[core] Expose dsl::compound() internally

This will be used in a follow up patch by the function to expression
conversion code.

src/core-files.json

@@ -673,6 +673,7 @@
         "mbgl/style/conversion/json.hpp": "src/mbgl/style/conversion/json.hpp",
         "mbgl/style/conversion/stringify.hpp": "src/mbgl/style/conversion/stringify.hpp",
         "mbgl/style/custom_tile_loader.hpp": "src/mbgl/style/custom_tile_loader.hpp",
+        "mbgl/style/expression/dsl_impl.hpp": "src/mbgl/style/expression/dsl_impl.hpp",
         "mbgl/style/expression/util.hpp": "src/mbgl/style/expression/util.hpp",
         "mbgl/style/image_impl.hpp": "src/mbgl/style/image_impl.hpp",
         "mbgl/style/layer_impl.hpp": "src/mbgl/style/layer_impl.hpp",

src/mbgl/style/expression/dsl.cpp

@@ -1,4 +1,5 @@
 #include <mbgl/style/expression/dsl.hpp>
+#include <mbgl/style/expression/dsl_impl.hpp>
 #include <mbgl/style/expression/error.hpp>
 #include <mbgl/style/expression/literal.hpp>
 #include <mbgl/style/expression/assertion.hpp>
@@ -14,18 +15,13 @@ namespace style {
 namespace expression {
 namespace dsl {
 
-static std::unique_ptr<Expression> compound(const char* op, std::vector<std::unique_ptr<Expression>> args) {
+std::unique_ptr<Expression> compound(const char* op, std::vector<std::unique_ptr<Expression>> args) {
     ParsingContext ctx;
     ParseResult result =  createCompoundExpression(op, std::move(args), ctx);
     assert(result);
     return std::move(*result);
 }
 
-template <class... Args>
-static std::unique_ptr<Expression> compound(const char* op, Args... args) {
-    return compound(op, vec(std::move(args)...));
-}
-
 std::unique_ptr<Expression> error(std::string message) {
     return std::make_unique<Error>(std::move(message));
 }

src/mbgl/style/expression/dsl_impl.hpp

@@ -0,0 +1,21 @@
+#pragma once
+
+#include <mbgl/style/expression/expression.hpp>
+#include <mbgl/util/color.hpp>
+
+namespace mbgl {
+namespace style {
+namespace expression {
+namespace dsl {
+
+std::unique_ptr<Expression> compound(const char* op, std::vector<std::unique_ptr<Expression>> args);
+
+template <class... Args>
+std::unique_ptr<Expression> compound(const char* op, Args... args) {
+    return compound(op, vec(std::move(args)...));
+}
+
+} // namespace dsl
+} // namespace expression
+} // namespace style
+} // namespace mbgl