correct several regressions traced through synaps-cad

Author: timschmidt

src/computable/node.rs

@@ -4050,7 +4050,9 @@ impl Computable {
             ) {
                 match (left_sign, right_sign) {
                     (Sign::Plus, Sign::Minus) if left_msd > right_msd => Some(Sign::Plus),
-                    (Sign::Minus, Sign::Plus) if right_msd > left_msd => Some(Sign::Minus),
+                    (Sign::Plus, Sign::Minus) if right_msd > left_msd => Some(Sign::Minus),
+                    (Sign::Minus, Sign::Plus) if left_msd > right_msd => Some(Sign::Minus),
+                    (Sign::Minus, Sign::Plus) if right_msd > left_msd => Some(Sign::Plus),
                     (Sign::Plus, Sign::Plus) => Some(Sign::Plus),
                     (Sign::Minus, Sign::Minus) => Some(Sign::Minus),
                     (Sign::NoSign, Sign::NoSign) => Some(Sign::NoSign),

src/real/convert.rs

@@ -375,6 +375,8 @@ impl Real {
         use crate::real::Class;
 
         if matches!(self.class, Class::One)
+            && self.computable.is_none()
+            && self.signal.is_none()
             && let fast @ Some(_) = self.rational.to_f64_lossy()
         {
             // Exact rationals can often be rounded to f64 without touching the
@@ -761,6 +763,17 @@ mod tests {
         assert!((approx + std::f64::consts::SQRT_2).abs() < 1e-15);
     }
 
+    #[test]
+    fn borrowed_f64_lossy_keeps_positive_translation_over_negative_symbolic_tail() {
+        let sqrt_two = Real::from(2).sqrt().unwrap();
+        let value = Real::from(10) + (Real::from(1) - sqrt_two);
+
+        let approx = value.to_f64_lossy().unwrap();
+
+        assert!(approx.is_sign_positive());
+        assert!((approx - (11.0 - std::f64::consts::SQRT_2)).abs() < 1e-12);
+    }
+
     #[test]
     #[cfg(all(feature = "cached-f32-approx", feature = "cached-f64-approx"))]
     fn primitive_approx_cache_fills_and_upgrades() {