What's the reason for letting the semantics of a=a++ be undefined?

Question

a = a++;

is undefined behaviour in C. The question I am asking is : why?

I mean, I get that it might be hard to provide a

Answer 1

Updating the same object twice without an intervening sequence point is Undefined Behaviour because ...

• because that makes compiler writers happier
• because it allows implementations to define it anyway
• because it doesn't force a specific constraint when it isn't needed
• ...