From 5e9008dfb7be19a51855009f67b57738c7dea5d2 Mon Sep 17 00:00:00 2001
From: William Throwe <wtt6@cornell.edu>
Date: Sat, 22 Aug 2015 20:06:25 -0400
Subject: [PATCH] Fix undefined behavior in f64::sqrt

This fixes a reappearance of bug #9987 introduced in
1ddee8070d3cb83609b1f71c29e3deda3d30fd51, which caused
f64::tests::test_sqrt_domain to fail (at least on some systems).
---
 src/libstd/num/f64.rs | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/src/libstd/num/f64.rs b/src/libstd/num/f64.rs
index 14c540e819e9..252a941d86c8 100644
--- a/src/libstd/num/f64.rs
+++ b/src/libstd/num/f64.rs
@@ -454,7 +454,11 @@ impl f64 {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
     pub fn sqrt(self) -> f64 {
-        unsafe { intrinsics::sqrtf64(self) }
+        if self < 0.0 {
+            NAN
+        } else {
+            unsafe { intrinsics::sqrtf64(self) }
+        }
     }
 
     /// Returns `e^(self)`, (the exponential function).