Marking .abs() method for floating point types as obsolete in Swift 4 (#5810)

* Marking .abs() method for floating point types as obsolete in Swift 4

...and deprecated in Swift 3.1

Author: moiseev

stdlib/public/SDK/CoreGraphics/CGFloat.swift.gyb

@@ -336,9 +336,10 @@ extension CGFloat {
     fatalError("unavailable")
   }
 
-  @available(*, unavailable, message: "Please use the `abs(_:)` free function")
+  @available(swift, deprecated: 3.1, obsoleted: 4.0, message: "Please use the `abs(_:)` free function")
+  @_transparent
   public static func abs(_ x: CGFloat) -> CGFloat {
-    fatalError("unavailable")
+    return x.magnitude
   }
 }
 

stdlib/public/core/FloatingPointTypes.swift.gyb

@@ -1056,9 +1056,10 @@ public postfix func -- (lhs: inout ${Self}) -> ${Self} {
 }
 
 extension ${Self} {
-  @available(*, unavailable, message: "Please use the `abs(_:)` free function")
+  @available(swift, deprecated: 3.1, obsoleted: 4.0, message: "Please use the `abs(_:)` free function")
+  @_transparent
   public static func abs(_ x: ${Self}) -> ${Self} {
-    fatalError("unavailable")
+    return x.magnitude
   }
 }
 

test/api-digester/source-stability.swift.expected

@@ -7,11 +7,5 @@ Protocol MutableIndexable has been removed (deprecated)
 Protocol RandomAccessIndexable has been removed (deprecated)
 Protocol RangeReplaceableIndexable has been removed (deprecated)
 
-/* Moved declarations */
-// rdar://28456614 Swift 3 breaking change: static function 'abs' removed from Double/Float/Float80/CGFloat
-Func Double.abs(_:) has been moved to Func abs(_:)
-Func Float.abs(_:) has been moved to Func abs(_:)
-Func Float80.abs(_:) has been moved to Func abs(_:)
-
 /* Function type change */
 Func UnsafePointer.withMemoryRebound(to:capacity:_:) has 3rd parameter type change from (UnsafeMutablePointer<T>) throws -> Result to (UnsafePointer<T>) throws -> Result