addition between number and unary ops

src/value/ops.rs

@@ -9,6 +9,8 @@ impl Value {
     pub fn add(self, mut other: Self, span: Span) -> SassResult<Self> {
         if let Self::Paren(..) = other {
             other = other.eval(span)?.node
+        } else if let Self::UnaryOp(..) = other {
+            other = other.eval(span)?.node
         }
         let precedence = Op::Plus.precedence();
         Ok(match self {

tests/addition.rs

@@ -263,3 +263,8 @@ test!(
     "a {\n  color: 1 + false;\n}\n",
     "a {\n  color: 1false;\n}\n"
 );
+test!(
+    num_plus_unary_not,
+    "a {\n  color: 1 + not 2;\n}\n",
+    "a {\n  color: 1false;\n}\n"
+);