Duration.seconds, .milliseconds, and .microseconds should preserve exact values

stdlib/public/core/Duration.swift

@@ -114,6 +114,23 @@ extension Duration {
     return Duration(_attoseconds:
       _Int128(seconds).multiplied(by: 1_000_000_000_000_000_000 as UInt64))
   }
+
+  /// Construct a `Duration` given a duration and scale, taking care so that
+  /// exact integer durations are preserved exactly.
+  internal init(_ duration: Double, scale: UInt64) {
+    // Split the duration into integral and fractional parts, as we need to
+    // handle them slightly differently to ensure that integer values are
+    // never rounded if `scale` is representable as Double.
+    let integralPart = duration.rounded(.towardZero)
+    let fractionalPart = integralPart - duration
+    self.init(_attoseconds:
+      // This term may trap due to overflow, but it cannot round, so if the
+      // input `seconds` is an exact integer, we get an exact integer result.
+      _Int128(integralPart).multiplied(by: scale) +
+      // This term may round, but cannot overflow.
+      _Int128((fractionalPart * Double(scale)).rounded())
+    )
+  }
 
   /// Construct a `Duration` given a number of seconds represented as a 
   /// `Double` by converting the value into the closest attosecond scale value.
@@ -123,7 +140,7 @@ extension Duration {
   /// - Returns: A `Duration` representing a given number of seconds.
   @available(SwiftStdlib 5.7, *)
   public static func seconds(_ seconds: Double) -> Duration {
-    return Duration(_attoseconds: _Int128(seconds * 1_000_000_000_000_000_000))
+    Duration(seconds, scale: 1_000_000_000_000_000_000)
   }
 
   /// Construct a `Duration` given a number of milliseconds represented as a 
@@ -148,8 +165,7 @@ extension Duration {
   /// - Returns: A `Duration` representing a given number of milliseconds.
   @available(SwiftStdlib 5.7, *)
   public static func milliseconds(_ milliseconds: Double) -> Duration {
-    return Duration(_attoseconds:
-      _Int128(milliseconds * 1_000_000_000_000_000))
+    Duration(milliseconds, scale: 1_000_000_000_000_000)
   }
 
   /// Construct a `Duration` given a number of microseconds represented as a 
@@ -174,8 +190,7 @@ extension Duration {
   /// - Returns: A `Duration` representing a given number of microseconds.
   @available(SwiftStdlib 5.7, *)
   public static func microseconds(_ microseconds: Double) -> Duration {
-    return Duration(_attoseconds:
-      _Int128(microseconds * 1_000_000_000_000))
+    Duration(microseconds, scale: 1_000_000_000_000)
   }
 
   /// Construct a `Duration` given a number of nanoseconds represented as a 

test/stdlib/Duration.swift

@@ -0,0 +1,46 @@
+// RUN: %target-run-simple-swift
+// REQUIRES: executable_test
+
+import StdlibUnittest
+
+var suite = TestSuite("DurationTests")
+defer { runAllTests() }
+
+if #available(SwiftStdlib 5.7, *) {
+  suite.test("seconds from Double") {
+    for _ in 0 ..< 100 {
+      let integerValue = Double(Int64.random(in: 0 ... 0x7fff_ffff_ffff_fc00))
+      let (sec, attosec) = Duration.seconds(integerValue).components
+      expectEqual(sec, Int64(integerValue))
+      expectEqual(attosec, 0)
+    }
+    // Value that overflows conversion from Double -> Int64, but should be
+    // representable as a number of seconds:
+    let huge: Double = 1.7e20
+    let duration = Duration.seconds(huge)
+    // Divide by 1000 to get back to a duration with representable components:
+    let smallerDuration = duration / 1000
+    expectEqual(smallerDuration.components, (170_000_000_000_000_000, 0))
+    // Now check that the components of the original value trap:
+    expectCrashLater()
+    let _ = duration.components
+  }
+
+  suite.test("milliseconds from Double") {
+    for _ in 0 ..< 100 {
+      let integerValue = Double(Int64.random(in: 0 ... 0x7fff_ffff_ffff_fc00))
+      let (sec, attosec) = Duration.milliseconds(integerValue).components
+      expectEqual(sec, Int64(integerValue) / 1000)
+      expectEqual(attosec, Int64(integerValue) % 1000 * 1_000_000_000_000_000)
+    }
+  }
+
+  suite.test("microseconds from Double") {
+    for _ in 0 ..< 100 {
+      let integerValue = Double(Int64.random(in: 0 ... 0x7fff_ffff_ffff_fc00))
+      let (sec, attosec) = Duration.microseconds(integerValue).components
+      expectEqual(sec, Int64(integerValue) / 1_000_000)
+      expectEqual(attosec, Int64(integerValue) % 1_000_000 * 1_000_000_000_000)
+    }
+  }
+}