Without releasing an official album, Charli XCX has sort of had a big 2017. She co-wrote a song with Blondie and wowed Debbie Harry, the kind of honor most alterna-pop stars dream of. This spring's album-length XCX "mixtape" Number 1 Angel was uneven, but it showcased Charli's curatorial eye and had some real sparklers: "Dreamer," with Starrah and RAYE, "Babygirl" with Uffie, "Lipgloss" with CupcakKe. This summer's delightful single "Boys," the first Charli track in some time to achieve actual popular audience, proved she could still make an idiosyncratic feel-good earworm when she felt like it. Today sees the release of Charli XCX's second 10-track, non-album collection this year, Pop 2, produced with PC Music's A.G. Cook (the two have a prolific creative partnership) and easyFun. Again, it features a small crowd of guests, among them Tove Lo, Mykki Blanco, Caroline Polachek, repeat appearances from CupcakKe and MØ, and a three-way international collaboration with Kim Petras and Jay Park on the weirdly irresistible "Unlock It." But by far the most highly anticipated was Pop 2's opening track, "Backseat," co-starring fellow superstar of unpopular pop Carly Rae Jepsen. Aesthetically, the primary theme of Pop 2 is Charli's ongoing love affair with the kooky, fractured artificiality of PC Music's signature sound. It's not even a bad stylistic match for the subject matter of "Backseat," which is about uncertainty in a long-term relationship, and the decision to turn away from those doubts rather than face them. Heartsick uncertainties are stock in trade for Jepsen, who's a friend of PC Music herself. She's a perfect fit here, even though the final product can't cash the check that "Charli XCX ft. Carly Rae Jepsen" wrote. Between the two, Jepsen is the more convincing romantic. Hearing someone whose own songwriting tends toward coy singing explicitly (even if feeling ambiguously) about a kiss and a touch is a genuine surprise. "Backseat" lifts off when Jepsen sings, "And so I turn it up," but sinks in a flood of "all alone" trills. ("Unlock It" shares a similar disproportioned cut-and-paste technique, warping Charli's repeated "lock it, unlock it" into a pattern of cracking Ks, and it works better there.) Jepsen sells her part of "Backseat" so well that the remainder of the song can't help but feel like a letdown. The already odd structure unravels after the second chorus, and by the final third, Charli and her co-producers are noodling with music box noises and recycling vocal clips in a way that would probably feel more productive during a live DJ set. On record, "Backseat" just isn't quite clever enough to carry it. Still, maybe you, like Charli and Carly's twinned narrators, are just looking for a distraction from a dimmed relationship. If so, "Backseat" is plenty shiny and metallic enough to do that.