Convert to expression body fix: do not allow = Unit for constructors

idea/src/org/jetbrains/kotlin/idea/intentions/ConvertToExpressionBodyIntention.kt

@@ -98,12 +98,14 @@ class ConvertToExpressionBodyIntention : SelfTargetingOffsetIndependentIntention
 
     private fun calcValue(declaration: KtDeclarationWithBody): KtExpression? {
         val body = declaration.blockExpression() ?: return null
-        return calcValue(body)
+        return calcValue(body, emptyAllowed = declaration !is KtConstructor<*>)
     }
 
-    private fun calcValue(body: KtBlockExpression): KtExpression? {
+    private fun calcValue(body: KtBlockExpression, emptyAllowed: Boolean = true): KtExpression? {
         val bodyStatements = body.statements
-        if (bodyStatements.isEmpty()) return KtPsiFactory(body).createExpression("Unit")
+        if (bodyStatements.isEmpty()) {
+            return if (emptyAllowed) KtPsiFactory(body).createExpression("Unit") else null
+        }
         val statement = bodyStatements.singleOrNull() ?: return null
         when (statement) {
             is KtReturnExpression -> {

idea/testData/intentions/convertToExpressionBody/constructorWithEmptyBody.kt

@@ -0,0 +1,5 @@
+// IS_APPLICABLE: false
+
+class C {
+    constructor()<caret> {}
+}

idea/tests/org/jetbrains/kotlin/idea/intentions/IntentionTestGenerated.java

@@ -4903,6 +4903,12 @@ public class IntentionTestGenerated extends AbstractIntentionTest {
             doTest(fileName);
         }
 
+        @TestMetadata("constructorWithEmptyBody.kt")
+        public void testConstructorWithEmptyBody() throws Exception {
+            String fileName = KotlinTestUtils.navigationMetadata("idea/testData/intentions/convertToExpressionBody/constructorWithEmptyBody.kt");
+            doTest(fileName);
+        }
+
         @TestMetadata("declaration.kt")
         public void testDeclaration() throws Exception {
             String fileName = KotlinTestUtils.navigationMetadata("idea/testData/intentions/convertToExpressionBody/declaration.kt");