"Redundant lambda arrow" inspection: Also report arrow with single underscore parameter #KT-24728 Fixed

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

@@ -11,21 +11,34 @@ import com.intellij.codeInspection.ProblemDescriptor
 import com.intellij.codeInspection.ProblemHighlightType
 import com.intellij.codeInspection.ProblemsHolder
 import com.intellij.openapi.project.Project
+import com.intellij.openapi.util.TextRange
 import com.intellij.psi.PsiElementVisitor
+import org.jetbrains.kotlin.psi.KtFunctionLiteral
 import org.jetbrains.kotlin.psi.lambdaExpressionVisitor
+import org.jetbrains.kotlin.psi.psiUtil.endOffset
+import org.jetbrains.kotlin.psi.psiUtil.startOffset
+import org.jetbrains.kotlin.resolve.calls.util.isSingleUnderscore
 
 class RedundantLambdaArrowInspection : AbstractKotlinInspection() {
     override fun buildVisitor(holder: ProblemsHolder, isOnTheFly: Boolean): PsiElementVisitor {
         return lambdaExpressionVisitor { lambdaExpression ->
             val functionLiteral = lambdaExpression.functionLiteral
-            if (functionLiteral.valueParameters.isNotEmpty()) return@lambdaExpressionVisitor
             val arrow = functionLiteral.arrow ?: return@lambdaExpressionVisitor
+            val parameters = functionLiteral.valueParameters
+            val singleParameter = parameters.singleOrNull()
+            if (parameters.isNotEmpty() && singleParameter?.isSingleUnderscore != true) return@lambdaExpressionVisitor
 
+            val startOffset = functionLiteral.startOffset
             holder.registerProblem(
-                    arrow,
+                holder.manager.createProblemDescriptor(
+                    functionLiteral,
+                    TextRange((singleParameter?.startOffset ?: arrow.startOffset) - startOffset, arrow.endOffset - startOffset),
                     "Redundant lambda arrow",
                     ProblemHighlightType.LIKE_UNUSED_SYMBOL,
-                    DeleteFix())
+                    isOnTheFly,
+                    DeleteFix()
+                )
+            )
         }
     }
 
@@ -33,9 +46,10 @@ class RedundantLambdaArrowInspection : AbstractKotlinInspection() {
         override fun getFamilyName() = "Remove arrow"
 
         override fun applyFix(project: Project, descriptor: ProblemDescriptor) {
-            val element = descriptor.psiElement
+            val element = descriptor.psiElement as? KtFunctionLiteral ?: return
             FileModificationService.getInstance().preparePsiElementForWrite(element)
-            element.delete()
+            element.valueParameterList?.delete()
+            element.arrow?.delete()
         }
     }
 }

idea/testData/inspectionsLocal/redundantLambdaArrow/underscore.kt

@@ -0,0 +1,5 @@
+fun foo(f: () -> Unit) {}
+
+fun bar() {
+    foo { <caret>_ -> }
+}

idea/testData/inspectionsLocal/redundantLambdaArrow/underscore.kt.after

@@ -0,0 +1,5 @@
+fun foo(f: () -> Unit) {}
+
+fun bar() {
+    foo { <caret>}
+}

idea/tests/org/jetbrains/kotlin/idea/inspections/LocalInspectionTestGenerated.java

@@ -3969,6 +3969,11 @@ public class LocalInspectionTestGenerated extends AbstractLocalInspectionTest {
         public void testSimple() throws Exception {
             runTest("idea/testData/inspectionsLocal/redundantLambdaArrow/simple.kt");
         }
+
+        @TestMetadata("underscore.kt")
+        public void testUnderscore() throws Exception {
+            runTest("idea/testData/inspectionsLocal/redundantLambdaArrow/underscore.kt");
+        }
     }
 
     @TestMetadata("idea/testData/inspectionsLocal/redundantObjectTypeCheck")