Remove `absl::weak_equality` and `absl::strong_equality`.

The corresponding `std` types were removed before C++20 was finalized:
https://wg21.link/P1959R0.

They are unused. The language mechanisms and recommendations changed since they
were originally proposed. In particular:

* An explicitly defaulted `operator==` is defined in terms of memberwise
  `operator==` rather than sibling `operator<=>`.

* An `operator!=` can be implicitly added to an overload set in terms of
  `operator==` rather than `operator<=>`.

* A class which has equality but not ordering defined should provide
  `operator==` rather than `operator<=>`.

PiperOrigin-RevId: 587834267
Change-Id: I2c2513c13f3485b9edc6a345dca4a577d8e65167

2 files changed, 4 insertions, 221 deletions

diff --git a/absl/types/compare.h b/absl/types/compare.h
index 2b89b69c..ab0543c0 100644
--- a/absl/types/compare.h
+++ b/absl/types/compare.h
@@ -16,15 +16,15 @@
 // compare.h
 // -----------------------------------------------------------------------------
 //
-// This header file defines the `absl::weak_equality`, `absl::strong_equality`,
-// `absl::partial_ordering`, `absl::weak_ordering`, and `absl::strong_ordering`
-// types for storing the results of three way comparisons.
+// This header file defines the `absl::partial_ordering`, `absl::weak_ordering`,
+// and `absl::strong_ordering` types for storing the results of three way
+// comparisons.
 //
 // Example:
 //   absl::weak_ordering compare(const std::string& a, const std::string& b);
 //
 // These are C++11 compatible versions of the C++20 corresponding types
-// (`std::weak_equality`, etc.) and are designed to be drop-in replacements
+// (`std::partial_ordering`, etc.) and are designed to be drop-in replacements
 // for code compliant with C++20.
 
 #ifndef ABSL_TYPES_COMPARE_H_
@@ -122,20 +122,6 @@ enum class ncmp : value_type { unordered = -127 };
 // in the header file (for performance) without using inline variables (which
 // aren't available in C++11).
 template <typename T>
-struct weak_equality_base {
-  ABSL_COMPARE_INLINE_BASECLASS_DECL(equivalent);
-  ABSL_COMPARE_INLINE_BASECLASS_DECL(nonequivalent);
-};
-
-template <typename T>
-struct strong_equality_base {
-  ABSL_COMPARE_INLINE_BASECLASS_DECL(equal);
-  ABSL_COMPARE_INLINE_BASECLASS_DECL(nonequal);
-  ABSL_COMPARE_INLINE_BASECLASS_DECL(equivalent);
-  ABSL_COMPARE_INLINE_BASECLASS_DECL(nonequivalent);
-};
-
-template <typename T>
 struct partial_ordering_base {
   ABSL_COMPARE_INLINE_BASECLASS_DECL(less);
   ABSL_COMPARE_INLINE_BASECLASS_DECL(equivalent);
@@ -160,104 +146,6 @@ struct strong_ordering_base {
 
