SR-11902: Improve Error Message for Auto-synthesized Equatable Conformance on Classes

1. Added note for 'Hashable' and 'Equatable' protocol synthesis for classes.

Author: kitaisreal

include/swift/AST/DiagnosticsSema.def

@@ -2710,6 +2710,9 @@ NOTE(missing_member_type_conformance_prevents_synthesis, none,
      "protocol %2, preventing synthesized conformance "
      "of %3 to %2",
      (unsigned, Type, Type, Type))
+NOTE(classes_equatable_hashable_synthesis,none,
+    "Currently 'Equatable' and 'Hashable' synthesis is not supported for classes "
+    "because of inheritance.", ())
 
 // Dynamic Self
 ERROR(dynamic_self_non_method,none,

lib/Sema/DerivedConformanceEquatableHashable.cpp

@@ -165,6 +165,11 @@ void diagnoseFailedDerivation(DeclContext *DC, NominalTypeDecl *nominal,
           nominal->getDeclaredInterfaceType());
     }
   }
+
+  if (auto *classDecl = dyn_cast<ClassDecl>(nominal)) {
+    ctx.Diags.diagnose(classDecl->getLoc(),
+                       diag::classes_equatable_hashable_synthesis);
+  }
 }
 
 /// Creates a named variable based on a prefix character and a numeric index.