• Compiler flag: -enforce-exclusivity=<value>
• Build setting: SWIFT_ENFORCE_EXCLUSIVE_ACCESS

Xcode 10.1 & Swift 4.2

• Compiler default for non optimized builds if you pass no argument is the same as if you passed checked
• Compiler default for optimized builds if you pass no argument is the same as if you passed unchecked

Configuration	Compiler Flag Value	Build Setting Description	Build Setting Value	Notes
Debug	checked	Full Enforcement (Run-time Checks in All Builds)	on
Debug	checked	Full Enforcement (Run-time Checks in Debug Builds Only)	debug-only	default
Debug	unchecked	Compile-time Enforcement	compile-time
Debug	none	No Enforcement	none
Release	checked	Full Enforcement (Run-time Checks in All Builds)	on
Release	no flag passed	Full Enforcement (Run-time Checks in Debug Builds Only)	debug-only	default
Release	unchecked	Compile-time Enforcement	compile-time
Release	none	No Enforcement	none

Xcode 10.2 (beta 3) & Swift 5

• Compiler default if you pass no argument is the same as if you passed checked

Configuration	Compiler Flag Value	Build Setting Description	Build Setting Value	Notes
Debug	checked	Full Enforcement (Run-time Checks in All Builds)	on	default
Debug	checked	Full Enforcement (Run-time Checks in Debug Builds Only)	debug-only
Debug	unchecked	Compile-time Enforcement Only	off
Release	checked	Full Enforcement (Run-time Checks in All Builds)	on	default
Release	unchecked	Full Enforcement (Run-time Checks in Debug Builds Only)	debug-only
Release	unchecked	Compile-time Enforcement Only	off