Resolve an issue where the resolution of operator<< overloads would…

… attempt to instantiate the incomplete `testing::internal::Secret` type.

PiperOrigin-RevId: 543799815
Change-Id: Ic0a4f48d825bef26cb8cc74d8a0117b3a5ef3f14

googletest/include/gtest/internal/gtest-port.h

@@ -925,9 +925,11 @@ using std::tuple_size;
 namespace internal {
 
 // A secret type that Google Test users don't know about.  It has no
-// definition on purpose.  Therefore it's impossible to create a
+// accessible constructors on purpose.  Therefore it's impossible to create a
 // Secret object, which is what we want.
-class Secret;
+class Secret {
+  Secret(const Secret&) = delete;
+};
 
 // A helper for suppressing warnings on constant condition.  It just
 // returns 'condition'.

googletest/test/gtest_unittest.cc

@@ -67,11 +67,27 @@ TEST(CommandLineFlagsTest, CanBeAccessedInCodeOnceGTestHIsIncluded) {
 #include <string>
 #include <type_traits>
 #include <unordered_set>
+#include <utility>
 #include <vector>
 
 #include "gtest/gtest-spi.h"
 #include "src/gtest-internal-inl.h"
 
+struct ConvertibleGlobalType {
+  // The inner enable_if is to ensure invoking is_constructible doesn't fail.
+  // The outer enable_if is to ensure the overload resolution doesn't encounter
+  // an ambiguity.
+  template <
+      class T,
+      std::enable_if_t<
+          false, std::enable_if_t<std::is_constructible<T>::value, int>> = 0>
+  operator T() const;  // NOLINT(google-explicit-constructor)
+};
+void operator<<(ConvertibleGlobalType&, int);
+static_assert(sizeof(decltype(std::declval<ConvertibleGlobalType&>()
+                              << 1)(*)()) > 0,
+              "error in operator<< overload resolution");
+
 namespace testing {
 namespace internal {