Handle negative numbers in sqrt properly.

Closes #9987.

src/libcore/num/f32.rs

@@ -259,7 +259,11 @@ impl Float for f32 {
 
     #[inline]
     fn sqrt(self) -> f32 {
-        unsafe { intrinsics::sqrtf32(self) }
+        if self < 0.0 {
+            NAN
+        } else {
+            unsafe { intrinsics::sqrtf32(self) }
+        }
     }
 
     #[inline]

src/libcore/num/f64.rs

@@ -266,7 +266,11 @@ impl Float for f64 {
 
     #[inline]
     fn sqrt(self) -> f64 {
-        unsafe { intrinsics::sqrtf64(self) }
+        if self < 0.0 {
+            NAN
+        } else {
+            unsafe { intrinsics::sqrtf64(self) }
+        }
     }
 
     #[inline]
@@ -377,4 +381,3 @@ impl Float for f64 {
         self * (value / 180.0)
     }
 }
-

src/libcore/num/mod.rs

@@ -1501,6 +1501,8 @@ pub trait Float: Signed + Primitive {
     fn frac_1_sqrt2() -> Self;
 
     /// Take the square root of a number.
+    ///
+    /// Returns NaN if `self` is not a non-negative number.
     fn sqrt(self) -> Self;
     /// Take the reciprocal (inverse) square root of a number, `1/sqrt(x)`.
     fn rsqrt(self) -> Self;

src/libstd/num/f32.rs

@@ -787,4 +787,15 @@ mod tests {
         assert_eq!(NEG_INFINITY.integer_decode(), (8388608u64, 105i16, -1i8));
         assert_eq!(NAN.integer_decode(), (12582912u64, 105i16, 1i8));
     }
+
+    #[test]
+    fn test_sqrt_domain() {
+        assert!(NAN.sqrt().is_nan());
+        assert!(NEG_INFINITY.sqrt().is_nan());
+        assert!((-1.0f32).sqrt().is_nan());
+        assert_eq!((-0.0f32).sqrt(), -0.0);
+        assert_eq!(0.0f32.sqrt(), 0.0);
+        assert_eq!(1.0f32.sqrt(), 1.0);
+        assert_eq!(INFINITY.sqrt(), INFINITY);
+    }
 }

src/libstd/num/f64.rs

@@ -789,4 +789,15 @@ mod tests {
         assert_eq!(NEG_INFINITY.integer_decode(), (4503599627370496, 972, -1));
         assert_eq!(NAN.integer_decode(), (6755399441055744u64, 972i16, 1i8));
     }
+
+    #[test]
+    fn test_sqrt_domain() {
+        assert!(NAN.sqrt().is_nan());
+        assert!(NEG_INFINITY.sqrt().is_nan());
+        assert!((-1.0f64).sqrt().is_nan());
+        assert_eq!((-0.0f64).sqrt(), -0.0);
+        assert_eq!(0.0f64.sqrt(), 0.0);
+        assert_eq!(1.0f64.sqrt(), 1.0);
+        assert_eq!(INFINITY.sqrt(), INFINITY);
+    }
 }