Use std::is_convertible in EnsurePtrConvertibleArgDecl

This is to make EnsurePtrConvertibleArgDecl(T, const T) to fail
for https://codereview.chromium.org/2220543002.

This CL also adds unit tests for EnsurePtrConvertibleArgDecl.

BUG=

Review-Url: https://codereview.chromium.org/2220443004
Cr-Commit-Position: refs/heads/master@{#418638}

third_party/WebKit/Source/wtf/TypeTraits.h

@@ -218,23 +218,23 @@ struct RemoveTemplate<OuterTemplate<T>, OuterTemplate> {
 // Here, we use a template specialization for same type case to allow incomplete
 // types.
 
-template <typename T, typename U> struct IsBaseOf {
-    static const bool value = std::is_base_of<T, U>::value;
+template <typename T, typename U> struct IsConvertible {
+    static const bool value = std::is_convertible<T, U>::value;
 };
 
-template <typename T> struct IsBaseOf<T, T> {
+template <typename T> struct IsConvertible<T, T> {
     static const bool value = true;
 };
 
 #define EnsurePtrConvertibleArgDecl(From, To) \
-    typename std::enable_if<WTF::IsBaseOf<To, From>::value>::type* = nullptr
+    typename std::enable_if<WTF::IsConvertible<From*, To*>::value>::type* = nullptr
 #define EnsurePtrConvertibleArgDefn(From, To) \
-    typename std::enable_if<WTF::IsBaseOf<To, From>::value>::type*
+    typename std::enable_if<WTF::IsConvertible<From*, To*>::value>::type*
 #else
 #define EnsurePtrConvertibleArgDecl(From, To) \
-    typename std::enable_if<std::is_base_of<To, From>::value>::type* = nullptr
+    typename std::enable_if<std::is_convertible<From*, To*>::value>::type* = nullptr
 #define EnsurePtrConvertibleArgDefn(From, To) \
-    typename std::enable_if<std::is_base_of<To, From>::value>::type*
+    typename std::enable_if<std::is_convertible<From*, To*>::value>::type*
 #endif
 
 } // namespace WTF

third_party/WebKit/Source/wtf/TypeTraitsTest.cpp

@@ -180,6 +180,42 @@ static_assert(!IsTriviallyDefaultConstructible<DefaultConstructorDeleted>::value
 static_assert(!IsTriviallyDestructible<DestructorDeleted>::value,
     "DestructorDeleted must not be trivially destructible.");
 
+template<typename T>
+class Wrapper
+{
+public:
+    template<typename U>
+    Wrapper(const Wrapper<U>&, EnsurePtrConvertibleArgDecl(U, T))
+    {
+    }
+};
+
+class ForwardDeclarationOnlyClass;
+
+static_assert(std::is_convertible<Wrapper<TestDerivedClass>, Wrapper<TestDerivedClass>>::value,
+    "EnsurePtrConvertibleArgDecl<T, T> should pass");
+
+static_assert(std::is_convertible<Wrapper<TestDerivedClass>, Wrapper<const TestDerivedClass>>::value,
+    "EnsurePtrConvertibleArgDecl<T, const T> should pass");
+
+static_assert(!std::is_convertible<Wrapper<const TestDerivedClass>, Wrapper<TestDerivedClass>>::value,
+    "EnsurePtrConvertibleArgDecl<const T, T> should not pass");
+
+static_assert(std::is_convertible<Wrapper<ForwardDeclarationOnlyClass>, Wrapper<ForwardDeclarationOnlyClass>>::value,
+    "EnsurePtrConvertibleArgDecl<T, T> should pass if T is not a complete type");
+
+static_assert(std::is_convertible<Wrapper<ForwardDeclarationOnlyClass>, Wrapper<const ForwardDeclarationOnlyClass>>::value,
+    "EnsurePtrConvertibleArgDecl<T, const T> should pass if T is not a complete type");
+
+static_assert(!std::is_convertible<Wrapper<const ForwardDeclarationOnlyClass>, Wrapper<ForwardDeclarationOnlyClass>>::value,
+    "EnsurePtrConvertibleArgDecl<const T, T> should not pass if T is not a complete type");
+
+static_assert(std::is_convertible<Wrapper<TestDerivedClass>, Wrapper<TestBaseClass<int>>>::value,
+    "EnsurePtrConvertibleArgDecl<U, T> should pass if U is a subclass of T");
+
+static_assert(!std::is_convertible<Wrapper<TestBaseClass<int>>, Wrapper<TestDerivedClass>>::value,
+    "EnsurePtrConvertibleArgDecl<U, T> should not pass if U is a base class of T");
+
 } // anonymous namespace
 
 } // namespace WTF