emit proper error for map as lhs of addition

src/value/ops.rs

@@ -256,7 +256,14 @@ impl Value {
         }
         let precedence = Op::Plus.precedence();
         Ok(match self {
-            Self::Function(..) | Self::ArgList(..) | Self::Map(..) => todo!(),
+            Self::Map(..) => {
+                return Err((
+                    format!("{} isn't a valid CSS value.", self.inspect(span)?),
+                    span,
+                )
+                    .into())
+            }
+            Self::Function(..) | Self::ArgList(..) => todo!(),
             Self::Important | Self::True | Self::False => match other {
                 Self::String(s, QuoteKind::Quoted) => Value::String(
                     format!("{}{}", self.to_css_string(span)?, s),

tests/error.rs

@@ -124,7 +124,10 @@ error!(
 );
 error!(toplevel_hash, "#", "Error: expected \"{\".");
 error!(toplevel_at, "@", "Error: Expected identifier.");
-error!(toplevel_ampersand, "& {}", "Error: Top-level selectors may not contain the parent selector \"&\".");
+error!(
+    toplevel_ampersand,
+    "& {}", "Error: Top-level selectors may not contain the parent selector \"&\"."
+);
 error!(toplevel_backslash, "\\", "Error: expected \"}\".");
 error!(toplevel_var_no_colon, "$r", "Error: expected \":\".");
 error!(bar_in_value, "a {color: a|b;}", "Error: expected \";\".");
@@ -164,3 +167,7 @@ error!(
     operator_mul,
     "a {color: 5 - *;}", "Error: Expected expression."
 );
+error!(
+    map_lhs_add,
+    "a {color: (a: b) + 1;}", "Error: (a: b) isn't a valid CSS value."
+);