Keep arrow when both parameter list and block of function literal are…

… empty (pinterest#2469)

Removal of the arrow in this case would lead to a compilation error as the function literal is changed to a normal block when removing the arrow.

Closes pinterest#2465

ktlint-ruleset-standard/src/main/kotlin/com/pinterest/ktlint/ruleset/standard/rules/FunctionLiteralRule.kt

@@ -374,7 +374,7 @@ public class FunctionLiteralRule :
         require(arrow.elementType == ARROW)
         arrow
             .prevSibling { it.elementType == VALUE_PARAMETER_LIST }
-            ?.takeIf { it.findChildByType(VALUE_PARAMETER) == null }
+            ?.takeIf { it.findChildByType(VALUE_PARAMETER) == null && arrow.isFollowedByNonEmptyBlock() }
             ?.let {
                 emit(arrow.startOffset, "Arrow is redundant when parameter list is empty", true)
                 if (autoCorrect) {
@@ -387,6 +387,11 @@ public class FunctionLiteralRule :
             }
     }
 
+    private fun ASTNode.isFollowedByNonEmptyBlock(): Boolean {
+        require(elementType == ARROW)
+        return nextSibling { it.elementType == BLOCK }?.firstChildNode != null
+    }
+
     private fun visitBlock(
         block: ASTNode,
         autoCorrect: Boolean,

ktlint-ruleset-standard/src/test/kotlin/com/pinterest/ktlint/ruleset/standard/rules/FunctionLiteralRuleTest.kt

@@ -465,4 +465,13 @@ class FunctionLiteralRuleTest {
             .hasLintViolation(1, 42, "Arrow is redundant when parameter list is empty")
             .isFormattedAs(formattedCode)
     }
+
+    @Test
+    fun `Issue 2465 - Given a function literal without parameters and with an empty block then do not remove the arrow`() {
+        val code =
+            """
+            fun foo(block: () -> Unit): Unit = foo { -> }
+            """.trimIndent()
+        functionLiteralRuleAssertThat(code).hasNoLintViolations()
+    }
 }