Support `AbslStringify` with `DCHECK_EQ`.

`AbslStringify` is the [recommended](abseil.io/tips/215) way to make a type printable. However, the simple expression `DCHECK_EQ(x, y)` fails when either argument implements it, and not `operator<<`.

PiperOrigin-RevId: 634261367
Change-Id: Ic42666c286cf172c9482abbd28194da828706c71

5 files changed, 240 insertions, 14 deletions

diff --git a/absl/log/BUILD.bazel b/absl/log/BUILD.bazel
index dd192a10..b13cf4d4 100644
--- a/absl/log/BUILD.bazel
+++ b/absl/log/BUILD.bazel
@@ -357,6 +357,8 @@ cc_library(
         "//absl/base:core_headers",
         "//absl/log/internal:test_helpers",
         "//absl/status",
+        "//absl/strings",
+        "//absl/strings:string_view",
         "@com_google_googletest//:gtest",
     ],
 )
diff --git a/absl/log/CMakeLists.txt b/absl/log/CMakeLists.txt
index 59aa19f3..43844651 100644
--- a/absl/log/CMakeLists.txt
+++ b/absl/log/CMakeLists.txt
@@ -789,6 +789,8 @@ absl_cc_test(
     absl::core_headers
     absl::log_internal_test_helpers
     absl::status
+    absl::strings
+    absl::string_view
     GTest::gmock_main
 )
 
@@ -832,6 +834,8 @@ absl_cc_test(
     absl::core_headers
     absl::log_internal_test_helpers
     absl::status
+    absl::strings
+    absl::string_view
     GTest::gmock_main
 )
 
diff --git a/absl/log/check_test_impl.inc b/absl/log/check_test_impl.inc
index d5c0aee4..64318108 100644
--- a/absl/log/check_test_impl.inc
+++ b/absl/log/check_test_impl.inc
@@ -31,6 +31,8 @@
 #include "absl/base/config.h"
 #include "absl/log/internal/test_helpers.h"
 #include "absl/status/status.h"
+#include "absl/strings/string_view.h"
+#include "absl/strings/substitute.h"
 
 // NOLINTBEGIN(misc-definitions-in-headers)
 
@@ -521,6 +523,162 @@ TEST(CHECKDeathTest, TestUserDefinedStreaming) {
           "Check failed: v1 == v2 (ComparableType{1} vs. ComparableType{2})"));
 }
 
