Several of the examples shown as valid C are not.

For example, this:

    const int foo = 1;
    int* bar = &foo;
    *bar = 2;

is said to have undefined behavior, but in fact the initialization of `bar` is a constraint violation, requiring a diagnostic. (Some compilers will issue a non-fatal warning, which is allowed by the C standard but IMHO is unfortunate.)

Another example: it says that this:

    const size_t buffer_size = 5;
    int buffer[buffer_size];

will not compile in C, but it's valid at block scope in C99, which introduced variable-length arrays. (C11 made them optional.)

"In C, this would compile, albeit likely with warnings about implicit conversion:"

    int main() {
        auto x = "actually an int";
        return x;
    }

The "implicit int" rule was dropped in C99, and even before that the language did not define an implicit conversion from char* to int. Again, some compilers might support it with a warning, but it's a constraint violation.