Sequence points - is this gcc warning a bug?

Question

Take this toy program:

int main() 
{
  int a = 1;
  a = ++a ;
}

Compilation under gcc 9.3 (or even trunk) with -Wall -std=c++17 outputs:

<source>:4:5: warning: operation on 'a' may be undefined [-Wsequence-point]
    4 |   a = ++a ;
      |   ~~^~~~~

I'm aware that in the past this indeed was an issue, but a different answer to the same SO question points to the part of the C++17 standard that was fixed to indicate that in an assignment operations the rhs is sequenced before the actual assignment:

8.18. ... In all cases, the assignment is sequenced after the value computation of the right and left operands, and before the value computation of the assignment expression.

So is this warning indeed bogus or am I missing something?

That answer says that c++17 'proposes' to do so, not that it actually does. — Cortex0101, Mar 30 '20 at 19:09
interesting to see a question around this issue that is not asking what has been asked a million times before, +1 — 463035818_is_not_an_ai, Mar 30 '20 at 19:09
@Cortex - The quote I put is from the C++17 standard directly - I verified this was integrated. — Ofek Shilon, Mar 30 '20 at 19:13
The historical name of the warning option aside, they’re not called “sequence points” anymore. Now things are just sequenced (before/after each other) or they’ve not. — Davis Herring, Mar 31 '20 at 02:29

score 8 · Accepted Answer · answered Mar 30 '20 at 19:16

Yes, this is a bug. Per [expr.ass]/1

The assignment operator (=) and the compound assignment operators all group right-to-left. All require a modifiable lvalue as their left operand and return an lvalue referring to the left operand. The result in all cases is a bit-field if the left operand is a bit-field. In all cases, the assignment is sequenced after the value computation of the right and left operands, and before the value computation of the assignment expression. The right operand is sequenced before the left operand. With respect to an indeterminately-sequenced function call, the operation of a compound assignment is a single evaluation. [ Note: Therefore, a function call shall not intervene between the lvalue-to-rvalue conversion and the side effect associated with any single compound assignment operator. — end note ]

^{emphasis mine}

There is now a sequence point between the increment and the assignment and the code has well defined behavior. Their warning heuristics needs to be updated to take this new feature into account.

@OfekShilon No problem. Glad to help – NathanOliver Mar 30 '20 at 19:47 — NathanOliver, Mar 30 '20 at 19:47

score 4 · Answer 2 · edited Mar 31 '20 at 09:21

4

As already mentioned in the other answer, the code is well-behaved and the message a false-positive for C++17, but I want to add that GCC is purposefully still warning about it.

The GCC documentation states about the -Wsequence-point warning flag (enabled by -Wall):

The C++17 standard will define the order of evaluation of operands in more cases: in particular it requires that the right-hand side of an assignment be evaluated before the left-hand side, so the above examples are no longer undefined. But this option will still warn about them, to help people avoid writing code that is undefined in C and earlier revisions of C++.

So it is intended that this program gives that warning.

edited Mar 31 '20 at 09:21

Ofek Shilon

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answered Mar 31 '20 at 08:52

walnut

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Thanks for pointing that out! It actually looks very much like a bug disguised as a feature. This warning should have been eliminated on -std=C++17, and if people need conformance to earlier standards they can compile to it. "Helping people avoid code that is UB in older standards" is a weird design goal at best. – Ofek Shilon Mar 31 '20 at 09:25

Sequence points - is this gcc warning a bug?

2 Answers2