Prohibit using kotlin.Result as a return type in most cases

#KT-26659 In Progress

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

@@ -334,6 +334,10 @@ public interface Errors {
     DiagnosticFactory1<PsiElement, String> RESERVED_MEMBER_INSIDE_INLINE_CLASS = DiagnosticFactory1.create(ERROR);
     DiagnosticFactory0<PsiElement> SECONDARY_CONSTRUCTOR_WITH_BODY_INSIDE_INLINE_CLASS = DiagnosticFactory0.create(ERROR);
 
+    // Result class
+
+    DiagnosticFactory0<PsiElement> RESULT_CLASS_IN_RETURN_TYPE = DiagnosticFactory0.create(ERROR);
+
     // Secondary constructors
 
     DiagnosticFactory0<KtConstructorDelegationReferenceExpression> CYCLIC_CONSTRUCTOR_DELEGATION_CALL = DiagnosticFactory0.create(ERROR);

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

@@ -665,6 +665,8 @@ public class DefaultErrorMessages {
         MAP.put(RESERVED_MEMBER_INSIDE_INLINE_CLASS, "Member with the name ''{0}'' is reserved for future releases", STRING);
         MAP.put(SECONDARY_CONSTRUCTOR_WITH_BODY_INSIDE_INLINE_CLASS, "Secondary constructors with bodies are reserved for for future releases");
 
+        MAP.put(RESULT_CLASS_IN_RETURN_TYPE, "'kotlin.Result' cannot be used as a return type");
+
         MAP.put(VARIANCE_ON_TYPE_PARAMETER_NOT_ALLOWED, "Variance annotations are only allowed for type parameters of classes and interfaces");
         MAP.put(BOUND_ON_TYPE_ALIAS_PARAMETER_NOT_ALLOWED, "Bounds are not allowed on type alias parameters");
 

compiler/frontend/src/org/jetbrains/kotlin/resolve/TargetPlatform.kt

@@ -120,7 +120,8 @@ private val DEFAULT_DECLARATION_CHECKERS = listOf(
     InlineClassDeclarationChecker,
     PropertiesWithBackingFieldsInsideInlineClass(),
     AnnotationClassTargetAndRetentionChecker(),
-    ReservedMembersAndConstructsForInlineClass()
+    ReservedMembersAndConstructsForInlineClass(),
+    ResultClassInReturnTypeChecker()
 )
 
 private val DEFAULT_CALL_CHECKERS = listOf(

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

@@ -0,0 +1,56 @@
+/*
+ * Copyright 2010-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
+ * that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.resolve.checkers
+
+import org.jetbrains.kotlin.builtins.KotlinBuiltIns
+import org.jetbrains.kotlin.descriptors.*
+import org.jetbrains.kotlin.diagnostics.Errors
+import org.jetbrains.kotlin.diagnostics.reportDiagnosticOnce
+import org.jetbrains.kotlin.psi.KtCallableDeclaration
+import org.jetbrains.kotlin.psi.KtDeclaration
+import org.jetbrains.kotlin.types.KotlinType
+
+class ResultClassInReturnTypeChecker : DeclarationChecker {
+    companion object {
+        private const val RESULT_NAME = "Result"
+    }
+
+    override fun check(declaration: KtDeclaration, descriptor: DeclarationDescriptor, context: DeclarationCheckerContext) {
+        if (declaration !is KtCallableDeclaration || descriptor !is CallableMemberDescriptor) return
+
+        val returnType = descriptor.returnType ?: return
+        if (isForbiddenReturnType(returnType, descriptor)) {
+            val typeReferenceOrDeclarationName = declaration.typeReference ?: declaration.nameIdentifier ?: return
+            context.trace.reportDiagnosticOnce(Errors.RESULT_CLASS_IN_RETURN_TYPE.on(typeReferenceOrDeclarationName))
+        }
+    }
+
+    private fun isForbiddenReturnType(returnType: KotlinType, declarationDescriptor: DeclarationDescriptor): Boolean {
+        val descriptor = returnType.constructor.declarationDescriptor ?: return false
+        if (!descriptor.isResultClass()) return false
+
+        if (declarationDescriptor is PropertyDescriptor || declarationDescriptor is PropertyGetterDescriptor) {
+            val visibility = (declarationDescriptor as DeclarationDescriptorWithVisibility).effectiveVisibility()
+            return when (visibility) {
+                is EffectiveVisibility.Private, is EffectiveVisibility.Local,
+                is EffectiveVisibility.InternalOrPackage, is EffectiveVisibility.InternalProtected,
+                is EffectiveVisibility.InternalProtectedBound -> false
+
+                is EffectiveVisibility.Public, is EffectiveVisibility.Protected,
+                is EffectiveVisibility.ProtectedBound -> true
+            }
+        }
+
+        return true
+    }
+
+    private fun DeclarationDescriptor.isResultClass(): Boolean {
+        val container = containingDeclaration ?: return false
+        return container is PackageFragmentDescriptor &&
+                container.fqName == KotlinBuiltIns.BUILT_INS_PACKAGE_FQ_NAME &&
+                name.asString() == RESULT_NAME
+    }
+}