1 files changed, 45 insertions, 1 deletions
diff --git a/third_party/base/template_util.h b/third_party/base/template_util.h
index ab660940f0..719f3f1c3a 100644
--- a/third_party/base/template_util.h
+++ b/third_party/base/template_util.h
@@ -21,7 +21,8 @@ typedef integral_constant<bool, true> true_type;
 typedef integral_constant<bool, false> false_type;
 
 template <class T, class U> struct is_same : public false_type {};
-template <class T> struct is_same<T,T> : true_type {};
+template <class T>
+struct is_same<T, T> : true_type {};
 
 template<bool B, class T = void>
 struct enable_if {};
@@ -29,6 +30,49 @@ struct enable_if {};
 template<class T>
 struct enable_if<true, T> { typedef T type; };
 
+namespace internal {
+
+// Types YesType and NoType are guaranteed such that sizeof(YesType) <
+// sizeof(NoType).
+typedef char YesType;
+
+struct NoType {
+  YesType dummy[2];
+};
+
+// This class is an implementation detail for is_convertible, and you
+// don't need to know how it works to use is_convertible. For those
+// who care: we declare two different functions, one whose argument is
+// of type To and one with a variadic argument list. We give them
+// return types of different size, so we can use sizeof to trick the
+// compiler into telling us which function it would have chosen if we
+// had called it with an argument of type From.  See Alexandrescu's
+// _Modern C++ Design_ for more details on this sort of trick.
+
+struct ConvertHelper {
+  template <typename To>
+  static YesType Test(To);
+
+  template <typename To>
+  static NoType Test(...);
+
+  template <typename From>
+  static From& Create();
+};
+
+}  // namespace internal
+
+// Inherits from true_type if From is convertible to To, false_type otherwise.
+//
+// Note that if the type is convertible, this will be a true_type REGARDLESS
+// of whether or not the conversion would emit a warning.
+template <typename From, typename To>
+struct is_convertible
+    : integral_constant<bool,
+                        sizeof(internal::ConvertHelper::Test<To>(
+                            internal::ConvertHelper::Create<From>())) ==
+                            sizeof(internal::YesType)> {};
+
 }  // namespace base
 }  // namespace pdfium