The C standard, since 1989, has said that attempting to modify the array object corresponding to a string literal has undefined behavior. Whether it "works" or not is not the issue.

The problem is that it's currently legal to pass a string literal to a function expecting a (non-const) pointer-to-char argument. As long as the function doesn't try to write through the pointer, there's no undefined behavior. (If the function does try to write through the pointer, the behavior is undefined, but no compile-time diagnostic is required.) If a future version of C made string literals const, such a program would become invalid (a constraint violation requiring a diagnostic). Such code was common in pre-ANSI C, before const was introduced to the language.

The following is currently valid C. The corresponding C++ code would be invalid. The proposal would make it invalid in C, with the cost of breaking some existing code, and the advantage of catching certain errors at compile time.

    #include <stdio.h>

    void print_message(char *message) {
        puts(message);
        // *message = '\0'; // would have undefined behavior
    }

    int main(void) {
        print_message("hello");
    }