Cleanup SystemTimeZone tests

Cleans up the DateFormatter test, improves the information about how it doesn’t catch issues when the system time zone is already GMT, and adds a more focused test for the SystemTimeZone name issue that can catch it when GMT is the system time zone.

Author: Kaiede

TestFoundation/TestDateFormatter.swift

@@ -25,7 +25,7 @@ class TestDateFormatter: XCTestCase {
             ("test_setLocaleToNil", test_setLocaleToNil),
             ("test_setTimeZoneToNil", test_setTimeZoneToNil),
             ("test_setTimeZone", test_setTimeZone),
-            ("test_ExpectedTimeZone", test_ExpectedTimeZone),
+            ("test_expectedTimeZone", test_expectedTimeZone),
         ]
     }
 
@@ -377,7 +377,7 @@ class TestDateFormatter: XCTestCase {
         XCTAssertEqual(f.timeZone, losAngeles)
     }
 
-    func test_ExpectedTimeZone() {
+    func test_expectedTimeZone() {
         let gmt = TimeZone(abbreviation: DEFAULT_TIMEZONE)
         let newYork = TimeZone(identifier: "America/New_York")!
         let losAngeles = TimeZone(identifier: "America/Los_Angeles")!
@@ -388,24 +388,24 @@ class TestDateFormatter: XCTestCase {
 
         let f = DateFormatter()
         f.dateFormat = "z"
+        f.locale = Locale(identifier: "en_US_POSIX")
 
         // Case 1: TimeZone.current
+        // This case can catch some issues that cause TimeZone.current to be
+        // treated like GMT, but it doesn't work if TimeZone.current is GMT.
+        // If you do find an issue like this caused by this first case,
+        // it would benefit from a more specific test that fails when
+        // TimeZone.current is GMT as well.
+        // (ex. TestTimeZone.test_systemTimeZoneName)
         f.timeZone = TimeZone.current
-        XCTAssertEqual(f.string(from: now), f.timeZone.abbreviation())
+        XCTAssertEqual(f.string(from: now), TimeZone.current.abbreviation())
 
         // Case 2: New York
         f.timeZone = newYork
-        XCTAssertEqual(f.string(from: now), f.timeZone.abbreviation())
+        XCTAssertEqual(f.string(from: now), newYork.abbreviation())
 
         // Case 3: Los Angeles
         f.timeZone = losAngeles
-        XCTAssertEqual(f.string(from: now), f.timeZone.abbreviation())
-
-        guard gmt != TimeZone.current else {
-            print("Inconclusive: This test checks to see if the formatter produces the same TZ as TimeZone.current")
-            print("When it fails, TimeZone.current formats as GMT instead of normal.")
-            print("Unfortunately, we can't use GMT as TimeZone.current for this test to be conclusive.")
-            return
-        }
+        XCTAssertEqual(f.string(from: now), losAngeles.abbreviation())
     }
 }

TestFoundation/TestTimeZone.swift

@@ -7,6 +7,8 @@
 // See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
 //
 
+import CoreFoundation
+
 class TestTimeZone: XCTestCase {
 
     static var allTests: [(String, (TestTimeZone) -> () throws -> Void)] {
@@ -29,6 +31,7 @@ class TestTimeZone: XCTestCase {
 
             ("test_customMirror", test_tz_customMirror),
             ("test_knownTimeZones", test_knownTimeZones),
+            ("test_systemTimeZoneName", test_systemTimeZoneName),
         ]
     }
 
@@ -198,4 +201,17 @@ class TestTimeZone: XCTestCase {
             XCTAssertNotNil(TimeZone(identifier: tz), "Cant instantiate valid timeZone: \(tz)")
         }
     }
+
+    func test_systemTimeZoneName() {
+        // Ensure that the system time zone creates names the same way as creating them with an identifier.
+        // If it isn't the same, bugs in DateFormat can result, but in this specific case, the bad length
+        // is only visible to CoreFoundation APIs, and the Swift versions hide it, making it hard to detect.
+        let timeZone = CFTimeZoneCopySystem()
+        let timeZoneName = CFTimeZoneGetName(timeZone)
+
+        let createdTimeZone = TimeZone(identifier: TimeZone.current.identifier)!
+
+        XCTAssertEqual(CFStringGetLength(timeZoneName), TimeZone.current.identifier.count)
+        XCTAssertEqual(CFStringGetLength(timeZoneName), createdTimeZone.identifier.count)
+    }
 }