Fix negation of 0 length Decimal (swiftlang#15986)

A Decimal value with _length 0 and _isNegative set to 1 is interpreted as a NaN. The 'negate()' function however, flipped the _isNegative flag without regard for the _length 0 case. This meant that -0 would become NaN. The fix checks for the _length 0 special case. In NSDecimalSubtract() the same check was performed. Since this now happens in negate(), it is removed from the NSDecimalSubtract() function.

Author: mortenbekditlevsen

stdlib/public/SDK/Foundation/Decimal.swift

@@ -59,6 +59,7 @@ extension Decimal {
     public mutating func formTruncatingRemainder(dividingBy other: Decimal) { fatalError("Decimal does not yet fully adopt FloatingPoint") }
 
     public mutating func negate() {
+        guard _length != 0 else { return }
         _isNegative = _isNegative == 0 ? 1 : 0
     }
 

test/stdlib/TestDecimal.swift

@@ -272,6 +272,9 @@ class TestDecimal : TestDecimalSuper {
         expectEqual(.minus, d.sign)
         d.negate()
         expectEqual(.plus, d.sign)
+        var e = Decimal(0)
+        e.negate()
+        expectEqual(e, 0)
         expectTrue(Decimal(3.5).isEqual(to: Decimal(3.5)))
         expectTrue(Decimal.nan.isEqual(to: Decimal.nan))
         expectTrue(Decimal(1.28).isLess(than: Decimal(2.24)))