Fix style parsing bug for constant expressions (#11606)

* Fix style parsing bug for constant expressions

Closes #10849

* Ignore tests for unported GL JS change

Refs https://github.com/mapbox/mapbox-gl-js/pull/6429

* Fuller fix

* Update mapbox-gl-js

2 files changed, 3 insertions, 2 deletions

diff --git a/include/mbgl/style/conversion/data_driven_property_value.hpp b/include/mbgl/style/conversion/data_driven_property_value.hpp
index 5f4395f35a..b4297816df 100644
--- a/include/mbgl/style/conversion/data_driven_property_value.hpp
+++ b/include/mbgl/style/conversion/data_driven_property_value.hpp
@@ -44,8 +44,6 @@ struct Converter<DataDrivenPropertyValue<T>> {
             } 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());
diff --git a/include/mbgl/style/expression/let.hpp b/include/mbgl/style/expression/let.hpp
index 75d2adda62..d0210d8bba 100644
--- a/include/mbgl/style/expression/let.hpp
+++ b/include/mbgl/style/expression/let.hpp
@@ -70,6 +70,9 @@ public:
 
     mbgl::Value serialize() const override;
     std::string getOperator() const override { return "var"; }
+    
+    const std::shared_ptr<Expression>& getBoundExpression() const { return value; }
+    
 private:
     std::string name;
     std::shared_ptr<Expression> value;