I forgot the passcode

You will lose all data on the device. If you have an iCloud backup, then the data will be restored and the phone will be usable just like it was before. The 2 support articles that cover this are:

If you forgot your iPhone passcode - Apple Support

If you see an iPhone or iPad unavailable message or Security Lockout screen - Apple Support

Herolike Said:

"I forgot the passcode: I forgot the passcode of my other iPhone, but I have iCloud. How can I log in without losing my data?"

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Resetting the Passcode:

Take this in for a Genius Bar Appointment at your local Apple Store. Bring a proof pf purchase with you.

In agreement with the first link. "If you forgot your iPhone passcode - Apple Support".