[SR-15132][SR-15134] Fix Decimal implementation of significand APIs

Darwin/Foundation-swiftoverlay-Tests/TestDecimal.swift

@@ -558,6 +558,27 @@ class TestDecimal : XCTestCase {
         XCTAssertEqual(answer,num,"\(ones) / 9 = \(answer) \(num)")
     }
 
+    func test_Significand() {
+        var x = -42 as Decimal
+        XCTAssertEqual(x.significand.sign, .plus)
+        var y = Decimal(sign: .plus, exponent: 0, significand: x)
+        XCTAssertEqual(y, -42)
+        y = Decimal(sign: .minus, exponent: 0, significand: x)
+        XCTAssertEqual(y, 42)
+
+        x = 42 as Decimal
+        XCTAssertEqual(x.significand.sign, .plus)
+        y = Decimal(sign: .plus, exponent: 0, significand: x)
+        XCTAssertEqual(y, 42)
+        y = Decimal(sign: .minus, exponent: 0, significand: x)
+        XCTAssertEqual(y, -42)
+
+        let a = Decimal.leastNonzeroMagnitude
+        XCTAssertEqual(Decimal(sign: .plus, exponent: -10, significand: a), 0)
+        let b = Decimal.greatestFiniteMagnitude
+        XCTAssertTrue(Decimal(sign: .plus, exponent: 10, significand: b).isNaN)
+    }
+
     func test_SimpleMultiplication() {
         var multiplicand = Decimal()
         multiplicand._isNegative = 0

Darwin/Foundation-swiftoverlay/Decimal.swift

@@ -467,13 +467,13 @@ extension Decimal {
     }
 
     public init(sign: FloatingPointSign, exponent: Int, significand: Decimal) {
-        self.init(
-            _exponent: Int32(exponent) + significand._exponent,
-            _length: significand._length,
-            _isNegative: sign == .plus ? 0 : 1,
-            _isCompact: significand._isCompact,
-            _reserved: 0,
-            _mantissa: significand._mantissa)
+        self = significand
+        let error = withUnsafeMutablePointer(to: &self) {
+            NSDecimalMultiplyByPowerOf10($0, $0, Int16(exponent), .plain)
+        }
+        if error == .underflow { self = 0 }
+        // We don't need to check for overflow because `Decimal` cannot represent infinity.
+        if sign == .minus { negate() }
     }
 
     public init(signOf: Decimal, magnitudeOf magnitude: Decimal) {
@@ -492,7 +492,7 @@ extension Decimal {
 
     public var significand: Decimal {
         return Decimal(
-            _exponent: 0, _length: _length, _isNegative: _isNegative, _isCompact: _isCompact,
+            _exponent: 0, _length: _length, _isNegative: 0, _isCompact: _isCompact,
             _reserved: 0, _mantissa: _mantissa)
     }
 

Sources/Foundation/Decimal.swift

@@ -695,13 +695,13 @@ extension Decimal {
     }
 
     public init(sign: FloatingPointSign, exponent: Int, significand: Decimal) {
-        self.init(
-            _exponent: Int32(exponent) + significand._exponent,
-            _length: significand._length,
-            _isNegative: sign == .plus ? 0 : 1,
-            _isCompact: significand._isCompact,
-            _reserved: 0,
-            _mantissa: significand._mantissa)
+        self = significand
+        let error = withUnsafeMutablePointer(to: &self) {
+            NSDecimalMultiplyByPowerOf10($0, $0, Int16(exponent), .plain)
+        }
+        if error == .underflow { self = 0 }
+        // We don't need to check for overflow because `Decimal` cannot represent infinity.
+        if sign == .minus { negate() }
     }
 
     public init(signOf: Decimal, magnitudeOf magnitude: Decimal) {
@@ -720,7 +720,7 @@ extension Decimal {
 
     public var significand: Decimal {
         return Decimal(
-            _exponent: 0, _length: _length, _isNegative: _isNegative, _isCompact: _isCompact,
+            _exponent: 0, _length: _length, _isNegative: 0, _isCompact: _isCompact,
             _reserved: 0, _mantissa: _mantissa)
     }
 

Tests/Foundation/Tests/TestDecimal.swift

@@ -7,6 +7,9 @@
 // See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
 //
 
+import Foundation
+import XCTest
+
 class TestDecimal: XCTestCase {
 
     func test_NSDecimalNumberInit() {
@@ -808,6 +811,27 @@ class TestDecimal: XCTestCase {
         XCTAssertEqual(zero3.description, "0")
     }
 
+    func test_Significand() {
+        var x = -42 as Decimal
+        XCTAssertEqual(x.significand.sign, .plus)
+        var y = Decimal(sign: .plus, exponent: 0, significand: x)
+        XCTAssertEqual(y, -42)
+        y = Decimal(sign: .minus, exponent: 0, significand: x)
+        XCTAssertEqual(y, 42)
+
+        x = 42 as Decimal
+        XCTAssertEqual(x.significand.sign, .plus)
+        y = Decimal(sign: .plus, exponent: 0, significand: x)
+        XCTAssertEqual(y, 42)
+        y = Decimal(sign: .minus, exponent: 0, significand: x)
+        XCTAssertEqual(y, -42)
+
+        let a = Decimal.leastNonzeroMagnitude
+        XCTAssertEqual(Decimal(sign: .plus, exponent: -10, significand: a), 0)
+        let b = Decimal.greatestFiniteMagnitude
+        XCTAssertTrue(Decimal(sign: .plus, exponent: 10, significand: b).isNaN)
+    }
+
     func test_SimpleMultiplication() {
         var multiplicand = Decimal()
         multiplicand._isNegative = 0
@@ -1460,6 +1484,7 @@ class TestDecimal: XCTestCase {
             ("test_RepeatingDivision", test_RepeatingDivision),
             ("test_Round", test_Round),
             ("test_ScanDecimal", test_ScanDecimal),
+            ("test_Significand", test_Significand),
             ("test_SimpleMultiplication", test_SimpleMultiplication),
             ("test_SmallerNumbers", test_SmallerNumbers),
             ("test_ULP", test_ULP),