Question: "If You Ever Leave Me" is a vocal duet between Barbra Streisand and Vince Gill, The song was written by which American adult contemporary and pop/rock singer, songwriter, musician and record producer who has sold over 30 million records?

Context articles: If You Ever Leave Me
The Broadway Album
I Finally Found Someone
Guilty (Barbra Streisand and Barry Gibb song)
What Kind of Fool (disambiguation)
What Kind of Fool
Barbra Streisand...and Other Musical Instruments
Barbra Streisand's Greatest Hits Volume 2
Richard Bennett (guitarist)
Richard Marx

"If You Ever Leave Me" is a vocal duet between Barbra Streisand and Vince Gill.
 The song was written by Richard Marx and produced by David Foster and Richard Marx.
 It first appeared as a single on Streisand's 1999 album, A Love Like Ours.
The Broadway Album is the twenty-fourth studio album by director, composer, actress and singer Barbra Streisand, released by Columbia Records on November 5, 1985.
 Consisting mainly of classic show tunes, the album marked a major shift in Barbra Streisand's career.
 Streisand had spent ten years appearing in musicals and singing standards on her albums in the 1960s.
 Beginning with the album "Stoney End" in 1971 and ending with the album "Emotion" in 1984, Streisand sang mostly rock and disco-oriented songs for Columbia records.
 Noted Broadway composer Stephen Sondheim personally penned additional lyrics for the songs "Putting It Together" and "Send in the Clowns" on request of the singer.
 The album, originally released on the Columbia label and subsequently re-released by Columbia and Sony Records, was a critical and commercial success.
 First certified gold by the RIAA on January 13, 1986, it reached four times platinum on January 31, 1995.
"I Finally Found Someone" is a song duet from 1996 with Canadian artist Bryan Adams and American artist Barbra Streisand.
 The song was part of the soundtrack of Streisand's self-directed movie "The Mirror Has Two Faces" and was nominated for an Oscar.
 It reached No. 8 on the "Billboard" Hot 100, and #2 on the Hot Adult Contemporary chart.
 "I Finally Found Someone" gave Streisand her first significant hit in almost a decade and her first top 10 hit on the Hot 100 (and first gold single) since 1981.
 The song was included on an Australian issue of Adams' album, "18 Til I Die".
"Guilty" is a vocal duet between Barbra Streisand and Barry Gibb.
 The song was written by all three Bee Gees: Barry, Robin & Maurice Gibb.
 Released as a single from Streisand's 1980 album of the same name.
 "Guilty" peaked at #3 on the US "Billboard" Hot 100 pop chart and #5 on the adult contemporary chart.
 In the UK, the song reached #34 on the UK Singles Chart.
 The single was certified gold by the RIAA.
 In addition, "Guilty" won a Grammy Award in the category Best Pop Vocal Performance, Duo or Group.
 The song also appeared on the 2001 Bee Gees compilation, "".
"What Kind of Fool" is a 1981 vocal duet between Barbra Streisand and Barry Gibb.
"What Kind of Fool" is the title of a vocal duet from 1981 (see 1981 in music) by singers Barbra Streisand and Barry Gibb.
 The song was written by Gibb and Albhy Galuten.
Barbra Streisand...and Other Musical Instruments (also called And Other Musical Instruments) is the fourteenth studio album by American singer Barbra Streisand.
 It was released on November 2, 1973 by Columbia Records.
 The album was made available following a 1973 live television special promoted to improve Streisand's image and sound.
 With world music as the primary genre, the album's instrumentation varies greatly; even items such as kitchen utensils were used to create melodies and beats.
 With a majority of the songs on the album being cover songs, Streisand also re-recorded various tracks that originated earlier in her career.
 Her manager was credited as the album's sole and executive producer.
Barbra Streisand's Greatest Hits Volume 2 is the second greatest hits album recorded by American vocalist Barbra Streisand.
 It was released on November 15, 1978 by Columbia Records.
 The album is a compilation consisting of ten commercially successful singles from the singer's releases in the 1970s, with a majority of them being cover songs.
 It also features a new version of "You Don't Bring Me Flowers", which was released as the collection's only single on October 7, 1978.
 Originating on Streisand's previous album, "Songbird", the new rendition is a duet with Neil Diamond who had also recorded the song for his 1978 album of the same name.
 The idea for the duet originated from DJ Gary Guthrie who sold the idea to the record label for $5 million.
Richard Bennett (born July 22, 1951) is an American guitarist and record producer.
 As a touring sideman, he performed with Neil Diamond for seventeen years and Mark Knopfler since 1994.
 As a session musician, he has worked with Billy Joel, Barbra Streisand, Rodney Crowell, and Vince Gill.
 He has produced albums for Steve Earle, Emmylou Harris, Marty Stuart, and Kim Richey.
Richard Noel Marx (born September 16, 1963) is an American adult contemporary and pop/rock singer, songwriter, musician and record producer who has sold over 30 million records.
 He had a stream of hit singles in the late 1980s and 1990s, including "Endless Summer Nights," "Right Here Waiting," "Now and Forever," "Hazard" and "At the Beginning" with Donna Lewis.
 Although some of his major hit songs were ballads, many of his songs have had a classic rock style, such as "Don't Mean Nothing," "Should've Known Better," "Satisfied," and "Too Late to Say Goodbye."