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Modifying a const variable using a pointer

suggest change 
int main (void)
{
    const int foo_readonly = 10;
    int *foo_ptr;

    foo_ptr = (int *)&foo_readonly; /* (1) This casts away the const qualifier */
    *foo_ptr = 20; /* This is undefined behavior */

    return 0;
}

Quoting ISO/IEC 9899:201x, section 6.7.3 §2:

If an attempt is made to modify an object defined with a const-qualified type through use of an lvalue with non-const-qualified type, the behavior is undefined. […]
  1. In GCC this can throw the following warning: warning: assignment discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
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Undefined behavior:
* Undefined behavior
* Dereferencing a pointer to variable beyond its lifetime
* Copying overlapping memory
* Signed integer overflow
* Use of an uninitialized variable
* Read value of pointer that was freed
* Data race
* Modify string literal
* Using incorrect format specifier in printf
* Modifying any object more than once between two sequence points
* Passing a null pointer to printf s conversion
* Bit shifting using negative counts or beyond the width of the type
* Freeing memory twice
* Accessing memory beyond allocated chunk
* Returning from a function thats declared with Noreturn or noreturn function specifier
* Addition or subtraction of pointer not properly bounded
* Modifying a const variable using a pointer
* Dereferencing a null pointer
* Reading an uninitialized object that is not backed by memory
* Using fflush on an input stream
* Division by zero
* Missing return statement in value returning function
* Inconsistent linkage of identifiers
* Conversion between pointer types produces incorrectly aligned result
* Modifying the string returned by getenv strerror and setlocale functions
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