base/memory: Remove base::internal::DereferencedPointerType

Replaces base::internal::DereferencedPointerType with the std
templates in <type_traits>.

Bug: 1073933
Change-Id: I108fc52d5f3834b0749f95342a76578ef1c10306
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2302880Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Bartek Nowierski <bartekn@chromium.org>
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#790318}

base/memory/checked_ptr.h

@@ -343,15 +343,6 @@ struct CheckedPtr2OrMTEImpl {
 
 #endif  // defined(ARCH_CPU_64_BITS) && !defined(OS_NACL)
 
-template <typename T>
-struct DereferencedPointerType {
-  using Type = decltype(*std::declval<T*>());
-};
-// This explicitly doesn't define any type aliases, since dereferencing void is
-// invalid.
-template <>
-struct DereferencedPointerType<void> {};
-
 }  // namespace internal
 
 // DO NOT USE! EXPERIMENTAL ONLY! This is helpful for local testing!
@@ -423,11 +414,10 @@ class CheckedPtr {
     return wrapped_ptr_ != Impl::GetWrappedNullPtr();
   }
 
-  // Use SFINAE to avoid defining |operator*| for T=void, which wouldn't compile
-  // due to |void&|.
   template <typename U = T,
-            typename V = typename internal::DereferencedPointerType<U>::Type>
-  ALWAYS_INLINE V& operator*() const {
+            typename Unused = std::enable_if_t<
+                !std::is_void<typename std::remove_cv<U>::type>::value>>
+  ALWAYS_INLINE U& operator*() const {
     return *GetForDereference();
   }
   ALWAYS_INLINE T* operator->() const { return GetForDereference(); }

base/memory/checked_ptr_unittest.nc

@@ -73,7 +73,7 @@ void WontCompile() {
   std::ignore = static_cast<Unrelated*>(ptr);
 }
 
-#elif defined(NCTEST_VOID_DEREFERENCE) // [r"ISO C\+\+ does not allow indirection on operand of type 'const void \*' \[-Wvoid-ptr-dereference\]"]
+#elif defined(NCTEST_VOID_DEREFERENCE) // [r"indirection requires pointer operand \('CheckedPtr<const void>' invalid\)"]
 
 void WontCompile() {
   const char foo[] = "42";