JSONDecoder: Check that a parsed float does not return infinity.

- The behaviour of the floating point string parsing for Float and
  Double was changed to return +/-infinity instead of nil on overflow.
  (swiftlang/swift#34339)

- Add an isFinite check to ensure that the result of a floating point
  conversion still fits in the type, matching current behaviour.

Author: spevans

Sources/Foundation/JSONDecoder.swift

@@ -404,7 +404,7 @@ extension JSONDecoderImpl: Decoder {
         as type: T.Type) throws -> T
     {
         if case .number(let number) = value {
-            guard let floatingPoint = T(number) else {
+            guard let floatingPoint = T(number), floatingPoint.isFinite else {
                 var path = self.codingPath
                 if let additionalKey = additionalKey {
                     path.append(additionalKey)

Tests/Foundation/Tests/TestJSONEncoder.swift

@@ -601,14 +601,23 @@ class TestJSONEncoder : XCTestCase {
 
     func test_codingOfFloat() {
         test_codingOf(value: Float(1.5), toAndFrom: "1.5")
+
+        // Check value too large fails to decode.
+        XCTAssertThrowsError(try JSONDecoder().decode(Float.self, from: "1e100".data(using: .utf8)!))
     }
 
     func test_codingOfDouble() {
         test_codingOf(value: Double(1.5), toAndFrom: "1.5")
+
+        // Check value too large fails to decode.
+        XCTAssertThrowsError(try JSONDecoder().decode(Double.self, from: "100e323".data(using: .utf8)!))
     }
 
     func test_codingOfDecimal() {
         test_codingOf(value: Decimal.pi, toAndFrom: "3.14159265358979323846264338327950288419")
+
+        // Check value too large fails to decode.
+        XCTAssertThrowsError(try JSONDecoder().decode(Decimal.self, from: "100e200".data(using: .utf8)!))
     }
 
     func test_codingOfString() {