Improve support of custom equals in inline classes

- Ensure that typed equals parameter's type is a star projection of
  corresponding inline class

- Make possible to declare typed equals that returns 'Nothing'

- Forbid type parameters in typed equals operator declaration

^KT-54909 fixed
^KT-54910 fixed

compiler/frontend/src/org/jetbrains/kotlin/diagnostics/Errors.java

@@ -426,7 +426,7 @@ public interface Errors {
     DiagnosticFactory0<PsiElement> VALUE_CLASS_CANNOT_BE_CLONEABLE = DiagnosticFactory0.create(ERROR);
     DiagnosticFactory0<PsiElement> INLINE_CLASS_DEPRECATED = DiagnosticFactory0.create(WARNING);
     DiagnosticFactory0<KtContextReceiverList> INLINE_CLASS_CANNOT_HAVE_CONTEXT_RECEIVERS = DiagnosticFactory0.create(ERROR);
-    DiagnosticFactory1<KtNamedFunction, String> INEFFICIENT_EQUALS_OVERRIDING_IN_INLINE_CLASS =
+    DiagnosticFactory1<KtNamedFunction, KotlinType> INEFFICIENT_EQUALS_OVERRIDING_IN_INLINE_CLASS =
             DiagnosticFactory1.create(WARNING, DECLARATION_NAME);
 
     // Result class

compiler/frontend/src/org/jetbrains/kotlin/diagnostics/rendering/DefaultErrorMessages.java

@@ -802,8 +802,8 @@ public class DefaultErrorMessages {
         MAP.put(INLINE_CLASS_DEPRECATED, "'inline' modifier is deprecated. Use 'value' instead");
         MAP.put(INLINE_CLASS_CANNOT_HAVE_CONTEXT_RECEIVERS, "Inline classes cannot have context receivers");
         MAP.put(INEFFICIENT_EQUALS_OVERRIDING_IN_INLINE_CLASS,
-                "Overriding ''equals'' from ''Any'' in inline class alongside with lack of ''equals(other: {0}): Boolean'' leads to boxing on every equality comparison",
-                STRING);
+                "Overriding ''equals'' from ''Any'' in inline class without operator ''equals(other: {0}): Boolean'' leads to boxing on every equality comparison",
+                RENDER_TYPE);
 
         MAP.put(RESULT_CLASS_IN_RETURN_TYPE, "'kotlin.Result' cannot be used as a return type");
         MAP.put(RESULT_CLASS_WITH_NULLABLE_OPERATOR, "Expression of type ''kotlin.Result'' cannot be used as a left operand of ''{0}''", STRING);

compiler/frontend/src/org/jetbrains/kotlin/resolve/checkers/InlineClassDeclarationChecker.kt

@@ -22,6 +22,7 @@ import org.jetbrains.kotlin.types.KotlinType
 import org.jetbrains.kotlin.types.typeUtil.isNothing
 import org.jetbrains.kotlin.types.typeUtil.isTypeParameter
 import org.jetbrains.kotlin.types.typeUtil.isUnit
+import org.jetbrains.kotlin.types.typeUtil.replaceArgumentsWithStarProjections
 
 private val javaLangCloneable = FqNameUnsafe("java.lang.Cloneable")
 
@@ -169,9 +170,20 @@ object InlineClassDeclarationChecker : DeclarationChecker {
         fun KtClass.namedFunctions() = declarations.filterIsInstance<KtNamedFunction>()
 
         if (context.languageVersionSettings.supportsFeature(LanguageFeature.CustomEqualsInInlineClasses)) {
+            val typedEquals = declaration.namedFunctions().firstOrNull { isTypedEquals(it) }
+
+            if (typedEquals?.typeParameters?.isNotEmpty() == true) {
+                trace.report(Errors.TYPE_PARAMETERS_NOT_ALLOWED.on(typedEquals))
+            }
+
             declaration.namedFunctions().singleOrNull { isUntypedEquals(it) }?.apply {
-                if (declaration.namedFunctions().none { isTypedEquals(it) }) {
-                    trace.report(Errors.INEFFICIENT_EQUALS_OVERRIDING_IN_INLINE_CLASS.on(this@apply, descriptor.name.asString()))
+                if (typedEquals == null) {
+                    trace.report(
+                        Errors.INEFFICIENT_EQUALS_OVERRIDING_IN_INLINE_CLASS.on(
+                            this@apply,
+                            descriptor.defaultType.replaceArgumentsWithStarProjections()
+                        )
+                    )
                 }
             }
         }