C++
gcc HEAD 11.0.0 20200918 (experimental)
$ g++ prog.cc -Wall -Wextra -I/opt/wandbox/boost-1.73.0/gcc-head/include -std=c++2a
prog.cc:3:5: error: stray '\' in program 3 | int \ug0e4 = 0; // C4429 | ^