Join lines could "convert to expression body" #KT-15769 Fixed (#1304)

idea/src/org/jetbrains/kotlin/idea/joinLines/JoinBlockIntoSingleStatementHandler.kt

@@ -19,9 +19,11 @@ package org.jetbrains.kotlin.idea.joinLines
 import com.intellij.codeInsight.editorActions.JoinRawLinesHandlerDelegate
 import com.intellij.openapi.editor.Document
 import com.intellij.psi.PsiFile
+import org.jetbrains.kotlin.idea.inspections.UseExpressionBodyInspection
 import org.jetbrains.kotlin.idea.intentions.MergeIfsIntention
 import org.jetbrains.kotlin.lexer.KtTokens
 import org.jetbrains.kotlin.psi.*
+import org.jetbrains.kotlin.psi.psiUtil.startOffset
 
 class JoinBlockIntoSingleStatementHandler : JoinRawLinesHandlerDelegate {
     override fun tryJoinRawLines(document: Document, file: PsiFile, start: Int, end: Int): Int {
@@ -36,8 +38,12 @@ class JoinBlockIntoSingleStatementHandler : JoinRawLinesHandlerDelegate {
 
         val block = brace.parent as? KtBlockExpression ?: return -1
         val statement = block.statements.singleOrNull() ?: return -1
+
         val parent = block.parent
-        if (parent !is KtContainerNode && parent !is KtWhenEntry) return -1
+        val useExpressionBodyInspection = UseExpressionBodyInspection(convertEmptyToUnit = false)
+        val oneLineReturnFunction = (parent as? KtDeclarationWithBody)?.takeIf { useExpressionBodyInspection.isActiveFor(it) }
+        if (parent !is KtContainerNode && parent !is KtWhenEntry && oneLineReturnFunction == null) return -1
+
         if (block.node.getChildren(KtTokens.COMMENTS).isNotEmpty()) return -1 // otherwise we will loose comments
 
         // handle nested if's
@@ -61,8 +67,14 @@ class JoinBlockIntoSingleStatementHandler : JoinRawLinesHandlerDelegate {
             }
         }
 
-        val newStatement = block.replace(statement)
+        return if (oneLineReturnFunction != null) {
-        return newStatement.textRange!!.startOffset
+            useExpressionBodyInspection.simplify(oneLineReturnFunction, false)
+            oneLineReturnFunction.bodyExpression!!.startOffset
+        }
+        else {
+            val newStatement = block.replace(statement)
+            newStatement.textRange!!.startOffset
+        }
     }
 
     override fun tryJoinLines(document: Document, file: PsiFile, start: Int, end: Int) = -1

idea/testData/joinLines/removeBraces/FunctionWithOneLineReturn.kt

@@ -0,0 +1,3 @@
+fun foo(): Int {<caret>
+    return 1
+}

idea/testData/joinLines/removeBraces/FunctionWithOneLineReturn.kt.after

@@ -0,0 +1 @@
+fun foo(): Int =<caret> 1

idea/tests/org/jetbrains/kotlin/idea/intentions/declarations/JoinLinesTestGenerated.java

@@ -359,6 +359,12 @@ public class JoinLinesTestGenerated extends AbstractJoinLinesTest {
             doTest(fileName);
         }
 
+        @TestMetadata("FunctionWithOneLineReturn.kt")
+        public void testFunctionWithOneLineReturn() throws Exception {
+            String fileName = KotlinTestUtils.navigationMetadata("idea/testData/joinLines/removeBraces/FunctionWithOneLineReturn.kt");
+            doTest(fileName);
+        }
+
         @TestMetadata("If.kt")
         public void testIf() throws Exception {
             String fileName = KotlinTestUtils.navigationMetadata("idea/testData/joinLines/removeBraces/If.kt");