https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111953
Bug ID: 111953 Summary: stringify operator # rejects "\" but not "\ \" as input Product: gcc Version: unknown Status: UNCONFIRMED Severity: normal Priority: P3 Component: preprocessor Assignee: unassigned at gcc dot gnu.org Reporter: stephan.stiller at outlook dot com Target Milestone: --- The code #define str(s) # s str(\ \) (full source file) should lead to UB (the result "\ \" is not a valid string literal) but doesn't. For a slightly different second line str(\) GCC does in fact give a warning warning: invalid string literal, ignoring final '\' for both C17 (with warnings enabled) gcc -E -std=c17 -pedantic -Wall -Wextra and "gnu17"-C: gcc -E ##### Here is some technical data (output of "gcc -v"): Using built-in specs. COLLECT_GCC=gcc COLLECT_LTO_WRAPPER=C:/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/12.2.0/lto-wrapper.exe Target: x86_64-w64-mingw32 Configured with: ../gcc-12.2.0/configure --prefix=/mingw64 --with-local-prefix=/mingw64/local --build=x86_64-w64-mingw32 --host=x86_64-w64-mingw32 --target=x86_64-w64-mingw32 --with-native-system-header-dir=/mingw64/include --libexecdir=/mingw64/lib --enable-bootstrap --enable-checking=release --with-arch=nocona --with-tune=generic --enable-languages=c,lto,c++,fortran,ada,objc,obj-c++,jit --enable-shared --enable-static --enable-libatomic --enable-threads=posix --enable-graphite --enable-fully-dynamic-string --enable-libstdcxx-filesystem-ts --enable-libstdcxx-time --disable-libstdcxx-pch --enable-lto --enable-libgomp --disable-multilib --disable-rpath --disable-win32-registry --disable-nls --disable-werror --disable-symvers --with-libiconv --with-system-zlib --with-gmp=/mingw64 --with-mpfr=/mingw64 --with-mpc=/mingw64 --with-isl=/mingw64 --with-pkgversion='Rev10, Built by MSYS2 project' --with-bugurl=https://github.com/msys2/MINGW-packages/issues --with-gnu-as --with-gnu-ld --disable-libstdcxx-debug --with-boot-ldflags=-static-libstdc++ --with-stage1-ldflags=-static-libstdc++ Thread model: posix Supported LTO compression algorithms: zlib zstd gcc version 12.2.0 (Rev10, Built by MSYS2 project)