Variable list array initialization in C explained [duplicate]

Question

Give an a basic set of C code:

int a = 5;
int b[a] = {1,2,3,4,5};

Why does this generate this error?

$gcc -o demo demo.c
error: variable-sized object may not be initialized

I understand that it is a bad thing to do and the compiler won't let you. However as a teacher I am having a hard time coming up with an explanation as to why you can not and should not do this.

(Assume C99)

How would you explain what is happening here?

Answer 1

Why does this generate this error?

C standard doesn't allow this. Variable length arrays can't be initialized using brace-enclosed list of initializers.

Standard says:
C11-§6.7.9/3:

The type of the entity to be initialized shall be an array of unknown size or a complete object type that is not a variable length array type.