In the track, released on July 27, Bieber disses Mendes, with whom Baldwin was seen as recently as May at the 2018 Met Gala. "Him or me, be for real, baby, it's a no brainer," Bieber sings.
Some fans interpreted the lyrics as a direct burn on the Canadian singer, while others defended Bieber's words as "just a song."
"Why would someone ever diss Shawn especially when he's a fan of you? RIP Bieber your reputation is over," wrote one Twitter user.
What Justin just DISSED Shawn in "No Brainer"??? I'm so done... HES SO done... why would someone ever diss Shawn especially when he's a fan of you? Rip Bieber your reputation is over— Hailey Purrpawz (@purrpawz) August 1, 2018
"'No Brainer' isn't about Justin shading Shawn Mendes. It's just a song! Some fans are just literally trying to find ways to start a fan war," someone else argued.
No Brainer isn't about Justin shading Shawn Mendes. It's just a song! Some fans are just literally trying to find ways to start a fan war geeeesh.— Ren 😁 (@karenmanaay) August 1, 2018
Whether he was slammed or not, Mendes has made it clear in the press that he's fully supportive of Baldwin and Bieber's engagement.
"I texted Hailey the day of and I said congrats. That's what it is. I think everybody wants there to be more — there's not," he told the Australian show The Project at the time, according to HollywoodLife.
A source close to Mendes told HollywoodLife that he has no hard feelings over the track. "Shawn will take any shade Justin wants to send because he can handle it. Shawn feels like he should be above it all because that is the guy he is and if Justin wants to do or say anything, he has other more important things to worry about," the source said.
The insider added that Mendes is close to Bieber's ex, Selena Gomez, "and he respects her too much to get into a feud with Justin. He's heard all the stories from the past with Justin and Selena and he has chosen Selena in the battle for friendship and just doesn't want to get into petty feuds, it does nothing for him at all."
Bieber and Baldwin got engaged on July 7 after dating for mere weeks — although the couple had known each other for years and even dated in the past. They reportedly dated from 2015 to 2016, with the model even accompanying Bieber and his family on vacation to Anguilla. After they split in 2016, Bieber reportedly reached out to Baldwin while he was back with Gomez at the end of 2017, and they rekindled their romance in June 2018, months after he'd split again from Gomez.
Weeks later, Bieber popped the question to Baldwin at a resort in the Bahamas. Since then, the two have confirmed their engagement, and Baldwin has even been spotted out and about wearing her massive engagement ring. It's possible that the nuptials are just around the corner, as Baldwin's aunt, actress Kim Basinger, told Us Weekly that the two already have selected their bridal party.
"Alaia [Baldwin's sister] and Ireland, they're in the wedding. So believe me, I just — it's cool! I think it's sweet. It's a very sweet thing," Basinger said, adding that she is "very, very happy for Hailey. I think it's a good thing. I think Justin's a cool guy. I don't really know him at all, but Ireland does, you know? I think he's come through a rough road. He's a really cool kid. I pray for them. I hope they're cool. I hope they're happy! It's wild! I think it's wild!"