From a872123a73eef24415889e0184dc96ac068acd07 Mon Sep 17 00:00:00 2001
From: Chandler Carruth <chandlerc@gmail.com>
Date: Wed, 19 Aug 2026 01:00:13 +0000
Subject: [PATCH] Let `Printable` children default their comparison operators
 (#7642)

A defaulted `operator==` or `operator<=>` compares every base class
subobject, so children of `Printable` couldn't default their
comparisons: `Printable` had no comparison operators of its own, which
made the defaulted operator implicitly deleted. Children that want
member-wise comparison had to write it out by hand instead.

`Printable` is empty, so it now provides comparisons that always compare
equal. Its operands are constrained template parameter rather than
`const Printable&` so that they're only viable for comparing the base
class subobjects themselves. An overload taking `const Printable&` would
also be viable when comparing two `DerivedT` objects by converting them
to the base class, and would then both make children that provide no
comparison silently compare equal and displace the comparisons of
children that provide them through a conversion of their own, as
`EnumBase` does.

Some hand-written comparisons stay, for reasons unrelated to
`Printable`: using `= default` would change their meaning.

Adds `common/ostream_test.cpp`, which covers both the member and friend
forms of defaulting, the resulting comparison categories, and both of
the hazards above.

Assisted-by: Claude Code

---------

Co-authored-by: Richard Smith <richard@metafoo.co.uk>
---
 common/BUILD                           |  13 ++
 common/ostream.h                       |  23 ++-
 common/ostream_test.cpp                | 253 +++++++++++++++++++++++++
 common/raw_hashtable_test_helpers.h    |  10 +-
 toolchain/sem_ir/clang_decl.h          |  10 +-
 toolchain/sem_ir/declared_facet_type.h |  13 +-
 6 files changed, 293 insertions(+), 29 deletions(-)
 create mode 100644 common/ostream_test.cpp

diff --git a/common/BUILD b/common/BUILD
index c438381f5ff5..f7d08dabbb19 100644
--- a/common/BUILD
+++ b/common/BUILD
@@ -506,6 +506,19 @@ cc_library(
     ],
 )
 
+cc_test(
+    name = "ostream_test",
+    size = "small",
+    srcs = ["ostream_test.cpp"],
+    deps = [
+        ":ostream",
+        ":raw_string_ostream",
+        "//testing/base:gtest_main",
+        "@googletest//:gtest",
+        "@llvm-project//llvm:Support",
+    ],
+)
+
 cc_library(
     name = "pretty_stack_trace_function",
     hdrs = ["pretty_stack_trace_function.h"],
diff --git a/common/ostream.h b/common/ostream.h
index d9fd397bacbc..35b2209b401c 100644
--- a/common/ostream.h
+++ b/common/ostream.h
@@ -7,6 +7,7 @@
 
 // Libraries should include this header instead of raw_ostream.
 
+#include <compare>
 #include <concepts>
 #include <ostream>
 #include <type_traits>
@@ -17,11 +18,31 @@
 
