[core] Implement LatLng::null()

include/mbgl/util/geo.hpp

@@ -14,11 +14,15 @@ using ScreenCoordinate = vec2<double>;
 
 class LatLng {
 public:
+    struct null {};
+
     double latitude;
     double longitude;
 
     enum WrapMode : bool { Unwrapped, Wrapped };
 
+    LatLng(null) : latitude(std::numeric_limits<double>::quiet_NaN()), longitude(latitude) {}
+
     LatLng(double lat = 0, double lon = 0, WrapMode mode = Unwrapped)
         : latitude(lat), longitude(lon) { if (mode == Wrapped) wrap(); }
 

src/mbgl/map/transform.cpp

@@ -651,7 +651,7 @@ void Transform::setGestureInProgress(bool inProgress) {
 #pragma mark Conversion and projection
 
 ScreenCoordinate Transform::latLngToScreenCoordinate(const LatLng& latLng) const {
-    if (!latLng) return {};
+    if (!latLng) return ScreenCoordinate::null();
 
     // If the center and point coordinates are not in the same side of the
     // antimeridian, we need to unwrap the point longitude to make sure it can
@@ -672,7 +672,8 @@ ScreenCoordinate Transform::latLngToScreenCoordinate(const LatLng& latLng) const
 }
 
 LatLng Transform::screenCoordinateToLatLng(const ScreenCoordinate& point) const {
-    if (!point) return {};
+    if (!point) return LatLng::null();
+
     ScreenCoordinate flippedPoint = point;
     flippedPoint.y = state.height - flippedPoint.y;
     return state.screenCoordinateToLatLng(flippedPoint);

test/map/transform.cpp

@@ -79,6 +79,9 @@ TEST(Transform, InvalidLatLng) {
     ASSERT_DOUBLE_EQ(10, transform.getLatLng().latitude);
     ASSERT_DOUBLE_EQ(8, transform.getLatLng().longitude);
     ASSERT_DOUBLE_EQ(4, transform.getScale());
+
+    ASSERT_FALSE(transform.latLngToScreenCoordinate(LatLng::null()));
+    ASSERT_FALSE(transform.screenCoordinateToLatLng(ScreenCoordinate::null()));
 }