1 files changed, 5 insertions, 6 deletions
diff --git a/include/mbgl/style/expression/parsing_context.hpp b/include/mbgl/style/expression/parsing_context.hpp
index 596b136419..40da7c26d0 100644
--- a/include/mbgl/style/expression/parsing_context.hpp
+++ b/include/mbgl/style/expression/parsing_context.hpp
@@ -93,7 +93,7 @@ public:
         Parse the given style-spec JSON value as an expression.
     */
     ParseResult parseExpression(const mbgl::style::conversion::Convertible& value,
-                                optional<TypeAnnotationOption> = {});
+                                const optional<TypeAnnotationOption>& = {});
 
     /*
         Parse the given style-spec JSON value as an expression intended to be used
@@ -108,8 +108,8 @@ public:
     ParseResult parse(const mbgl::style::conversion::Convertible&,
                       std::size_t,
                       optional<type::Type> = {},
-                      optional<TypeAnnotationOption> = {});
-    
+                      const optional<TypeAnnotationOption>& = {});
+
     /*
         Parse a child expression.  For use by individual Expression::parse() methods.
     */
@@ -168,9 +168,8 @@ private:
         type (either Literal, or the one named in value[0]) and dispatching to the
         appropriate ParseXxxx::parse(const V&, ParsingContext) method.
     */
-    ParseResult parse(const mbgl::style::conversion::Convertible& value,
-                      optional<TypeAnnotationOption> = {});
-    
+    ParseResult parse(const mbgl::style::conversion::Convertible& value, const optional<TypeAnnotationOption>& = {});
+
     std::string key;
     optional<type::Type> expected;
     std::shared_ptr<detail::Scope> scope;