 }  // namespace compare_internal
 
-class weak_equality
-    : public compare_internal::weak_equality_base<weak_equality> {
-  explicit constexpr weak_equality(compare_internal::eq v) noexcept
-      : value_(static_cast<compare_internal::value_type>(v)) {}
-  friend struct compare_internal::weak_equality_base<weak_equality>;
-
- public:
-  ABSL_COMPARE_INLINE_SUBCLASS_DECL(weak_equality, equivalent);
-  ABSL_COMPARE_INLINE_SUBCLASS_DECL(weak_equality, nonequivalent);
-
-  // Comparisons
-  friend constexpr bool operator==(
-      weak_equality v, compare_internal::OnlyLiteralZero) noexcept {
-    return v.value_ == 0;
-  }
-  friend constexpr bool operator!=(
-      weak_equality v, compare_internal::OnlyLiteralZero) noexcept {
-    return v.value_ != 0;
-  }
-  friend constexpr bool operator==(compare_internal::OnlyLiteralZero,
-                                   weak_equality v) noexcept {
-    return 0 == v.value_;
-  }
-  friend constexpr bool operator!=(compare_internal::OnlyLiteralZero,
-                                   weak_equality v) noexcept {
-    return 0 != v.value_;
-  }
-  friend constexpr bool operator==(weak_equality v1,
-                                   weak_equality v2) noexcept {
-    return v1.value_ == v2.value_;
-  }
-  friend constexpr bool operator!=(weak_equality v1,
-                                   weak_equality v2) noexcept {
-    return v1.value_ != v2.value_;
-  }
-
- private:
-  compare_internal::value_type value_;
-};
-ABSL_COMPARE_INLINE_INIT(weak_equality, equivalent,
-                         compare_internal::eq::equivalent);
-ABSL_COMPARE_INLINE_INIT(weak_equality, nonequivalent,
-                         compare_internal::eq::nonequivalent);
-
-class strong_equality
-    : public compare_internal::strong_equality_base<strong_equality> {
-  explicit constexpr strong_equality(compare_internal::eq v) noexcept
-      : value_(static_cast<compare_internal::value_type>(v)) {}
-  friend struct compare_internal::strong_equality_base<strong_equality>;
-
- public:
-  ABSL_COMPARE_INLINE_SUBCLASS_DECL(strong_equality, equal);
-  ABSL_COMPARE_INLINE_SUBCLASS_DECL(strong_equality, nonequal);
-  ABSL_COMPARE_INLINE_SUBCLASS_DECL(strong_equality, equivalent);
-  ABSL_COMPARE_INLINE_SUBCLASS_DECL(strong_equality, nonequivalent);
-
-  // Conversion
-  constexpr operator weak_equality() const noexcept {  // NOLINT
-    return value_ == 0 ? weak_equality::equivalent
-                       : weak_equality::nonequivalent;
-  }
-  // Comparisons
-  friend constexpr bool operator==(
-      strong_equality v, compare_internal::OnlyLiteralZero) noexcept {
-    return v.value_ == 0;
-  }
-  friend constexpr bool operator!=(
-      strong_equality v, compare_internal::OnlyLiteralZero) noexcept {
-    return v.value_ != 0;
-  }
-  friend constexpr bool operator==(compare_internal::OnlyLiteralZero,
-                                   strong_equality v) noexcept {
-    return 0 == v.value_;
-  }
-  friend constexpr bool operator!=(compare_internal::OnlyLiteralZero,
-                                   strong_equality v) noexcept {
-    return 0 != v.value_;
-  }
-  friend constexpr bool operator==(strong_equality v1,
-                                   strong_equality v2) noexcept {
-    return v1.value_ == v2.value_;
-  }
-  friend constexpr bool operator!=(strong_equality v1,
-                                   strong_equality v2) noexcept {
-    return v1.value_ != v2.value_;
-  }
-
- private:
-  compare_internal::value_type value_;
-};
-ABSL_COMPARE_INLINE_INIT(strong_equality, equal, compare_internal::eq::equal);
-ABSL_COMPARE_INLINE_INIT(strong_equality, nonequal,
-                         compare_internal::eq::nonequal);
-ABSL_COMPARE_INLINE_INIT(strong_equality, equivalent,
-                         compare_internal::eq::equivalent);
-ABSL_COMPARE_INLINE_INIT(strong_equality, nonequivalent,
-                         compare_internal::eq::nonequivalent);
-
 class partial_ordering
     : public compare_internal::partial_ordering_base<partial_ordering> {
   explicit constexpr partial_ordering(compare_internal::eq v) noexcept
@@ -279,11 +167,6 @@ class partial_ordering
   ABSL_COMPARE_INLINE_SUBCLASS_DECL(partial_ordering, greater);
   ABSL_COMPARE_INLINE_SUBCLASS_DECL(partial_ordering, unordered);
 
