Improve NaN handling in absl::Duration arithmetic.

PiperOrigin-RevId: 774936932
Change-Id: Ibde499b8c9825b4357edf71cfcb9c45eb75f4702

absl/time/duration.cc

diff --git a/absl/time/duration.cc b/absl/time/duration.cc
index 38c4b63..fb7c90a 100644
--- a/absl/time/duration.cc
+++ b/absl/time/duration.cc
@@ -469,7 +469,7 @@
 
 Duration& Duration::operator*=(double r) {
   if (time_internal::IsInfiniteDuration(*this) || !IsFinite(r)) {
-    const bool is_neg = std::signbit(r) != (rep_hi_.Get() < 0);
+    const bool is_neg = std::isnan(r) || std::signbit(r) != (rep_hi_.Get() < 0);
     return *this = is_neg ? -InfiniteDuration() : InfiniteDuration();
   }
   return *this = ScaleDouble<std::multiplies>(*this, r);
@@ -485,7 +485,7 @@
 
 Duration& Duration::operator/=(double r) {
   if (time_internal::IsInfiniteDuration(*this) || !IsValidDivisor(r)) {
-    const bool is_neg = std::signbit(r) != (rep_hi_.Get() < 0);
+    const bool is_neg = std::isnan(r) || std::signbit(r) != (rep_hi_.Get() < 0);
     return *this = is_neg ? -InfiniteDuration() : InfiniteDuration();
   }
   return *this = ScaleDouble<std::divides>(*this, r);

absl/time/duration_test.cc

diff --git a/absl/time/duration_test.cc b/absl/time/duration_test.cc
index 1e3fe67..164ad6b 100644
--- a/absl/time/duration_test.cc
+++ b/absl/time/duration_test.cc
@@ -841,18 +841,18 @@
 
 TEST(Duration, NaN) {
   // Note that IEEE 754 does not define the behavior of a nan's sign when it is
-  // copied, so the code below allows for either + or - InfiniteDuration.
+  // copied. We return -InfiniteDuration in either case.
 #define TEST_NAN_HANDLING(NAME, NAN)           \
   do {                                         \
     const auto inf = absl::InfiniteDuration(); \
     auto x = NAME(NAN);                        \
-    EXPECT_TRUE(x == inf || x == -inf);        \
+    EXPECT_TRUE(x == -inf);                    \
     auto y = NAME(42);                         \
     y *= NAN;                                  \
-    EXPECT_TRUE(y == inf || y == -inf);        \
+    EXPECT_TRUE(y == -inf);                    \
     auto z = NAME(42);                         \
     z /= NAN;                                  \
-    EXPECT_TRUE(z == inf || z == -inf);        \
+    EXPECT_TRUE(z == -inf);                    \
   } while (0)
 
   const double nan = std::numeric_limits<double>::quiet_NaN();