Hi Jonathan, Thanks for the investigation and the explanation. Is there anything left for me to do here? I.e. should this go into a dedicated bug report? And in case of yes, should I open one? Thanks, Thomas On Wed, 27 Nov 2024 at 13:24, Jonathan Wakely <jwakely.gcc@xxxxxxxxx> wrote: > On Tue, 26 Nov 2024 at 14:55, Jonathan Wakely <jwakely.gcc@xxxxxxxxx> > wrote: > > > > On Tue, 26 Nov 2024 at 14:07, Thomas Madlener <thomas.madlener@xxxxxxxxx> > wrote: > > >> > > >> > // But this DOES > > >> > if (!(a1 = a2)) return 1; > > >> > > >> > > >> In this case, if the use of == was an accident and you had intended to > > >> write !(a1 == a2) then the code would look the same (except for the > > >> missing '=' character). The "extra" parentheses here would have to be > > >> present because of the ! operator, so unlike the first case above, > > >> they are not redundant. > > > > > > > > > Thanks for the quick reply and explanation. That makes sense, and I > didn't think about that. > > > > > >> The types involved (i.e. the fact that A::operator= is being called) > > >> are not relevant at all, as far as I know. The warning operates purely > > >> on the syntax and warns about the use of = where maybe == was > > >> intended. > > > > > > > > > OK. In that case the warning makes sense. I thought there was a bit > more semantic analysis going on, but that would probably be rather hard to > implement. > > > > > >> Using if ((!(a1 = a2))) suppresses the warning for this case. > > > > > > > > > Thanks for the hint. I was for now going for if (auto assign = (a1 = > a2); !assign) which works, but is quite a bit of additional noise compared > to your solution. > > > > > > Given your explanations and the proposed fix, I am actually happy with > just implementing that. But I am also curious why it doesn't happen with > c++17. > > > > I'm thoroughly puzzled, but I'll follow up when I get an answer. > > The different behaviour with C++20 is because of the default > comparisons feature in C++20, where operator!= can be synthesized from > operator== as needed. The different rules for == and != in C++20 alter > how G++ handles !(expr) and cause it to look into the expression, and > then the warning code sees (a1 = a2) and triggers the warning. I don't > think the difference was intended. >
