[stdlib] Fix a Float16-to-integer conversion bug (swiftlang#33893)

* [stdlib] Fix a Float16-to-integer conversion bug

* [stdlib] Add a test for 'Int(exactly: Float16.infinity)'

Author: xwu

stdlib/public/core/IntegerTypes.swift.gyb

@@ -1195,9 +1195,11 @@ public struct ${Self}
     // undefined if it overflows.
 %     if not (FloatBits == 16 and bits >= 32): # Float16 is always in-range for 32- and 64-bit ints.
     guard source > ${str(lower)}.0 && source < ${str(upper)}.0 else {
+%     else:
+    guard source.isFinite else {
+%     end
       return nil
     }
-%     end
     guard source == source.rounded(.towardZero) else {
       return nil
     }

test/stdlib/FloatingPoint.swift.gyb

@@ -260,6 +260,16 @@ FloatingPoint.test("Float/staticProperties") {
 }
 
 // Tests the float and int conversions work correctly. Each case is special.
+
+#if !os(macOS) && !(os(iOS) && targetEnvironment(macCatalyst))
+if #available(iOS 14.0, watchOS 7.0, tvOS 14.0, *) {
+  FloatingPoint.test("Float16/Int") {
+    expectNil(Int(exactly: Float16.infinity))
+    expectNil(Int(exactly: -Float16.infinity))
+  }
+}
+#endif
+
 FloatingPoint.test("Float/UInt8") {
   expectEqual(UInt8.min, UInt8(Float(UInt8.min)))
   expectEqual(UInt8.max, UInt8(Float(UInt8.max)))