A building boom in Nashville, Tennessee, has sent rents soaring and filled the sky with construction cranes. Down on the streets, officials say homeless young adults are paying the price as an affordable housing shortage intensifies.Now the city is one of several nationwide that’s trying to size up the problem with a new way to count its homeless youth. The Oasis Center for at-risk youth hosted a slumber party, complete with a talent show and karaoke.It happened on the one night each year when most cities count their homeless populations by canvassing streets and shelters.At Oasis, in exchange for coming in to be counted, homeless teens and 20-somethings could spend the night; receive free medical care, clothing and a meal; and get some much-needed TLC from staffers like Casey Woods.She’d greet new arrivals with hugs and praise, like when Kalwin Perrin came in from the cold. “How are you?” she asked. “Oh, you look so handsome. I’m so proud of you!”Perrin was no stranger to the center. This year, he was asked to emcee the talent show. “Dancing, singing and some spoken word — I know I’ll see some spoken word,” he said while preparing.While Perrin’s housing is unstable, his energy is unchecked. He sang, danced, cracked jokes and led a revue-style rendition of “Proud Mary,” complete with synchronized dancing by several staffers and some of his homeless peers.Of course, there is a serious side to the “Youth Count.”Homeless young adults are difficult to find, since some rarely sleeping on the streets or in shelters. They often choose to couch hop or get motel rooms together. And they go uncounted. So while Nashville officials estimate there are as many as 3,000 homeless young people up to age 24, only about 200 were counted last year.Counting the homeless accurately helps a city win a larger share of federal grants. Oasis also learns about their situations through surveys, in part so the city can respond. Perrin, for example, tells staff he’s slept in several places in...