Fix conversion range check for floating-point to unsigned

When converting floating-point to any integer type, the fractional part
is discarded, so any value in the range (-1.0,-0.00) (exclusive) will
truncate to -0.0, and so will fit in an unsigned destination type.

See https://en.cppreference.com/w/cpp/language/implicit_conversion#Floating.E2.80.93integral_conversions.

Change-Id: Id8a5263d486cc3cd25670da27c417c66e28fccdd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2488717Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819633}

base/numerics/safe_conversions_impl.h

@@ -378,9 +378,17 @@ struct DstRangeRelationToSrcRangeImpl<Dst,
     using SrcLimits = std::numeric_limits<Src>;
     using DstLimits = NarrowingRange<Dst, Src, Bounds>;
     using Promotion = decltype(Src() + Dst());
+    bool ge_zero = false;
+    // Converting floating-point to integer will discard fractional part, so
+    // values in (-1.0, -0.0) will truncate to 0 and fit in Dst.
+    if (std::is_floating_point<Src>::value) {
+      ge_zero = value > Src(-1);
+    } else {
+      ge_zero = value >= Src(0);
+    }
     return RangeCheck(
-        value >= Src(0) && (DstLimits::lowest() == 0 ||
-                            static_cast<Dst>(value) >= DstLimits::lowest()),
+        ge_zero && (DstLimits::lowest() == 0 ||
+                    static_cast<Dst>(value) >= DstLimits::lowest()),
         static_cast<Promotion>(SrcLimits::max()) <=
                 static_cast<Promotion>(DstLimits::max()) ||
             static_cast<Promotion>(value) <=

base/safe_numerics_unittest.cc

@@ -1535,6 +1535,11 @@ TEST(SafeNumerics, IsValueInRangeForNumericType) {
   EXPECT_FALSE(IsValueInRangeForNumericType<uint32_t>(
       std::numeric_limits<int64_t>::lowest()));
 
+  // Converting to integer types will discard the fractional part first, so -0.9
+  // will be truncated to -0.0.
+  EXPECT_TRUE(IsValueInRangeForNumericType<uint32_t>(-0.9));
+  EXPECT_FALSE(IsValueInRangeForNumericType<uint32_t>(-1.0));
+
   EXPECT_TRUE(IsValueInRangeForNumericType<int32_t>(0));
   EXPECT_TRUE(IsValueInRangeForNumericType<int32_t>(1));
   EXPECT_TRUE(IsValueInRangeForNumericType<int32_t>(2));
@@ -1569,6 +1574,11 @@ TEST(SafeNumerics, IsValueInRangeForNumericType) {
   EXPECT_FALSE(IsValueInRangeForNumericType<uint64_t>(
       std::numeric_limits<int64_t>::lowest()));
 
+  // Converting to integer types will discard the fractional part first, so -0.9
+  // will be truncated to -0.0.
+  EXPECT_TRUE(IsValueInRangeForNumericType<uint64_t>(-0.9));
+  EXPECT_FALSE(IsValueInRangeForNumericType<uint64_t>(-1.0));
+
   EXPECT_TRUE(IsValueInRangeForNumericType<int64_t>(0));
   EXPECT_TRUE(IsValueInRangeForNumericType<int64_t>(1));
   EXPECT_TRUE(IsValueInRangeForNumericType<int64_t>(2));