Make withSign(NaN) behave as in JVM

#KT-4900

Where only the sign of the result is undefined, but the absolute value is unchanged.

js/js.libraries/src/core/math.kt

@@ -463,11 +463,11 @@ public inline val Double.sign: Double get() = nativeMath.sign(this)
  * If [sign] is `NaN` the sign of the result is undefined.
  */
 @SinceKotlin("1.2")
-public fun Double.withSign(sign: Double): Double =
-        this.absoluteValue * when(sign) {
-            0.0 -> sign(1 / sign)
-            else -> sign(sign)
-        }
+public fun Double.withSign(sign: Double): Double {
+    val thisSignBit = js("Kotlin").doubleSignBit(this).unsafeCast<Int>()
+    val newSignBit = js("Kotlin").doubleSignBit(sign).unsafeCast<Int>()
+    return if (thisSignBit == newSignBit) this else -this
+}
 
 /**
  * Returns this value with the sign bit same as of the [sign] value.

js/js.libraries/src/js/misc.js

@@ -83,6 +83,12 @@ function imul(a, b) {
         bufInt32[0] = value;
         return bufFloat32[0];
     };
+
+    // returns zero value for number with positive sign bit and non-zero value for number with negative sign bit.
+    Kotlin.doubleSignBit = function(value) {
+        bufFloat64[0] = value;
+        return bufInt32[1] & 0x80000000;
+    }
 })();
 
 

libraries/stdlib/test/numbers/MathTest.kt

@@ -301,7 +301,7 @@ class DoubleMathTest {
             for (b in allValues) {
                 val r = a.withSign(b)
                 assertEquals(a.absoluteValue, r.absoluteValue)
-                assertEquals(b.sign, r.sign)
+                assertEquals(b.sign, r.sign, "expected $a with sign bit of $b to have sign ${b.sign}")
             }
 
             val rp0 = a.withSign(0.0)
@@ -315,6 +315,9 @@ class DoubleMathTest {
             val ri = a.withSign(-1)
             assertEquals(-1.0, ri.sign)
             assertEquals(a.absoluteValue, ri.absoluteValue)
+
+            val rn = a.withSign(Double.NaN)
+            assertEquals(a.absoluteValue, rn.absoluteValue)
         }
     }