-  // Conversion
-  constexpr operator weak_equality() const noexcept {  // NOLINT
-    return value_ == 0 ? weak_equality::equivalent
-                       : weak_equality::nonequivalent;
-  }
   // Comparisons
   friend constexpr bool operator==(
       partial_ordering v, compare_internal::OnlyLiteralZero) noexcept {
@@ -367,10 +250,6 @@ class weak_ordering
   ABSL_COMPARE_INLINE_SUBCLASS_DECL(weak_ordering, greater);
 
   // Conversions
-  constexpr operator weak_equality() const noexcept {  // NOLINT
-    return value_ == 0 ? weak_equality::equivalent
-                       : weak_equality::nonequivalent;
-  }
   constexpr operator partial_ordering() const noexcept {  // NOLINT
     return value_ == 0 ? partial_ordering::equivalent
                        : (value_ < 0 ? partial_ordering::less
@@ -458,13 +337,6 @@ class strong_ordering
   ABSL_COMPARE_INLINE_SUBCLASS_DECL(strong_ordering, greater);
 
   // Conversions
-  constexpr operator weak_equality() const noexcept {  // NOLINT
-    return value_ == 0 ? weak_equality::equivalent
-                       : weak_equality::nonequivalent;
-  }
-  constexpr operator strong_equality() const noexcept {  // NOLINT
-    return value_ == 0 ? strong_equality::equal : strong_equality::nonequal;
-  }
   constexpr operator partial_ordering() const noexcept {  // NOLINT
     return value_ == 0 ? partial_ordering::equivalent
                        : (value_ < 0 ? partial_ordering::less
diff --git a/absl/types/compare_test.cc b/absl/types/compare_test.cc
index 8095baf9..455cdbba 100644
--- a/absl/types/compare_test.cc
+++ b/absl/types/compare_test.cc
@@ -26,45 +26,6 @@ namespace {
 // to an int, which can't be converted to the unspecified zero type.
 bool Identity(bool b) { return b; }
 
-TEST(Compare, WeakEquality) {
-  EXPECT_TRUE(Identity(weak_equality::equivalent == 0));
-  EXPECT_TRUE(Identity(0 == weak_equality::equivalent));
-  EXPECT_TRUE(Identity(weak_equality::nonequivalent != 0));
-  EXPECT_TRUE(Identity(0 != weak_equality::nonequivalent));
-  const weak_equality values[] = {weak_equality::equivalent,
-                                  weak_equality::nonequivalent};
-  for (const auto& lhs : values) {
-    for (const auto& rhs : values) {
-      const bool are_equal = &lhs == &rhs;
-      EXPECT_EQ(lhs == rhs, are_equal);
-      EXPECT_EQ(lhs != rhs, !are_equal);
-    }
-  }
-}
-
-TEST(Compare, StrongEquality) {
-  EXPECT_TRUE(Identity(strong_equality::equal == 0));
-  EXPECT_TRUE(Identity(0 == strong_equality::equal));
-  EXPECT_TRUE(Identity(strong_equality::nonequal != 0));
-  EXPECT_TRUE(Identity(0 != strong_equality::nonequal));
-  EXPECT_TRUE(Identity(strong_equality::equivalent == 0));
-  EXPECT_TRUE(Identity(0 == strong_equality::equivalent));
-  EXPECT_TRUE(Identity(strong_equality::nonequivalent != 0));
-  EXPECT_TRUE(Identity(0 != strong_equality::nonequivalent));
-  const strong_equality values[] = {strong_equality::equal,
-                                    strong_equality::nonequal};
-  for (const auto& lhs : values) {
-    for (const auto& rhs : values) {
-      const bool are_equal = &lhs == &rhs;
-      EXPECT_EQ(lhs == rhs, are_equal);
-      EXPECT_EQ(lhs != rhs, !are_equal);
-    }
-  }
-  EXPECT_TRUE(Identity(strong_equality::equivalent == strong_equality::equal));
-  EXPECT_TRUE(
-      Identity(strong_equality::nonequivalent == strong_equality::nonequal));
-}
-
 TEST(Compare, PartialOrdering) {
   EXPECT_TRUE(Identity(partial_ordering::less < 0));
   EXPECT_TRUE(Identity(0 > partial_ordering::less));
@@ -147,30 +108,6 @@ TEST(Compare, StrongOrdering) {
 
