https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94695
--- Comment #1 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Marek Polacek <mpola...@gcc.gnu.org>: https://gcc.gnu.org/g:969baf03acd8124345617cea125b148568c7370a commit r11-3539-g969baf03acd8124345617cea125b148568c7370a Author: Marek Polacek <pola...@redhat.com> Date: Thu Sep 24 14:30:50 2020 -0400 c++: Implement -Wrange-loop-construct [PR94695] This new warning can be used to prevent expensive copies inside range-based for-loops, for instance: struct S { char arr[128]; }; void fn () { S arr[5]; for (const auto x : arr) { } } where auto deduces to S and then we copy the big S in every iteration. Using "const auto &x" would not incur such a copy. With this patch the compiler will warn: q.C:4:19: warning: loop variable 'x' creates a copy from type 'const S' [-Wrange-loop-construct] 4 | for (const auto x : arr) { } | ^ q.C:4:19: note: use reference type 'const S&' to prevent copying 4 | for (const auto x : arr) { } | ^ | & As per Clang, this warning is suppressed for trivially copyable types whose size does not exceed 64B. The tricky part of the patch was how to figure out if using a reference would have prevented a copy. To that point, I'm using the new function called ref_conv_binds_directly_p. This warning is enabled by -Wall. Further warnings of similar nature should follow soon. gcc/c-family/ChangeLog: PR c++/94695 * c.opt (Wrange-loop-construct): New option. gcc/cp/ChangeLog: PR c++/94695 * call.c (ref_conv_binds_directly_p): New function. * cp-tree.h (ref_conv_binds_directly_p): Declare. * parser.c (warn_for_range_copy): New function. (cp_convert_range_for): Call it. gcc/ChangeLog: PR c++/94695 * doc/invoke.texi: Document -Wrange-loop-construct. gcc/testsuite/ChangeLog: PR c++/94695 * g++.dg/warn/Wrange-loop-construct.C: New test.
