Simon Cowell -- who normally says exactly what's on his mind -- was holding back BIG TIME on the subject of LeAnn Rimes last night ... 'cause while he insisted the singer was sober, he DEFINITELY couldn't say it with a straight face.
Simon was outside Mixology at The Grove in L.A. last night, when we asked about LeAnn's bizarre duet with Carly Rose Sonenclar on "X Factor" this week ... the one that left people thinking she was hammered.
Simon told us he thought the performance was "interesting" ... and he didn't believe LeAnn was on drugs or alcohol at the time. Buuut ... the giant poop-eating grin on his face makes it hard to believe the words that were coming out of his mouth.
Still, Simon says ... LeAnn is welcome back to the show any time she wants.