+// A type that can be printed using AbslStringify.
+struct StringifiableType {
+  int x = 0;
+  explicit StringifiableType(int x) : x(x) {}
+  friend bool operator==(const StringifiableType& lhs,
+                         const StringifiableType& rhs) {
+    return lhs.x == rhs.x;
+  }
+  friend bool operator!=(const StringifiableType& lhs,
+                         const StringifiableType& rhs) {
+    return lhs.x != rhs.x;
+  }
+  friend bool operator<(const StringifiableType& lhs,
+                        const StringifiableType& rhs) {
+    return lhs.x < rhs.x;
+  }
+  friend bool operator>(const StringifiableType& lhs,
+                        const StringifiableType& rhs) {
+    return lhs.x > rhs.x;
+  }
+  friend bool operator<=(const StringifiableType& lhs,
+                         const StringifiableType& rhs) {
+    return lhs.x <= rhs.x;
+  }
+  friend bool operator>=(const StringifiableType& lhs,
+                         const StringifiableType& rhs) {
+    return lhs.x >= rhs.x;
+  }
+  template <typename Sink>
+  friend void AbslStringify(Sink& sink, const StringifiableType& obj) {
+    absl::Format(&sink, "StringifiableType{%d}", obj.x);
+  }
+
+  // Make sure no unintended copy happens.
+  StringifiableType(const StringifiableType&) = delete;
+};
+
+TEST(CHECKTest, TestUserDefinedAbslStringify) {
+  const StringifiableType v1(1);
+  const StringifiableType v2(2);
+
+  ABSL_TEST_CHECK_EQ(v1, v1);
+  ABSL_TEST_CHECK_NE(v1, v2);
+  ABSL_TEST_CHECK_LT(v1, v2);
+  ABSL_TEST_CHECK_LE(v1, v2);
+  ABSL_TEST_CHECK_GT(v2, v1);
+  ABSL_TEST_CHECK_GE(v2, v1);
+}
+
+TEST(CHECKDeathTest, TestUserDefinedAbslStringify) {
+  const StringifiableType v1(1);
+  const StringifiableType v2(2);
+
+  // Returns a matcher for the expected check failure message when comparing two
+  // values.
+  auto expected_output = [](int lhs, absl::string_view condition, int rhs) {
+    return HasSubstr(
+        absl::Substitute("Check failed: v$0 $1 v$2 (StringifiableType{$0} vs. "
+                         "StringifiableType{$2})",
+                         lhs, condition, rhs));
+  };
+  // Test comparisons where the check fails.
+  EXPECT_DEATH(ABSL_TEST_CHECK_EQ(v1, v2), expected_output(1, "==", 2));
+  EXPECT_DEATH(ABSL_TEST_CHECK_NE(v1, v1), expected_output(1, "!=", 1));
+  EXPECT_DEATH(ABSL_TEST_CHECK_LT(v2, v1), expected_output(2, "<", 1));
+  EXPECT_DEATH(ABSL_TEST_CHECK_LE(v2, v1), expected_output(2, "<=", 1));
+  EXPECT_DEATH(ABSL_TEST_CHECK_GT(v1, v2), expected_output(1, ">", 2));
+  EXPECT_DEATH(ABSL_TEST_CHECK_GE(v1, v2), expected_output(1, ">=", 2));
+}
+
+// A type that can be printed using both AbslStringify and operator<<.
+struct StringifiableStreamableType {
+  int x = 0;
+  explicit StringifiableStreamableType(int x) : x(x) {}
+
+  friend bool operator==(const StringifiableStreamableType& lhs,
+                         const StringifiableStreamableType& rhs) {
+    return lhs.x == rhs.x;
+  }
+  friend bool operator!=(const StringifiableStreamableType& lhs,
+                         const StringifiableStreamableType& rhs) {
+    return lhs.x != rhs.x;
+  }
+  template <typename Sink>
+  friend void AbslStringify(Sink& sink,
+                            const StringifiableStreamableType& obj) {
+    absl::Format(&sink, "Strigified{%d}", obj.x);
+  }
+  friend std::ostream& operator<<(std::ostream& out,
+                                  const StringifiableStreamableType& obj) {
+    return out << "Streamed{" << obj.x << "}";
+  }
+
+  // Avoid unintentional copy.
+  StringifiableStreamableType(const StringifiableStreamableType&) = delete;
+};
+
+TEST(CHECKDeathTest, TestStreamingPreferredOverAbslStringify) {
+  StringifiableStreamableType v1(1);
+  StringifiableStreamableType v2(2);
+
+  EXPECT_DEATH(
+      ABSL_TEST_CHECK_EQ(v1, v2),
+      HasSubstr("Check failed: v1 == v2 (Streamed{1} vs. Streamed{2})"));
+}
+
+// A type whose pointer can be passed to AbslStringify.
+struct PointerIsStringifiable {};
+template <typename Sink>
+void AbslStringify(Sink& sink, const PointerIsStringifiable* var) {
+  sink.Append("PointerIsStringifiable");
+}
+
+// Verifies that a pointer is printed as a number despite having AbslStringify
+// defined. Users may implement AbslStringify that dereferences the pointer, and
+// doing so as part of DCHECK would not be good.
+TEST(CHECKDeathTest, TestPointerPrintedAsNumberDespiteAbslStringify) {
+  const auto* p = reinterpret_cast<const PointerIsStringifiable*>(0x1234);
+
+#ifdef _MSC_VER
+  EXPECT_DEATH(
+      ABSL_TEST_CHECK_EQ(p, nullptr),
+      HasSubstr("Check failed: p == nullptr (0000000000001234 vs. (null))"));
+#else   // _MSC_VER
+  EXPECT_DEATH(ABSL_TEST_CHECK_EQ(p, nullptr),
+               HasSubstr("Check failed: p == nullptr (0x1234 vs. (null))"));
+#endif  // _MSC_VER
+}
+
+// An uncopyable object with operator<<.
+struct Uncopyable {
+  int x;
+  explicit Uncopyable(int x) : x(x) {}
+  Uncopyable(const Uncopyable&) = delete;
+  friend bool operator==(const Uncopyable& lhs, const Uncopyable& rhs) {
+    return lhs.x == rhs.x;
+  }
+  friend bool operator!=(const Uncopyable& lhs, const Uncopyable& rhs) {
+    return lhs.x != rhs.x;
+  }
+  friend std::ostream& operator<<(std::ostream& os, const Uncopyable& obj) {
+    return os << "Uncopyable{" << obj.x << "}";
+  }
+};
+
+// Test that an uncopyable object can be used.
+// Will catch us if implementation has an unintended copy.
+TEST(CHECKDeathTest, TestUncopyable) {
+  const Uncopyable v1(1);
+  const Uncopyable v2(2);
+
+  EXPECT_DEATH(
+      ABSL_TEST_CHECK_EQ(v1, v2),
+      HasSubstr("Check failed: v1 == v2 (Uncopyable{1} vs. Uncopyable{2})"));
+}
+
 }  // namespace absl_log_internal
 
