refactor: global to_string -> ToString

include/snapshot/internal/string_utility.h

@@ -13,6 +13,15 @@
 
 namespace snapshot {
 
+namespace internal {
+
+template <typename T, std::enable_if_t<internal::has_global_to_string_v<T>, bool> = true>
+inline std::string VisitGlobalToString(const T& t) {
+    return ToString(t);
+}
+
+}  // namespace internal
+
 class StringUtility {
     friend class StringUtilityTest;
 
@@ -24,7 +33,7 @@ class StringUtility {
 
     template <typename T, std::enable_if_t<internal::has_global_to_string_v<T>, bool> = true>
     static std::string ToString(const T& t) {
-        return to_string(t);
+        return internal::VisitGlobalToString(t);
     }
 
     template <typename T, std::enable_if_t<internal::has_class_internal_to_string_v<T>, bool> = true>

include/snapshot/types_check/has_global_to_string.h

@@ -9,7 +9,7 @@ template <typename T>
 class has_global_to_string {
 private:
     template <typename U>
-    static auto check(int) -> decltype(to_string(std::declval<U>()), std::true_type());
+    static auto check(int) -> decltype(ToString(std::declval<U>()), std::true_type());
 
     template <typename U>
     static std::false_type check(...);

test/internal/string_utility_test.cc

@@ -31,7 +31,7 @@ class CustomToString1 {
     int z;
 };
 
-string to_string(const CustomToString1& c) {
+string ToString(const CustomToString1& c) {
     return to_string(c.x) + " " + to_string(c.y) + " " + to_string(c.z) + "\n";
 }