 namespace Carbon {
 
-// CRTP base class for printable types. Children (DerivedT) must implement:
+// CRTP base class for printable types. Derived classes (DerivedT) must
+// implement:
 // - auto Print(llvm::raw_ostream& out) const -> void
 template <typename DerivedT>
 // NOLINTNEXTLINE(bugprone-crtp-constructor-accessibility)
 class Printable {
+  // Comparisons of the base class itself, which is empty and so always compares
+  // equal, allowing derived classes to default their own comparison operators.
+  //
+  // These are templated so that they are only used when the types of the
+  // arguments are exactly `Printable`, rather than a derived class, and are
+  // hidden friends so that they aren't candidates for unrelated comparisons.
+  template <typename T>
+    requires std::same_as<T, Printable>
+  friend constexpr auto operator==(const T& /*lhs*/, const T& /*rhs*/) noexcept
+      -> bool {
+    return true;
+  }
+  template <typename T>
+    requires std::same_as<T, Printable>
+  friend constexpr auto operator<=>(const T& /*lhs*/, const T& /*rhs*/) noexcept
+      -> std::strong_ordering {
+    return std::strong_ordering::equal;
+  }
+
   // Supports printing to llvm::raw_ostream.
   friend auto operator<<(llvm::raw_ostream& out, const DerivedT& obj)
       -> llvm::raw_ostream& {
diff --git a/common/ostream_test.cpp b/common/ostream_test.cpp
new file mode 100644
index 000000000000..8f6bba7eed5a
--- /dev/null
+++ b/common/ostream_test.cpp
@@ -0,0 +1,253 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include "common/ostream.h"
+
+#include <gmock/gmock.h>
+#include <gtest/gtest.h>
+
+#include <compare>
+#include <concepts>
+#include <limits>
+#include <sstream>
+#include <string>
+#include <type_traits>
+#include <utility>
+
+#include "common/raw_string_ostream.h"
+#include "llvm/ADT/STLExtras.h"
+#include "llvm/ADT/SmallVector.h"
+
+namespace Carbon::Testing {
+namespace {
+
+using ::testing::ElementsAre;
+
+// Whether two types can be compared with both `==` and `<=>`.
+template <typename LhsT, typename RhsT>
+concept Comparable = requires(const LhsT& lhs, const RhsT& rhs) {
+  lhs == rhs;
+  lhs <=> rhs;
+};
+
+// A child that defaults its comparisons with member declarations.
+struct Point : Printable<Point> {
+  int x;
+  int y;
+
+  constexpr Point(int x, int y) : x(x), y(y) {}
+
+  auto Print(llvm::raw_ostream& out) const -> void {
+    out << "(" << x << ", " << y << ")";
+  }
+
+  auto operator<=>(const Point& rhs) const = default;
+};
+
+// A child that defaults its comparisons with friend declarations, and whose
+// comparisons are neither trivial nor `noexcept`.
+struct Label : Printable<Label> {
+  std::string text;
+
+  explicit Label(std::string text) : text(std::move(text)) {}
+
+  auto Print(llvm::raw_ostream& out) const -> void { out << text; }
+
+  friend auto operator<=>(const Label& lhs, const Label& rhs) = default;
+};
+
+// A child that defaults equality without providing any ordering.
+struct Id : Printable<Id> {
+  int value;
+
+  constexpr explicit Id(int value) : value(value) {}
+
+  auto Print(llvm::raw_ostream& out) const -> void { out << "#" << value; }
+
+  auto operator==(const Id& rhs) const -> bool = default;
+};
+
+// A child whose defaulted comparison is only a partial ordering.
+struct Measure : Printable<Measure> {
+  double value;
+
+  constexpr explicit Measure(double value) : value(value) {}
+
+  auto Print(llvm::raw_ostream& out) const -> void { out << value; }
+
+  auto operator<=>(const Measure& rhs) const = default;
+};
+
+// A child that requests a weaker ordering than its members provide.
+struct Version : Printable<Version> {
+  int major;
+  int minor;
+
+  constexpr Version(int major, int minor) : major(major), minor(minor) {}
+
+  auto Print(llvm::raw_ostream& out) const -> void {
+    out << major << "." << minor;
+  }
+
+  auto operator<=>(const Version& rhs) const -> std::weak_ordering = default;
+};
+
+// A child that doesn't want to be compared at all.
+struct Opaque : Printable<Opaque> {
+  int value;
+
+  constexpr explicit Opaque(int value) : value(value) {}
+
+  auto Print(llvm::raw_ostream& out) const -> void { out << value; }
+};
+
+// A child that compares through an implicit conversion rather than through
+// operators of its own, the way `EnumBase` children do.
+class Level : public Printable<Level> {
+ public:
+  enum RawLevel { Low, High };
+
+  constexpr explicit Level(RawLevel value) : value_(value) {}
+
+  // NOLINTNEXTLINE(google-explicit-constructor)
+  explicit(false) constexpr operator RawLevel() const { return value_; }
+
+  auto Print(llvm::raw_ostream& out) const -> void {
+    out << (value_ == Low ? "low" : "high");
+  }
+
+ private:
+  RawLevel value_;
+};
+
+TEST(PrintableTest, Printing) {
+  RawStringOstream raw_out;
+  raw_out << Point(1, 2) << " " << Label("label");
+  EXPECT_EQ(raw_out.TakeStr(), "(1, 2) label");
+
+  std::ostringstream standard_out;
+  standard_out << Point(1, 2) << " " << Label("label");
