[Sema][Diagnostics] Add warning of synthesized == to non-synthesized <

Synthesized `==` is composed of member-wise `==`s, which is likely incorrect if the non-synthesized `<` is not composed of member-wise inequalities.

Author: WowbaggersLiquidLunch

include/swift/AST/DiagnosticsSema.def

@@ -1373,7 +1373,7 @@ ERROR(nominal_type_not_attribute,none,
 
 ERROR(mutating_invalid_global_scope,none, "%0 is only valid on methods",
       (SelfAccessKind))
-ERROR(mutating_invalid_classes,none, "%0 is not valid on %1s in " 
+ERROR(mutating_invalid_classes,none, "%0 is not valid on %1s in "
       "%select{classes|class-bound protocols}2",
       (SelfAccessKind, DescriptiveDeclKind, bool))
 
@@ -2547,7 +2547,7 @@ NOTE(overridden_near_match_here,none,
 ERROR(override_decl_extension,none,
       "%select{|non-@objc}0 %2 %3 %select{"
       "is declared in extension of %4 and cannot be overridden|"
-      "declared in %4 cannot be overridden from extension}1", 
+      "declared in %4 cannot be overridden from extension}1",
       (bool, bool, DescriptiveDeclKind, DeclName, DeclName))
 NOTE(overridden_here,none,
      "overridden declaration is here", ())
@@ -2922,6 +2922,14 @@ ERROR(broken_comparable_requirement,none,
       "Comparable protocol is broken: unexpected requirement", ())
 ERROR(broken_equatable_requirement,none,
       "Equatable protocol is broken: unexpected requirement", ())
+WARNING(synthesized_equatable_nonsythesized_comparable_fixit,none,
+      "compiler synthesized implementation of operator '==(lhs:rhs:)' "
+      "for 'Equatable' conformance for %0 may not match behavior of "
+      "custom implementation of operator '<(lhs:rhs:)'",
+      (Type))
+FIXIT(insert_equals_function_declaration,
+      "static func == (lhs: %0, rhs: $0) -> Bool {\n<#code#>\n}\n\n",
+      (Type))
 ERROR(broken_hashable_requirement,none,
       "Hashable protocol is broken: unexpected requirement", ())
 ERROR(broken_hashable_no_hasher,none,
@@ -3021,7 +3029,7 @@ NOTE(automatic_protocol_synthesis_unsupported,none,
     "automatic synthesis of '%0' is not supported for %select{classes|structs}1",
     (StringRef, unsigned))
 NOTE(comparable_synthesis_raw_value_not_allowed, none,
-     "enum declares raw type %0, preventing synthesized conformance of %1 to %2", 
+     "enum declares raw type %0, preventing synthesized conformance of %1 to %2",
      (Type, Type, Type))
 
 // Dynamic Self
@@ -3512,15 +3520,15 @@ ERROR(unordered_adjacent_operators,none,
 ERROR(missing_builtin_precedence_group,none,
       "broken standard library: missing builtin precedence group %0",
       (Identifier))
-WARNING(nan_comparison, none, 
+WARNING(nan_comparison, none,
        "comparison with '.nan' using %0 is always %select{false|true}1, use "
        "'%2.isNaN' to check if '%3' %select{is not a number|is a number}1",
        (Identifier, bool, StringRef, StringRef))
-WARNING(nan_comparison_without_isnan, none, 
-       "comparison with '.nan' using %0 is always %select{false|true}1", 
+WARNING(nan_comparison_without_isnan, none,
+       "comparison with '.nan' using %0 is always %select{false|true}1",
        (Identifier, bool))
-WARNING(nan_comparison_both_nan, none, 
-       "'.nan' %0 '.nan' is always %select{false|true}1", 
+WARNING(nan_comparison_both_nan, none,
+       "'.nan' %0 '.nan' is always %select{false|true}1",
        (StringRef, bool))
 
 // If you change this, also change enum TryKindForDiagnostics.
@@ -4317,7 +4325,7 @@ NOTE(note_add_nonisolated_to_decl,none,
      "add 'nonisolated' to %0 to make this %1 not isolated to the actor",
      (DeclName, DescriptiveDeclKind))
 NOTE(note_add_globalactor_to_function,none,
-     "add '@%0' to make %1 %2 part of global actor %3", 
+     "add '@%0' to make %1 %2 part of global actor %3",
      (StringRef, DescriptiveDeclKind, DeclName, Type))
 FIXIT(insert_globalactor_attr, "@%0 ", (Type))
 ERROR(not_objc_function_async,none,
@@ -4807,7 +4815,7 @@ ERROR(differentiable_function_type_invalid_result,none,
       "%select{| and satisfy '%0 == %0.TangentVector'}1, but the enclosing "
       "function type is '@differentiable%select{|(_linear)}1'",
       (StringRef, bool))
-ERROR(differentiable_function_type_no_differentiability_parameters, 
+ERROR(differentiable_function_type_no_differentiability_parameters,
       none,
       "'@differentiable' function type requires at least one differentiability "
       "parameter, i.e. a non-'@noDerivative' parameter whose type conforms to "

lib/Sema/DerivedConformanceEquatableHashable.cpp

@@ -454,6 +454,38 @@ ValueDecl *DerivedConformance::deriveEquatable(ValueDecl *requirement) {
 
   // Build the necessary decl.
   if (requirement->getBaseName() == "==") {
+    auto NominalType = Nominal->getDeclaredInterfaceType();
+    auto ComparableProtocol =
+        Context.getProtocol(KnownProtocolKind::Comparable);
+    auto ComparableConformance = TypeChecker::conformsToProtocol(
+        NominalType, ComparableProtocol, getParentModule());
+
+    // `Comparable` inherits from `Equatable`, but does not provide a default
+    // implementation of `==`, so for enums and structs, the compiler
+    // synthesises one from member-wise `==`s (SE-0185). If the user-implemented
+    // `<` is not composed of member-wise inequalities, then the synthesised
+    // `==` is almost certainly incorrect.
+    //
+    // Warn the user of synthesised `==` if the type has non-synthesised
+    // `Comparable conformance.
+    if (ComparableConformance) {
+      ConcreteDeclRef LessThanFunctionWitness =
+          ComparableConformance.getWitnessByName(NominalType,
+                                                 Context.Id_LessThanOperator);
+
+      auto LessThanFunctionDeclaration = LessThanFunctionWitness.getDecl();
+
+      if (LessThanFunctionDeclaration &&
+          !LessThanFunctionDeclaration->isImplicit()) {
+        LessThanFunctionDeclaration
+            ->diagnose(
+                diag::synthesized_equatable_nonsythesized_comparable_fixit,
+                NominalType)
+            .fixItInsert(LessThanFunctionDeclaration->getStartLoc(),
+                         diag::insert_equals_function_declaration, NominalType);
+      }
+    }
+
     if (auto ed = dyn_cast<EnumDecl>(Nominal)) {
       auto bodySynthesizer =
           !ed->hasCases()