How can such a block be created?

Question

css

Plz tell me how can i do this?
div1 should always be a square, regardless of the width and height of the parent block. And div2 should stretch, i.e. occupy all free space

height can be dynamic and is determined by the height of the parent block. Therefore, it is impossible to rigidly set the width and height of the square.

Answer 1

htmlbook.ru/layout

Answer 2

something like this, can be improved, but I'm too lazy https://jsfiddle.net/38Tnx/5499/

Answer 3

flexbox)