 // NOLINTEND(misc-definitions-in-headers)
diff --git a/absl/log/internal/check_op.cc b/absl/log/internal/check_op.cc
index f4b67647..23c4a3b2 100644
--- a/absl/log/internal/check_op.cc
+++ b/absl/log/internal/check_op.cc
@@ -16,6 +16,10 @@
 
 #include <string.h>
 
+#include <ostream>
+
+#include "absl/strings/string_view.h"
+
 #ifdef _MSC_VER
 #define strcasecmp _stricmp
 #else
@@ -113,6 +117,22 @@ DEFINE_CHECK_STROP_IMPL(CHECK_STRCASEEQ, strcasecmp, true)
 DEFINE_CHECK_STROP_IMPL(CHECK_STRCASENE, strcasecmp, false)
 #undef DEFINE_CHECK_STROP_IMPL
 
+namespace detect_specialization {
+
+StringifySink::StringifySink(std::ostream& os) : os_(os) {}
+
+void StringifySink::Append(absl::string_view text) { os_ << text; }
+
+void StringifySink::Append(size_t length, char ch) {
+  for (size_t i = 0; i < length; ++i) os_.put(ch);
+}
+
+void AbslFormatFlush(StringifySink* sink, absl::string_view text) {
+  sink->Append(text);
+}
+
+}  // namespace detect_specialization
+
 }  // namespace log_internal
 ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/log/internal/check_op.h b/absl/log/internal/check_op.h
index 0d24f4d1..21592207 100644
--- a/absl/log/internal/check_op.h
+++ b/absl/log/internal/check_op.h
@@ -24,9 +24,11 @@
 
 #include <stdint.h>
 
+#include <cstddef>
 #include <ostream>
 #include <sstream>
 #include <string>
+#include <type_traits>
 #include <utility>
 
 #include "absl/base/attributes.h"
@@ -35,6 +37,8 @@
 #include "absl/log/internal/nullguard.h"
 #include "absl/log/internal/nullstream.h"
 #include "absl/log/internal/strip.h"
+#include "absl/strings/has_absl_stringify.h"
+#include "absl/strings/string_view.h"
 
