Correctly parse constant expressions in dds style properties (#11282)

* Correctly parse constant expressions in dds style properties

Closes #10849

* Clarify

include/mbgl/style/conversion/data_driven_property_value.hpp

@@ -8,6 +8,8 @@
 #include <mbgl/style/expression/is_expression.hpp>
 #include <mbgl/style/expression/is_constant.hpp>
 #include <mbgl/style/expression/find_zoom_curve.hpp>
+#include <mbgl/style/expression/literal.hpp>
+#include <mbgl/style/expression/value.hpp>
 
 #include <unordered_set>
 
@@ -32,12 +34,25 @@ struct Converter<DataDrivenPropertyValue<T>> {
                 return {};
             }
             
-            if (isFeatureConstant(**expression)) {
+            bool featureConstant = isFeatureConstant(**expression);
+            bool zoomConstant = isZoomConstant(**expression);
+            
+            if (featureConstant && !zoomConstant) {
                 return DataDrivenPropertyValue<T>(CameraFunction<T>(std::move(*expression)));
-            } else if (isZoomConstant(**expression)) {
+            } else if (!featureConstant && zoomConstant) {
                 return DataDrivenPropertyValue<T>(SourceFunction<T>(std::move(*expression)));
-            } else {
+            } else if (!featureConstant && !zoomConstant) {
                 return DataDrivenPropertyValue<T>(CompositeFunction<T>(std::move(*expression)));
+            } else {
+                // If an expression is neither zoom- nor feature-dependent, it
+                // should have been reduced to a Literal when it was parsed.
+                auto literal = dynamic_cast<Literal*>(expression->get());
+                assert(literal);
+                optional<T> constant = fromExpressionValue<T>(literal->getValue());
+                if (!constant) {
+                    return {};
+                }
+                return DataDrivenPropertyValue<T>(*constant);
             }
         } else if (!isObject(value)) {
             optional<T> constant = convert<T>(value, error);

include/mbgl/style/expression/literal.hpp

@@ -32,6 +32,10 @@ public:
     std::vector<optional<Value>> possibleOutputs() const override {
         return {{ value }};
     }
+    
+    Value getValue() const {
+        return value;
+    }
 
 private:
     Value value;