Fix base_nocompile_tests on Windows.

The error message when downcasting a derived class that multiply
inherited from SupportsWeakPtr differed on Windows; instead of
producing a "no matching function" message, it produced the much more
confusing "use of member 'AsWeakPtrImpl' before its declaration is a
Microsoft extension [-Wmicrosoft-template]".

Change the way we detect the SupportsWeakPtr base to something that
produces the same error on all platforms (and, hopefully, one that is
clearer about its cause).

Bug: 1442625
Change-Id: I2de8d247c528a2afb36c3e65d4c4a2bca6f35bdc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4814291
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1188633}

base/memory/weak_ptr.h

@@ -176,29 +176,37 @@ class BASE_EXPORT WeakReferenceOwner {
 class SupportsWeakPtrBase {
  public:
   // A safe static downcast of a WeakPtr<Base> to WeakPtr<Derived>. This
-  // conversion will only compile if there is exists a Base which inherits
-  // from SupportsWeakPtr<Base>. See base::AsWeakPtr() below for a helper
-  // function that makes calling this easier.
+  // conversion will only compile if Derived singly inherits from
+  // SupportsWeakPtr<Base>. See base::AsWeakPtr() below for a helper function
+  // that makes calling this easier.
   //
   // Precondition: t != nullptr
   template<typename Derived>
   static WeakPtr<Derived> StaticAsWeakPtr(Derived* t) {
     static_assert(
         std::is_base_of<internal::SupportsWeakPtrBase, Derived>::value,
         "AsWeakPtr argument must inherit from SupportsWeakPtr");
-    return AsWeakPtrImpl<Derived>(t);
-  }
-
- private:
-  // This template function uses type inference to find a Base of Derived
-  // which is an instance of SupportsWeakPtr<Base>. We can then safely
-  // static_cast the Base* to a Derived*.
-  template <typename Derived, typename Base>
-  static WeakPtr<Derived> AsWeakPtrImpl(SupportsWeakPtr<Base>* t) {
-    WeakPtr<Base> weak = t->AsWeakPtr();
+    using Base = typename decltype(ExtractSinglyInheritedBase(t))::Base;
+    // Ensure SupportsWeakPtr<Base>::AsWeakPtr() is called even if the subclass
+    // hides or overloads it.
+    WeakPtr<Base> weak = static_cast<SupportsWeakPtr<Base>*>(t)->AsWeakPtr();
     return WeakPtr<Derived>(weak.CloneWeakReference(),
                             static_cast<Derived*>(weak.ptr_));
   }
+
+ private:
+  // This class can only be instantiated if the constructor argument inherits
+  // from SupportsWeakPtr<T> in exactly one way.
+  template <typename T>
+  struct ExtractSinglyInheritedBase;
+  template <typename T>
+  struct ExtractSinglyInheritedBase<SupportsWeakPtr<T>> {
+    using Base = T;
+    explicit ExtractSinglyInheritedBase(SupportsWeakPtr<T>*);
+  };
+  template <typename T>
+  ExtractSinglyInheritedBase(SupportsWeakPtr<T>*)
+      -> ExtractSinglyInheritedBase<SupportsWeakPtr<T>>;
 };
 
 // Forward declaration from safe_ptr.h.

base/memory/weak_ptr_unittest.nc

@@ -129,7 +129,7 @@ void WontCompile() {
   WeakPtr<Unrelated> ptr = AsWeakPtr(&f);
 }
 
-#elif defined(NCTEST_AMBIGUOUS_ANCESTORS)  // [r"fatal error: no matching function for call to 'AsWeakPtrImpl'"]
+#elif defined(NCTEST_AMBIGUOUS_ANCESTORS)  // [r"fatal error: no viable constructor or deduction guide for deduction of template arguments of 'ExtractSinglyInheritedBase'"]
 
 void WontCompile() {
   MultiplyDerivedProducer f;