 // `ABSL_LOG_INTERNAL_STRIP_STRING_LITERAL` wraps string literals that
 // should be stripped when `ABSL_MIN_LOG_LEVEL` exceeds `kFatal`.
@@ -287,6 +291,44 @@ decltype(detect_specialization::operator<<(std::declval<std::ostream&>(),
                                            std::declval<const T&>()))
 Detect(char);
 
+// A sink for AbslStringify which redirects everything to a std::ostream.
+class StringifySink {
+ public:
+  explicit StringifySink(std::ostream& os ABSL_ATTRIBUTE_LIFETIME_BOUND);
+
+  void Append(absl::string_view text);
+  void Append(size_t length, char ch);
+  friend void AbslFormatFlush(StringifySink* sink, absl::string_view text);
+
+ private:
+  std::ostream& os_;
+};
+
+// Wraps a type implementing AbslStringify, and implements operator<<.
+template <typename T>
+class StringifyToStreamWrapper {
+ public:
+  explicit StringifyToStreamWrapper(const T& v ABSL_ATTRIBUTE_LIFETIME_BOUND)
+      : v_(v) {}
+
+  friend std::ostream& operator<<(std::ostream& os,
+                                  const StringifyToStreamWrapper& wrapper) {
+    StringifySink sink(os);
+    AbslStringify(sink, wrapper.v_);
+    return os;
+  }
+
+ private:
+  const T& v_;
+};
+
+// This overload triggers when T implements AbslStringify.
+// StringifyToStreamWrapper is used to allow MakeCheckOpString to use
+// operator<<.
+template <typename T>
+std::enable_if_t<HasAbslStringify<T>::value,
+                 StringifyToStreamWrapper<T>>
+Detect(...);  // Ellipsis has lowest preference when int passed.
 }  // namespace detect_specialization
 
 template <typename T>
@@ -342,20 +384,20 @@ ABSL_LOG_INTERNAL_DEFINE_MAKE_CHECK_OP_STRING_EXTERN(const void*);
 // `(int, int)` override works around the issue that the compiler will not
 // instantiate the template version of the function on values of unnamed enum
 // type.
-#define ABSL_LOG_INTERNAL_CHECK_OP_IMPL(name, op)                        \
-  template <typename T1, typename T2>                                    \
-  inline constexpr ::std::string* name##Impl(const T1& v1, const T2& v2, \
-                                             const char* exprtext) {     \
-    using U1 = CheckOpStreamType<T1>;                                    \
-    using U2 = CheckOpStreamType<T2>;                                    \
-    return ABSL_PREDICT_TRUE(v1 op v2)                                   \
-               ? nullptr                                                 \
-               : ABSL_LOG_INTERNAL_CHECK_OP_IMPL_RESULT(U1, U2, v1, v2,  \
-                                                        exprtext);       \
-  }                                                                      \
-  inline constexpr ::std::string* name##Impl(int v1, int v2,             \
-                                             const char* exprtext) {     \
-    return name##Impl<int, int>(v1, v2, exprtext);                       \
+#define ABSL_LOG_INTERNAL_CHECK_OP_IMPL(name, op)                          \
+  template <typename T1, typename T2>                                      \
+  inline constexpr ::std::string* name##Impl(const T1& v1, const T2& v2,   \
+                                             const char* exprtext) {       \
+    using U1 = CheckOpStreamType<T1>;                                      \
+    using U2 = CheckOpStreamType<T2>;                                      \
+    return ABSL_PREDICT_TRUE(v1 op v2)                                     \
+               ? nullptr                                                   \
+               : ABSL_LOG_INTERNAL_CHECK_OP_IMPL_RESULT(U1, U2, U1(v1),    \
+                                                        U2(v2), exprtext); \
+  }                                                                        \
+  inline constexpr ::std::string* name##Impl(int v1, int v2,               \
+                                             const char* exprtext) {       \
+    return name##Impl<int, int>(v1, v2, exprtext);                         \
   }
 
 ABSL_LOG_INTERNAL_CHECK_OP_IMPL(Check_EQ, ==)