https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95368
--- Comment #2 from Rafael Avila de Espindola <rafael at espindo dot la> --- I just tested a few compilers with the testcase from comment 1: * clang version 11.0.0 (https://github.com/llvm/llvm-project.git 0796b170fb3bf38e6cc4e59746120b37c9a9cd9f): Accepts it. * g++ (GCC) 10.1.1 20200507 (Red Hat 10.1.1-1): Rejects it: test.cc:17:14: error: passing ‘const foo’ as ‘this’ argument discards qualifiers [-fpermissive] * gcc (GCC) 9.3.1 20200528 Rejects it: test.cc:17:14: error: passing ‘const foo’ as ‘this’ argument discards qualifiers