fighting with the optimizer, I happily drew attention to
this little nifty bitwise operator in any_if_ns function.

Explanation of the bug: bitwise inclusive OR operator is not a sequence point
(per chapter §5.13 of C++14 standard), that's why at compiling the expression
a() | b() | ... | z() optimizer is allowed to rearrange the execution order
of the functions a, b ... z.

There is high chance that a lot of people were misguided by the bug and chose
not to use sequential_or.

I vaguely remember how about three years ago I thought that I am dumb and/or
the documentation is wrong when I tried to use the sequential_or but ended
with some workaround.

There are three possible fixes:

• This one
• Make the original any_ns and any_if_ns strict ordered
  (could potentially make permutations operator slower)
• Break the any_ns and any_if_ns API and somehow generalize the code

[djowel]

Very interesting insights! I honestly haven't thought about this until now! Do we have tests for this?

[Kojoley]

There cannot be a test for this particular problem. For me it happened on sequential_or tests. The bug depends on compiler optimization strategy, inlining aggressiveness and phase of the moon .

[Kojoley]

Or if someone know a test for i++ + ++i problem - I can adopt it for our case.

Example:

#include <cstdio>

int main()
{
    return printf("Hello") | printf("World");
}

The example produces "WorldHello" for me on MSVC on any optimization strategy except Od and with any inlining strategy, but it prints "HelloWorld" on GCC and Clang with everything I have tried.

From my point of view if such a test exists - it requires some preprocessor pragma or compiler argument. Actually this is a good idea for an in-compiler-fuzzer, but again, the bug will appear on already existing tests and does not require a dedicated test.

[djowel]

OK, well, I haven't studied your solution yet, but could you give me a quick explanation on how the solution works?

[Kojoley]

It is very simple, the crucial part is b1c74d0.

[Kojoley]

I cannot setup Appveyor on the repository until the problem is fixed. Should I remove sequence_or tests from the test suit on CI?

[djowel]

I understand now. Sorry for the delay. Please go ahead.