American romantic comedies that even halfway work are so rare that one which manages a little pathos and maybe five actual belly-laughs should be cut a little slack.

Which could be Justin Long’s motto as a leading man. No, that’s not mean — or any meaner than the “Poor Man’s John Cusack” mantle he’s worn among the movie reviewing classes the past — oh — 20 years or so. And that’s only mean until you think about it and then agree with it.

Long is the hapless ex invited to his longtime love’s San Francisco nuptials. He was the guy she dated for eight years, “Literally, Right Before Aaron” whom Allison is about to marry.

Allison is played by Cobie Smulders (“How I Met Your Mother”). So you can understand the look of distress Adam (Long) wears first scene to last in this rom com.

She’s the one who got away. Everybody knows it. They can’t help but rub his nose in it.

“Hey, didya hear what I said to ‘SECOND PLACE’ over here?”

But in a situation that exists ONLY in the movies — going back way before “My Best Friend’s Wedding” — she wants Adam at her wedding, that whole “I don’t want you to hate me…I’m going to be at YOUR wedding, you HAVE to come to mine” thing.

Really? Quick show of hands, how often has that happened at a wedding you went to? Never mind — DKDC. Because if there’s one thing the movies teach us about this obviously bad idea that so many movie characters go for, it’s that it’s a bad idea for a reason.

Adam consults with friends (John Cho, Charlene Yi, the wisdom of the East?). He goes in spite of their advice. He takes time off from editing a goofy adventures-in-nature TV series (Peter Gallagher is the screwball host), packs his 1970s VW Beetle (a “car with character”) and heads north. He lies to people he runs into — an old college pal (Malcolm Barrett) and even his mom (Lea Thompson) about his reasons for being there.

And he pines pines pines for Allison, even though he has to see how the “good at everything” hunk Aaron (Ryan Hansen) she’s tying the knot with is a step or two up from him.

Actor-turned-writer/director Ryan Eggold finds his laughs around the periphery of Adam/Long’s perpetual look of sucker-punched shock. There’s the current girlfriend (Briga Heelan) Adam riffs an impromptu proposal to — “What else IS there, right?” — only to break up with her the very next breath.

Cho’s character’s amusing “intervention” involves tough love, bowling and a few dope slaps.

And Luis Guzman makes a third act appearance as the wise Latino cook who offers champagne and pet goldfish homilies to help Adam cope, upon seeing Adam’s stricken face.

“Somebody kill your cat?”

None of which really fixes him. Because, come on, it’s COBIE SMULDERS. If “How I Met Your Mother” taught us nothing else, it’s that she’s the Queen of Heartbreak — embodying it or causing it.

And if the movie finds its pathos and laughs around the edges, “Literally, Right Before Aaron” finds its easy if limited appeal outside the Hollywood mainstream, where “Home Again” or “Fifty Shades of S & M” is somebody’s idea of what a romantic comedy should be these days.

No. As Shakespeare declared, there shalt be no rom-com without a wedding, no wedding without a wedding-interrupted and no interruption that isn’t made funnier by the Great Luis Guzman.

MPAA Rating: unrated, adult themes, profanity

Cast: Justin Long, Cobie Smulders, Lea Thompson, Ryan Dana Delaney

Credits:  Written and directed by Ryan Eggold. A Screen Media release.

Running time: 1:45