Restrict the 1-argument ArrayView constructor to types with .size() and .data()

Before this change, the argument still needed those methods, but not
having them resulted in a compilation error. Now, it results in this
constructor being removed from the overload set.

This currently makes no difference, but I'm about to publish a CL that
breaks without this.

Review-Url: https://codereview.webrtc.org/2312473002
Cr-Commit-Position: refs/heads/master@{#14068}

webrtc/base/array_view.h

@@ -11,10 +11,37 @@
 #ifndef WEBRTC_BASE_ARRAY_VIEW_H_
 #define WEBRTC_BASE_ARRAY_VIEW_H_
 
+#include <type_traits>
+
 #include "webrtc/base/checks.h"
 
 namespace rtc {
 
+namespace internal {
+
+// (Internal; please don't use outside this file.) Determines if the given
+// class has zero-argument .data() and .size() methods whose return values are
+// convertible to T* and size_t, respectively.
+template <typename DS, typename T>
+class HasDataAndSize {
+ private:
+  template <
+      typename C,
+      typename std::enable_if<
+          std::is_convertible<decltype(std::declval<C>().data()), T*>::value &&
+          std::is_convertible<decltype(std::declval<C>().size()),
+                              size_t>::value>::type* = nullptr>
+  static int Test(int);
+
+  template <typename>
+  static char Test(...);
+
+ public:
+  static constexpr bool value = std::is_same<decltype(Test<DS>(0)), int>::value;
+};
+
+}  // namespace internal
+
 // Many functions read from or write to arrays. The obvious way to do this is
 // to use two arguments, a pointer to the first element and an element count:
 //
@@ -95,7 +122,9 @@ class ArrayView final {
   // or ArrayView<const T>, const std::vector<T> to ArrayView<const T>, and
   // rtc::Buffer to ArrayView<uint8_t> (with the same const behavior as
   // std::vector).
-  template <typename U>
+  template <typename U,
+            typename std::enable_if<
+                internal::HasDataAndSize<U, T>::value>::type* = nullptr>
   ArrayView(U& u) : ArrayView(u.data(), u.size()) {}
 
   // Indexing, size, and iteration. These allow mutation even if the ArrayView

webrtc/base/array_view_unittest.cc

@@ -20,8 +20,45 @@
 namespace rtc {
 
 namespace {
+
+namespace test_has_data_and_size {
+
+template <typename C, typename T>
+using DS = internal::HasDataAndSize<C, T>;
+
+template <typename DR, typename SR>
+struct Test1 {
+  DR data();
+  SR size();
+};
+static_assert(DS<Test1<int*, int>, int>::value, "");
+static_assert(DS<Test1<int*, int>, const int>::value, "");
+static_assert(DS<Test1<const int*, int>, const int>::value, "");
+static_assert(!DS<Test1<const int*, int>, int>::value, "");  // Wrong const.
+static_assert(!DS<Test1<char*, size_t>, int>::value, "");  // Wrong ptr type.
+
+struct Test2 {
+  int* data;
+  size_t size;
+};
+static_assert(!DS<Test2, int>::value, "");  // Because they aren't methods.
+
+struct Test3 {
+  int* data();
+};
+static_assert(!DS<Test3, int>::value, "");  // Because .size() is missing.
+
+class Test4 {
+  int* data();
+  size_t size();
+};
+static_assert(!DS<Test4, int>::value, "");  // Because methods are private.
+
+}  // namespace test_has_data_and_size
+
 template <typename T>
 void Call(ArrayView<T>) {}
+
 }  // namespace
 
 TEST(ArrayViewTest, TestConstructFromPtrAndArray) {