Fixes the "ambiguous overload" compiler error when a mock function takes an...

Fixes the "ambiguous overload" compiler error when a mock function takes an argument that supports streaming to basic_ostream<C, T>.

include/gmock/gmock-printers.h

@@ -116,8 +116,19 @@ class TypeWithoutFormatter<T, true> {
 // to simplify the implementation, as much code in 'internal' needs to
 // use << in STL, which would conflict with our own << were it defined
 // in 'internal'.
-template <typename T>
-::std::ostream& operator<<(::std::ostream& os, const T& x) {
+//
+// Note that this operator<< takes a generic std::basic_ostream<Char,
+// CharTraits> type instead of the more restricted std::ostream.  If
+// we define it to take an std::ostream instead, we'll get an
+// "ambiguous overloads" compiler error when trying to print a type
+// Foo that supports streaming to std::basic_ostream<Char,
+// CharTraits>, as the compiler cannot tell whether
+// operator<<(std::ostream&, const T&) or
+// operator<<(std::basic_stream<Char, CharTraits>, const Foo&) is more
+// specific.
+template <typename Char, typename CharTraits, typename T>
+::std::basic_ostream<Char, CharTraits>& operator<<(
+    ::std::basic_ostream<Char, CharTraits>& os, const T& x) {
   TypeWithoutFormatter<T, ::testing::internal::IsAProtocolMessage<T>::value>::
       PrintValue(x, &os);
   return os;

test/gmock-printers_test.cc

@@ -597,6 +597,64 @@ TEST(PrintWideStringTest, StringInStdNamespace) {
 }
 #endif  // GTEST_HAS_STD_WSTRING
 
+// Tests printing types that support generic streaming (i.e. streaming
+// to std::basic_ostream<Char, CharTraits> for any valid Char and
+// CharTraits types).
+
+// Tests printing a non-template type that supports generic streaming.
+
+class AllowsGenericStreaming {};
+
+template <typename Char, typename CharTraits>
+std::basic_ostream<Char, CharTraits>& operator<<(
+    std::basic_ostream<Char, CharTraits>& os,
+    const AllowsGenericStreaming& a) {
+  return os << "AllowsGenericStreaming";
+}
+
+TEST(PrintTypeWithGenericStreamingTest, NonTemplateType) {
+  AllowsGenericStreaming a;
+  EXPECT_EQ("AllowsGenericStreaming", Print(a));
+}
+
+// Tests printing a template type that supports generic streaming.
+
+template <typename T>
+class AllowsGenericStreamingTemplate {};
+
+template <typename Char, typename CharTraits, typename T>
+std::basic_ostream<Char, CharTraits>& operator<<(
+    std::basic_ostream<Char, CharTraits>& os,
+    const AllowsGenericStreamingTemplate<T>& a) {
+  return os << "AllowsGenericStreamingTemplate";
+}
+
+TEST(PrintTypeWithGenericStreamingTest, TemplateType) {
+  AllowsGenericStreamingTemplate<int> a;
+  EXPECT_EQ("AllowsGenericStreamingTemplate", Print(a));
+}
+
+// Tests printing a type that supports generic streaming and can be
+// implicitly converted to another printable type.
+
+template <typename T>
+class AllowsGenericStreamingAndImplicitConversionTemplate {
+ public:
+  operator bool() const { return false; }
+};
+
+template <typename Char, typename CharTraits, typename T>
+std::basic_ostream<Char, CharTraits>& operator<<(
+    std::basic_ostream<Char, CharTraits>& os,
+    const AllowsGenericStreamingAndImplicitConversionTemplate<T>& a) {
+  return os << "AllowsGenericStreamingAndImplicitConversionTemplate";
+}
+
+TEST(PrintTypeWithGenericStreamingTest, TypeImplicitlyConvertible) {
+  AllowsGenericStreamingAndImplicitConversionTemplate<int> a;
+  EXPECT_EQ("AllowsGenericStreamingAndImplicitConversionTemplate", Print(a));
+}
+
 // Tests printing STL containers.
 
 TEST(PrintStlContainerTest, EmptyDeque) {