llvm/clang/test/Sema/pre-c2x-compat.c

// RUN: %clang_cc1 %s -std=c2x -Wpre-c2x-compat -pedantic -fsyntax-only -verify

int digit_seps = 123'456; // expected-warning {{digit separators are incompatible with C standards before C23}}
unsigned char u8_char = u8'x'; // expected-warning {{unicode literals are incompatible with C standards before C23}}