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> a[i] = i++

And I would never excrete such a line of code without automatically thinking "hmmm.. does this really do what I think it does?"

If I see a[i] = i++ in a codebase I am immediately removing it as it looks ambiguous, possibly invalid and probably unpredictable.

Even if the behaviour is defined, relying on defined non-obvious behaviours means relying on subsequent programmers to have read the spec.

This is just a very particular case of calling a function with such arguments that the result will depend on the order of evaluation for the arguments.

This is an undefined operation in almost all programming languages.

That is not actually the case, a great many languages define the order of operations in a deterministic manner.

There is no intrinsic reason why a[i] = i++ has to result in undefined behavior.

High performance calling conventions depend on machine architecture.

This should be caught by the compiler and reported as an error.