Sequence points - is this gcc warning a bug?

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.

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.