Make GoogleTest handle std::monostate and valueless_by_exception() for std::variant similarly to how it handles std::nullopt

Fixes: #2066
PiperOrigin-RevId: 955981465
Change-Id: I6b34dc2e634d4b727a50fb3e21e5d8e25c39a13b

googletest/include/gtest/gtest-printers.h

diff --git a/googletest/include/gtest/gtest-printers.h b/googletest/include/gtest/gtest-printers.h
index a138157..fc0913f 100644
--- a/googletest/include/gtest/gtest-printers.h
+++ b/googletest/include/gtest/gtest-printers.h
@@ -945,13 +945,13 @@
 class [[nodiscard]] UniversalPrinter<std::optional<T>> {
  public:
   static void Print(const std::optional<T>& value, ::std::ostream* os) {
-    *os << '(';
     if (!value) {
-      *os << "nullopt";
+      UniversalPrint(std::nullopt, os);
     } else {
+      *os << '(';
       UniversalPrint(*value, os);
+      *os << ')';
     }
-    *os << ')';
   }
 };
 
@@ -961,27 +961,38 @@
   static void Print(std::nullopt_t, ::std::ostream* os) { *os << "(nullopt)"; }
 };
 
+struct UniversalPrinterVisitor {
+  template <typename T>
+  void operator()(const T& arg) const {
+    *os << "'" << GetTypeName<T>() << "(index = " << index << ")' with value ";
+    UniversalPrint(arg, os);
+  }
+  ::std::ostream* os;
+  std::size_t index;
+};
+
 // Printer for std::variant
 template <typename... T>
 class [[nodiscard]] UniversalPrinter<std::variant<T...>> {
  public:
   static void Print(const std::variant<T...>& value, ::std::ostream* os) {
-    *os << '(';
-    std::visit(Visitor{os, value.index()}, value);
-    *os << ')';
-  }
-
- private:
-  struct Visitor {
-    template <typename U>
-    void operator()(const U& u) const {
-      *os << "'" << GetTypeName<U>() << "(index = " << index
-          << ")' with value ";
-      UniversalPrint(u, os);
+    if (value.valueless_by_exception()) {
+      *os << "(valueless)";
+    } else {
+      *os << '(';
+      std::visit(UniversalPrinterVisitor{os, value.index()}, value);
+      *os << ')';
     }
-    ::std::ostream* os;
-    std::size_t index;
-  };
+  }
+};
+
+// Printer for std::monostate
+template <>
+class [[nodiscard]] UniversalPrinter<std::monostate> {
+ public:
+  static void Print(std::monostate, ::std::ostream* os) {
+    *os << "(monostate)";
+  }
 };
 
 // UniversalPrintArray(begin, len, os) prints an array of 'len'

googletest/test/googletest-printers-test.cc

diff --git a/googletest/test/googletest-printers-test.cc b/googletest/test/googletest-printers-test.cc
index 0b11f8e..928aab1 100644
--- a/googletest/test/googletest-printers-test.cc
+++ b/googletest/test/googletest-printers-test.cc
@@ -2030,6 +2030,26 @@
       PrintToString(Type(NonPrintable{})));
 }
 
+TEST(PrintVariantTest, Monostate) {
+  EXPECT_EQ("(monostate)", PrintToString(std::monostate()));
+
+#if GTEST_HAS_EXCEPTIONS
+  struct ThrowOnMove {
+    ThrowOnMove() = default;
+    ThrowOnMove(ThrowOnMove&& other) { *this = std::move(other); }
+    ThrowOnMove& operator=(ThrowOnMove&&) {
+      (void)std::vector<bool>().at(0);
+      return *this;
+    }
+  };
+  std::variant<std::monostate, ThrowOnMove> v = std::monostate();
+  EXPECT_EQ("('std::monostate(index = 0)' with value (monostate))",
+            PrintToString(v));
+  EXPECT_THROW(v = ThrowOnMove(), std::out_of_range);
+  EXPECT_EQ("(valueless)", PrintToString(v));
+#endif
+}
+
 #if GTEST_INTERNAL_HAS_COMPARE_LIB
 TEST(PrintOrderingTest, Basic) {
   EXPECT_EQ("(less)", PrintToString(std::strong_ordering::less));