 TEST(Compare, Conversions) {
   EXPECT_TRUE(
-      Identity(implicit_cast<weak_equality>(strong_equality::equal) == 0));
-  EXPECT_TRUE(
-      Identity(implicit_cast<weak_equality>(strong_equality::nonequal) != 0));
-  EXPECT_TRUE(
-      Identity(implicit_cast<weak_equality>(strong_equality::equivalent) == 0));
-  EXPECT_TRUE(Identity(
-      implicit_cast<weak_equality>(strong_equality::nonequivalent) != 0));
-
-  EXPECT_TRUE(
-      Identity(implicit_cast<weak_equality>(partial_ordering::less) != 0));
-  EXPECT_TRUE(Identity(
-      implicit_cast<weak_equality>(partial_ordering::equivalent) == 0));
-  EXPECT_TRUE(
-      Identity(implicit_cast<weak_equality>(partial_ordering::greater) != 0));
-  EXPECT_TRUE(
-      Identity(implicit_cast<weak_equality>(partial_ordering::unordered) != 0));
-
-  EXPECT_TRUE(implicit_cast<weak_equality>(weak_ordering::less) != 0);
-  EXPECT_TRUE(
-      Identity(implicit_cast<weak_equality>(weak_ordering::equivalent) == 0));
-  EXPECT_TRUE(
-      Identity(implicit_cast<weak_equality>(weak_ordering::greater) != 0));
-
-  EXPECT_TRUE(
       Identity(implicit_cast<partial_ordering>(weak_ordering::less) != 0));
   EXPECT_TRUE(
       Identity(implicit_cast<partial_ordering>(weak_ordering::less) < 0));
@@ -186,24 +123,6 @@ TEST(Compare, Conversions) {
       Identity(implicit_cast<partial_ordering>(weak_ordering::greater) >= 0));
 
   EXPECT_TRUE(
-      Identity(implicit_cast<weak_equality>(strong_ordering::less) != 0));
-  EXPECT_TRUE(
-      Identity(implicit_cast<weak_equality>(strong_ordering::equal) == 0));
-  EXPECT_TRUE(
-      Identity(implicit_cast<weak_equality>(strong_ordering::equivalent) == 0));
-  EXPECT_TRUE(
-      Identity(implicit_cast<weak_equality>(strong_ordering::greater) != 0));
-
-  EXPECT_TRUE(
-      Identity(implicit_cast<strong_equality>(strong_ordering::less) != 0));
-  EXPECT_TRUE(
-      Identity(implicit_cast<strong_equality>(strong_ordering::equal) == 0));
-  EXPECT_TRUE(Identity(
-      implicit_cast<strong_equality>(strong_ordering::equivalent) == 0));
-  EXPECT_TRUE(
-      Identity(implicit_cast<strong_equality>(strong_ordering::greater) != 0));
-
-  EXPECT_TRUE(
       Identity(implicit_cast<partial_ordering>(strong_ordering::less) != 0));
   EXPECT_TRUE(
       Identity(implicit_cast<partial_ordering>(strong_ordering::less) < 0));
@@ -360,14 +279,6 @@ TEST(DoThreeWayComparison, SanityTest) {
 
 #ifdef __cpp_inline_variables
 TEST(Compare, StaticAsserts) {
-  static_assert(weak_equality::equivalent == 0, "");
-  static_assert(weak_equality::nonequivalent != 0, "");
-
-  static_assert(strong_equality::equal == 0, "");
-  static_assert(strong_equality::nonequal != 0, "");
-  static_assert(strong_equality::equivalent == 0, "");
-  static_assert(strong_equality::nonequivalent != 0, "");
-
   static_assert(partial_ordering::less < 0, "");
   static_assert(partial_ordering::equivalent == 0, "");
   static_assert(partial_ordering::greater > 0, "");