Add inspection to detect public API with implicit type

idea/src/META-INF/plugin.xml

@@ -2761,6 +2761,15 @@
                      language="kotlin"
     />
 
+    <localInspection implementationClass="org.jetbrains.kotlin.idea.inspections.PublicApiImplicitTypeInspection"
+                     displayName="Public API declaration has implicit return type"
+                     groupPath="Kotlin"
+                     groupName="Other problems"
+                     enabledByDefault="false"
+                     level="WEAK WARNING"
+                     language="kotlin"
+    />
+
     <referenceImporter implementation="org.jetbrains.kotlin.idea.quickfix.KotlinReferenceImporter"/>
 
     <fileType.fileViewProviderFactory filetype="KJSM" implementationClass="com.intellij.psi.ClassFileViewProviderFactory"/>

idea/src/org/jetbrains/kotlin/idea/inspections/AbstractImplicitTypeInspection.kt

@@ -0,0 +1,22 @@
+/*
+ * 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.idea.inspections
+
+import org.jetbrains.kotlin.idea.intentions.SpecifyTypeExplicitlyIntention
+import org.jetbrains.kotlin.psi.KtCallableDeclaration
+
+abstract class AbstractImplicitTypeInspection(
+    additionalChecker: (KtCallableDeclaration, AbstractImplicitTypeInspection) -> Boolean
+) : IntentionBasedInspection<KtCallableDeclaration>(
+    SpecifyTypeExplicitlyIntention::class,
+    { element, inspection ->
+        with(inspection as AbstractImplicitTypeInspection) {
+            element.typeReference == null && additionalChecker(element, inspection)
+        }
+    }
+) {
+    override fun inspectionTarget(element: KtCallableDeclaration) = element.nameIdentifier
+}

idea/src/org/jetbrains/kotlin/idea/inspections/HasPlatformTypeInspection.kt

@@ -18,7 +18,6 @@ package org.jetbrains.kotlin.idea.inspections
 
 import com.intellij.codeInspection.IntentionWrapper
 import com.intellij.codeInspection.LocalQuickFix
-import com.intellij.codeInspection.ProblemHighlightType
 import com.intellij.codeInspection.ui.MultipleCheckboxOptionsPanel
 import org.jetbrains.kotlin.idea.intentions.SpecifyTypeExplicitlyIntention
 import org.jetbrains.kotlin.idea.intentions.isFlexibleRecursive
@@ -30,27 +29,33 @@ import org.jetbrains.kotlin.types.TypeUtils
 import javax.swing.JComponent
 
 class HasPlatformTypeInspection(
-        @JvmField var publicAPIOnly: Boolean = true,
-        @JvmField var reportPlatformArguments: Boolean = false
-) : IntentionBasedInspection<KtCallableDeclaration>(
-        SpecifyTypeExplicitlyIntention::class,
-        { element, inspection ->
-            with(inspection as HasPlatformTypeInspection) {
-                SpecifyTypeExplicitlyIntention.dangerousFlexibleTypeOrNull(element, this.publicAPIOnly, this.reportPlatformArguments) != null
-            }
+    @JvmField var publicAPIOnly: Boolean = true,
+    @JvmField var reportPlatformArguments: Boolean = false
+) : AbstractImplicitTypeInspection(
+    { element, inspection ->
+        with(inspection as HasPlatformTypeInspection) {
+            SpecifyTypeExplicitlyIntention.dangerousFlexibleTypeOrNull(
+                element,
+                this.publicAPIOnly,
+                this.reportPlatformArguments
+            ) != null
         }
+    }
 ) {
 
     override val problemText = "Declaration has type inferred from a platform call, which can lead to unchecked nullability issues. " +
-                               "Specify type explicitly as nullable or non-nullable."
+            "Specify type explicitly as nullable or non-nullable."
 
     override fun additionalFixes(element: KtCallableDeclaration): List<LocalQuickFix>? {
-        val type = SpecifyTypeExplicitlyIntention.dangerousFlexibleTypeOrNull(element, publicAPIOnly, reportPlatformArguments) ?: return null
+        val type = SpecifyTypeExplicitlyIntention.dangerousFlexibleTypeOrNull(
+            element, publicAPIOnly, reportPlatformArguments
+        ) ?: return null
 
         if (TypeUtils.isNullableType(type)) {
             val expression = element.node.findChildByType(KtTokens.EQ)?.psi?.getNextSiblingIgnoringWhitespaceAndComments()
             if (expression != null &&
-                (!reportPlatformArguments || !TypeUtils.makeNotNullable(type).isFlexibleRecursive())) {
+                (!reportPlatformArguments || !TypeUtils.makeNotNullable(type).isFlexibleRecursive())
+            ) {
                 return listOf(IntentionWrapper(AddExclExclCallFix(expression), element.containingFile))
             }
         }
@@ -58,8 +63,6 @@ class HasPlatformTypeInspection(
         return null
     }
 
-    override fun inspectionTarget(element: KtCallableDeclaration) = element.nameIdentifier
-
     override fun createOptionsPanel(): JComponent? {
         val panel = MultipleCheckboxOptionsPanel(this)
         panel.addCheckbox("Apply only to public or protected members", "publicAPIOnly")

idea/src/org/jetbrains/kotlin/idea/inspections/PublicApiImplicitTypeInspection.kt

@@ -0,0 +1,29 @@
+/*
+ * 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.idea.inspections
+
+import org.jetbrains.kotlin.descriptors.CallableMemberDescriptor
+import org.jetbrains.kotlin.descriptors.effectiveVisibility
+import org.jetbrains.kotlin.idea.caches.resolve.resolveToDescriptorIfAny
+import org.jetbrains.kotlin.psi.KtFunction
+import org.jetbrains.kotlin.psi.KtProperty
+import org.jetbrains.kotlin.psi.psiUtil.containingClassOrObject
+
+class PublicApiImplicitTypeInspection : AbstractImplicitTypeInspection(
+    { element, _ ->
+        element.containingClassOrObject?.isLocal != true &&
+                when (element) {
+                    is KtFunction -> !element.isLocal
+                    is KtProperty -> !element.isLocal
+                    else -> false
+                } && run {
+            val callableMemberDescriptor = element.resolveToDescriptorIfAny() as? CallableMemberDescriptor
+            callableMemberDescriptor?.effectiveVisibility()?.toVisibility()?.isPublicAPI == true
+        }
+    }
+) {
+    override val problemText = "For API stability, it's recommended to specify explicitly public & protected declaration types"
+}