SR-7481: NumberFormatter inconsistency on Linux

- For the numberStyle .currency, use a minimumIntegerDigits of 1.

- If minimumIntegerDigits is set to any value (including 0) before
  the numberStyle is set, preserve the original value.

(cherry picked from commit c8c16eb)

Author: spevans

Foundation/NumberFormatter.swift

@@ -140,7 +140,7 @@ open class NumberFormatter : Formatter {
         _setFormatterAttribute(formatter, attributeName: kCFNumberFormatterPlusSign, value: _plusSign?._cfObject)
         _setFormatterAttribute(formatter, attributeName: kCFNumberFormatterCurrencySymbol, value: _currencySymbol?._cfObject)
         _setFormatterAttribute(formatter, attributeName: kCFNumberFormatterExponentSymbol, value: _exponentSymbol?._cfObject)
-        _setFormatterAttribute(formatter, attributeName: kCFNumberFormatterMinIntegerDigits, value: _minimumIntegerDigits._bridgeToObjectiveC()._cfObject)
+        _setFormatterAttribute(formatter, attributeName: kCFNumberFormatterMinIntegerDigits, value: minimumIntegerDigits._bridgeToObjectiveC()._cfObject)
         _setFormatterAttribute(formatter, attributeName: kCFNumberFormatterMaxIntegerDigits, value: _maximumIntegerDigits._bridgeToObjectiveC()._cfObject)
         _setFormatterAttribute(formatter, attributeName: kCFNumberFormatterMinFractionDigits, value: _minimumFractionDigits._bridgeToObjectiveC()._cfObject)
         if _minimumFractionDigits <= 0 {
@@ -190,12 +190,15 @@ open class NumberFormatter : Formatter {
             case .currency, .currencyPlural, .currencyISOCode, .currencyAccounting:
                 _usesSignificantDigits = false
                 _usesGroupingSeparator = true
+                if _minimumIntegerDigits == nil {
+                    _minimumIntegerDigits = 1
+                }
                 _minimumFractionDigits = 2
 
             case .decimal:
                 _usesGroupingSeparator = true
                 _maximumFractionDigits = 3
-                if _minimumIntegerDigits == 0 {
+                if _minimumIntegerDigits == nil {
                     _minimumIntegerDigits = 1
                 }
                 if _groupingSize == 0 {
@@ -680,11 +683,14 @@ open class NumberFormatter : Formatter {
             _roundingIncrement = newValue
         }
     }
-    
-    internal var _minimumIntegerDigits: Int = 0
+
+    // Use an optional for _minimumIntegerDigits to track if the value is
+    // set BEFORE the .numberStyle is changed. This allows preserving a setting
+    // of 0.
+    internal var _minimumIntegerDigits: Int?
     open var minimumIntegerDigits: Int {
         get {
-            return _minimumIntegerDigits
+            return _minimumIntegerDigits ?? 0
         }
         set {
             _reset()

TestFoundation/TestNumberFormatter.swift

@@ -221,6 +221,28 @@ class TestNumberFormatter: XCTestCase {
         XCTAssertEqual(numberFormatter.minimumIntegerDigits, 3)
         formattedString = numberFormatter.string(from: 0.1)
         XCTAssertEqual(formattedString, "000.1")
+
+        numberFormatter.numberStyle = .currency
+        XCTAssertEqual(numberFormatter.minimumIntegerDigits, 3)
+
+        // If .minimumIntegerDigits is set to 0 before .numberStyle change, preserve the value
+        let currencyFormatter = NumberFormatter()
+        XCTAssertEqual(currencyFormatter.minimumIntegerDigits, 0)
+        currencyFormatter.minimumIntegerDigits = 0
+        currencyFormatter.numberStyle = .currency
+        XCTAssertEqual(currencyFormatter.minimumIntegerDigits, 0)
+        currencyFormatter.locale = Locale(identifier: "en_US")
+        formattedString = currencyFormatter.string(from: NSNumber(value: 0))
+        XCTAssertEqual(formattedString, "$.00")
+
+        // If .minimumIntegerDigits is not set before .numberStyle change, update the value
+        let currencyFormatter2 = NumberFormatter()
+        XCTAssertEqual(currencyFormatter2.minimumIntegerDigits, 0)
+        currencyFormatter2.numberStyle = .currency
+        XCTAssertEqual(currencyFormatter2.minimumIntegerDigits, 1)
+        currencyFormatter2.locale = Locale(identifier: "en_US")
+        formattedString = currencyFormatter2.string(from: NSNumber(value: 0))
+        XCTAssertEqual(formattedString, "$0.00")
     }
 
     func test_maximumIntegerDigits() {