https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82125
--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> --- >From PR 103583: Compiling the following with gcc -c: struct A { int *begin(); // int *end(); }; void foo(A a) { for (auto it : a) { } } shows two error messages: error: ‘begin’ was not declared in this scope error: ‘end’ was not declared in this scope The first error message is incorrect. If only one of 'begin' and 'end' is missing, GCC shouldn't print an error about the other one. * Comment 1 Jonathan Wakely 2021-12-06 14:30:35 UTC The error is technically correct. If your class had both begin and end, then the range-based for loop would use a.begin() and a.end(). But because it doesn't have both, the loop uses begin(a) and end(a), neither of which is defined. And that's what the error says. * Comment 2 Jonathan Wakely 2021-12-06 14:31:38 UTC See [stmt.ranged] p1. * Comment 3 Jonathan Wakely 2021-12-06 14:35:55 UTC I think the implementation doesn't do anything special to handle this case. It just looks for the members a.begin() and a.end() and then if they aren't found, it goes ahead with rewriting the code to use begin(a) and end(a), and any errors that result from that are shown straight to the user. Another option would be to do lookup for begin(a) and end(a) with errors suppressed, and if they aren't found, issue a custom diagnostic saying something like "no suitable begin/end pair available for range-based 'for' loop". Clang seems to do something similar: f.C:6:18: error: invalid range expression of type 'A'; no viable 'end' function available for (auto it : a) { } ^ ~