In "As I Walked Out One Evening," the line, "I'll love you, dear, I'll love you till China and Africa meet," is an example of what literary device?

The speaker is walking along a street and he hears a lover singing about his love. The lover uses a series of exaggerated statements (hyperbole) to express how much he loves. The key theme here is that he will love his significant other for an endless amount of time. Time is no match for his passion. He will love "her" until two separate lands meet (China and Africa). Even if Auden (or the lover in this poem) is talking about continental drift, Africa would "meet" South America before it would ever meet China. But the notion here is that China and Africa would probably never meet. Therefore, he would love her forever.

The lover's hyperbolic statements become even quirkier and more impossible. This is how he wants to express that his love is timeless. He adds that he will love until the stars squawk like geese and the oceans dry up. These grandiose statements of love are then brought back to earth with the real human experience of time:

But all the clocks in the city 

Began to whirr and chime: 

'O let not Time deceive you, 

You cannot conquer time. 

Using exaggerated, hyperbolic statements is the lover's attempt to do just that: to use love to escape or conquer time.