[6.0] rdar://132940984 (Regression: Swift Decoding of Double.greatestFiniteMagnitude as Int causes a crash) (#827)

Sources/FoundationEssentials/JSON/JSONDecoder.swift

@@ -988,17 +988,18 @@ extension JSONDecoderImpl: Decoder {
     static private func _slowpath_unwrapFixedWidthInteger<T: FixedWidthInteger>(as type: T.Type, json5: Bool, numberBuffer: BufferView<UInt8>, fullSource: BufferView<UInt8>, digitBeginning: BufferViewIndex<UInt8>, for codingPathNode: _CodingPathNode, _ additionalKey: (some CodingKey)?) throws -> T {
         // This is the slow path... If the fast path has failed. For example for "34.0" as an integer, we try to parse as either a Decimal or a Double and then convert back, losslessly.
         if let double = Double(prevalidatedBuffer: numberBuffer) {
+            // T.init(exactly:) guards against non-integer Double(s), but the parser may
+            // have already transformed the non-integer "1.0000000000000001" into 1, etc.
+            // Proper lossless behavior should be implemented by the parser.
+            guard let value = T(exactly: double) else {
+                throw JSONError.numberIsNotRepresentableInSwift(parsed: String(decoding: numberBuffer, as: UTF8.self))
+            }
+
             // The distance between Double(s) is >=2 from ±2^53.
             // 2^53 may represent either 2^53 or 2^53+1 rounded toward zero.
             // This code makes it so you don't get integer A from integer B.
             // Proper lossless behavior should be implemented by the parser.
             if double.magnitude < Double(sign: .plus, exponent: Double.significandBitCount + 1, significand: 1) {
-                // T.init(exactly:) guards against non-integer Double(s), but the parser may
-                // have already transformed the non-integer "1.0000000000000001" into 1, etc.
-                // Proper lossless behavior should be implemented by the parser.
-                guard let value = T(exactly: double) else {
-                    throw JSONError.numberIsNotRepresentableInSwift(parsed: String(decoding: numberBuffer, as: UTF8.self))
-                }
                 return value
             }
         }

Tests/FoundationEssentialsTests/JSONEncoderTests.swift

@@ -1450,6 +1450,11 @@ final class JSONEncoderTests : XCTestCase {
         _testRoundTrip(of: testValue)
     }
 
+    func test_decodeLargeDoubleAsInteger() {
+        let data = try! JSONEncoder().encode(Double.greatestFiniteMagnitude)
+        XCTAssertThrowsError(try JSONDecoder().decode(UInt64.self, from: data))
+    }
+
     func test_localeDecimalPolicyIndependence() {
         var currentLocale: UnsafeMutablePointer<CChar>? = nil
         if let localePtr = setlocale(LC_ALL, nil) {