Fix a parenthesizing bug in calculations (#1482)

See sass/sass#3147
See sass/sass#3148

Author: nex3

CHANGELOG.md

@@ -10,7 +10,10 @@
   is a `:host` or `:host-context` and the other is a selector that's guaranteed
   to be within the current shadow DOM. The `@extend` logic has been updated
   accordingly as well.
-  
+
+* Fix a bug where the right-hand operand of a `-` in a calculation could
+  incorrectly be stripped of parentheses.
+
 ### Dart API
 
 * `SassCalculation.plus()` now allows `SassString` arguments.

lib/src/visitor/serialize.dart

@@ -498,14 +498,25 @@ class _SerializeVisitor
       var right = value.right;
       var parenthesizeRight = right is CalculationInterpolation ||
           (right is CalculationOperation &&
-              (right.operator.precedence < value.operator.precedence ||
-                  value.operator == CalculationOperator.dividedBy));
+              _parenthesizeCalculationRhs(value.operator, right.operator));
       if (parenthesizeRight) _buffer.writeCharCode($lparen);
       _writeCalculationValue(right);
       if (parenthesizeRight) _buffer.writeCharCode($rparen);
     }
   }
 
+  /// Returns whether the right-hand operation of a calculation should be
+  /// parenthesized.
+  ///
+  /// In `a ? (b # c)`, `outer` is `?` and `right` is `#`.
+  bool _parenthesizeCalculationRhs(
+      CalculationOperator outer, CalculationOperator right) {
+    if (outer == CalculationOperator.dividedBy) return true;
+    if (outer == CalculationOperator.plus) return false;
+    return right == CalculationOperator.plus ||
+        right == CalculationOperator.minus;
+  }
+
   void visitColor(SassColor value) {
     // In compressed mode, emit colors in the shortest representation possible.
     if (_isCompressed && fuzzyEquals(value.alpha, 1)) {