+  EXPECT_EQ(standard_out.str(), "(1, 2) label");
+
+  EXPECT_EQ(PrintToString(Point(1, 2)), "(1, 2)");
+}
+
+TEST(PrintableTest, DefaultedEquality) {
+  EXPECT_EQ(Point(1, 2), Point(1, 2));
+  EXPECT_NE(Point(1, 2), Point(1, 3));
+  EXPECT_NE(Point(1, 2), Point(2, 2));
+
+  static_assert(Point(1, 2) == Point(1, 2));
+  static_assert(Point(1, 2) != Point(1, 3));
+
+  // The base class comparisons don't make defaulted comparisons throwing.
+  Point point(1, 2);
+  static_assert(noexcept(point == point));
+}
+
+TEST(PrintableTest, DefaultedOrdering) {
+  // Ordering is lexicographic in declaration order, with the empty base class
+  // contributing nothing.
+  EXPECT_LT(Point(1, 2), Point(1, 3));
+  EXPECT_LT(Point(1, 9), Point(2, 0));
+  EXPECT_LE(Point(1, 2), Point(1, 2));
+  EXPECT_GT(Point(2, 0), Point(1, 9));
+  EXPECT_GE(Point(1, 2), Point(1, 2));
+
+  EXPECT_EQ(Point(1, 2) <=> Point(1, 3), std::strong_ordering::less);
+  EXPECT_EQ(Point(1, 2) <=> Point(1, 2), std::strong_ordering::equal);
+  EXPECT_EQ(Point(1, 3) <=> Point(1, 2), std::strong_ordering::greater);
+
+  static_assert(std::totally_ordered<Point>);
+  static_assert(std::same_as<std::compare_three_way_result_t<Point>,
+                             std::strong_ordering>);
+  static_assert(Point(1, 2) < Point(1, 3));
+
+  Point point(1, 2);
+  static_assert(noexcept(point <=> point));
+}
+
+TEST(PrintableTest, DefaultedFriendComparison) {
+  EXPECT_EQ(Label("a"), Label("a"));
+  EXPECT_NE(Label("a"), Label("b"));
+  EXPECT_LT(Label("a"), Label("b"));
+  EXPECT_EQ(Label("a") <=> Label("b"), std::strong_ordering::less);
+
+  static_assert(std::totally_ordered<Label>);
+}
+
+TEST(PrintableTest, DefaultedEqualityWithoutOrdering) {
+  EXPECT_EQ(Id(1), Id(1));
+  EXPECT_NE(Id(1), Id(2));
+
+  static_assert(std::equality_comparable<Id>);
+  static_assert(!std::totally_ordered<Id>);
+  static_assert(!std::three_way_comparable<Id>);
+}
+
+TEST(PrintableTest, DefaultedComparisonCategories) {
+  static_assert(std::same_as<std::compare_three_way_result_t<Measure>,
+                             std::partial_ordering>);
+  static_assert(std::same_as<std::compare_three_way_result_t<Version>,
+                             std::weak_ordering>);
+
+  EXPECT_LT(Measure(1.0), Measure(2.0));
+  EXPECT_EQ(Measure(1.0) <=> Measure(2.0), std::partial_ordering::less);
+
+  // The base class comparing equal must not make unordered values ordered.
+  Measure nan(std::numeric_limits<double>::quiet_NaN());
+  EXPECT_EQ(nan <=> Measure(1.0), std::partial_ordering::unordered);
+  EXPECT_NE(nan, nan);
+
+  EXPECT_LT(Version(1, 0), Version(1, 1));
+  EXPECT_EQ(Version(1, 0) <=> Version(1, 1), std::weak_ordering::less);
+}
+
+TEST(PrintableTest, NoComparisonWithoutDefaulting) {
+  // Inheriting from `Printable` must not by itself make a type comparable, and
+  // in particular must not make distinct values compare equal.
+  static_assert(!std::equality_comparable<Opaque>);
+  static_assert(!std::totally_ordered<Opaque>);
+  static_assert(!std::three_way_comparable<Opaque>);
+
+  EXPECT_EQ(PrintToString(Opaque(1)), "1");
+}
+
+TEST(PrintableTest, NoComparisonBetweenBaseAndChild) {
+  // The base class comparisons are viable only between two base class
+  // subobjects, and so don't apply to comparing a child with one, whether or
+  // not the child provides comparisons of its own.
+  static_assert(Comparable<Printable<Opaque>, Printable<Opaque>>);
+  static_assert(!Comparable<Printable<Opaque>, Opaque>);
+  static_assert(!Comparable<Opaque, Printable<Opaque>>);
+  static_assert(!Comparable<Printable<Label>, Label>);
+  static_assert(!Comparable<Label, Printable<Label>>);
+}
+
+TEST(PrintableTest, ComparisonThroughConversion) {
+  // The base class comparisons must not displace comparisons that a child
+  // provides through a conversion.
+  EXPECT_TRUE(Level(Level::Low) == Level(Level::Low));
+  EXPECT_FALSE(Level(Level::Low) == Level(Level::High));
+  EXPECT_TRUE(Level(Level::Low) < Level(Level::High));
+  EXPECT_TRUE(Level(Level::High) == Level::High);
+}
+
+TEST(PrintableTest, ComparisonsUsableGenerically) {
+  llvm::SmallVector<Point> points = {Point(2, 1), Point(1, 2), Point(1, 1)};
+  llvm::sort(points);
+  EXPECT_THAT(points, ElementsAre(Point(1, 1), Point(1, 2), Point(2, 1)));
+
+  EXPECT_EQ(llvm::find(points, Point(1, 2)), points.begin() + 1);
+}
+
+TEST(PrintableTest, EmptyBaseClass) {
+  // The comparison support must not add any state to children.
+  static_assert(sizeof(Point) == 2 * sizeof(int));
+  static_assert(sizeof(Id) == sizeof(int));
+  static_assert(std::is_empty_v<Printable<Point>>);
+}
+
+}  // namespace
+}  // namespace Carbon::Testing
diff --git a/common/raw_hashtable_test_helpers.h b/common/raw_hashtable_test_helpers.h
index 874f9321191c..e4f42882a17c 100644
--- a/common/raw_hashtable_test_helpers.h
+++ b/common/raw_hashtable_test_helpers.h
@@ -70,15 +70,9 @@ struct MoveOnlyTestData : Printable<TestData> {
   }
   auto Print(llvm::raw_ostream& out) const -> void { out << value; }
 
-  friend auto operator==(const MoveOnlyTestData& lhs,
-                         const MoveOnlyTestData& rhs) -> bool {
-    return lhs.value == rhs.value;
-  }
-
   friend auto operator<=>(const MoveOnlyTestData& lhs,
-                          const MoveOnlyTestData& rhs) -> std::strong_ordering {
-    return lhs.value <=> rhs.value;
-  }
+                          const MoveOnlyTestData& rhs)
+      -> std::strong_ordering = default;
 
   friend auto CarbonHashValue(const MoveOnlyTestData& data, uint64_t seed)
       -> HashCode {
diff --git a/toolchain/sem_ir/clang_decl.h b/toolchain/sem_ir/clang_decl.h
index da765e34cd44..7983a19a16b4 100644
--- a/toolchain/sem_ir/clang_decl.h
+++ b/toolchain/sem_ir/clang_decl.h
@@ -89,11 +89,7 @@ struct ClangDeclSignature : public Printable<ClangDeclSignature> {
 
   auto Print(llvm::raw_ostream& out) const -> void;
 
-  auto operator==(const ClangDeclSignature& rhs) const -> bool {
-    return kind == rhs.kind && num_params == rhs.num_params &&
-           passing_modes == rhs.passing_modes &&
-           self_passing_mode == rhs.self_passing_mode;
-  }
+  auto operator==(const ClangDeclSignature& rhs) const -> bool = default;
 
   // Hashing for ClangDeclSignature.
   friend auto CarbonHashValue(const ClangDeclSignature& value, uint64_t seed)
@@ -139,9 +135,7 @@ struct ClangDeclKey : public Printable<ClangDeclKey> {
 
   auto Print(llvm::raw_ostream& out) const -> void;
 
-  auto operator==(const ClangDeclKey& rhs) const -> bool {
-    return decl == rhs.decl && signature_id == rhs.signature_id;
-  }
+  auto operator==(const ClangDeclKey& rhs) const -> bool = default;
 
   // Hashing for ClangDecl. See common/hashing.h.
   friend auto CarbonHashValue(const ClangDeclKey& value, uint64_t seed)
diff --git a/toolchain/sem_ir/declared_facet_type.h b/toolchain/sem_ir/declared_facet_type.h
index a87af1466e10..d7e5b3a8300d 100644
--- a/toolchain/sem_ir/declared_facet_type.h
+++ b/toolchain/sem_ir/declared_facet_type.h
@@ -130,18 +130,7 @@ struct DeclaredFacetType : Printable<DeclaredFacetType> {
   auto IsExtendedOnly() const -> bool;
 
   friend auto operator==(const DeclaredFacetType& lhs,
-                         const DeclaredFacetType& rhs) -> bool {
-    return lhs.extend_constraints == rhs.extend_constraints &&
-           lhs.self_impls_constraints == rhs.self_impls_constraints &&
-           lhs.extend_named_constraints == rhs.extend_named_constraints &&
-           lhs.self_impls_named_constraints ==
-               rhs.self_impls_named_constraints &&
-           lhs.type_impls_interfaces == rhs.type_impls_interfaces &&
-           lhs.type_impls_named_constraints ==
-               rhs.type_impls_named_constraints &&
-           lhs.rewrite_constraints == rhs.rewrite_constraints &&
-           lhs.other_requirements == rhs.other_requirements;
-  }
+                         const DeclaredFacetType& rhs) -> bool = default;
 };
 
 constexpr DeclaredFacetType::RewriteConstraint