FIR: permit tailrec calls in inline lambdas

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/declaration/FirTailrecFunctionChecker.kt

@@ -36,16 +36,8 @@ object FirTailrecFunctionChecker : FirSimpleFunctionChecker() {
         var catchScopeCount = 0
         var finallyScopeCount = 0
         var tailrecCount = 0
-        var lambdaScopeCount = 0
         graph.traverse(TraverseDirection.Forward, object : ControlFlowGraphVisitorVoid() {
             override fun visitNode(node: CFGNode<*>) {}
-            override fun visitPostponedLambdaEnterNode(node: PostponedLambdaEnterNode) {
-                lambdaScopeCount++
-            }
-
-            override fun visitPostponedLambdaExitNode(node: PostponedLambdaExitNode) {
-                lambdaScopeCount--
-            }
 
             override fun visitTryMainBlockEnterNode(node: TryMainBlockEnterNode) {
                 tryScopeCount++
@@ -81,30 +73,16 @@ object FirTailrecFunctionChecker : FirSimpleFunctionChecker() {
                     return
                 }
                 val dispatchReceiver = functionCall.dispatchReceiver
-                // A tailrec call does not support changing dispatchers. Here we report changing dispatch receiver if the dispatch receiver
+                val dispatchReceiverOwner = declaration.dispatchReceiverType?.toSymbol(context.session) as? FirClassSymbol<*>
-                // is present and not a `this` or a singleton. For the `this` check, we don't need to actually compare if the dispatch
+                val sameReceiver = dispatchReceiver is FirNoReceiverExpression ||
-                // receiver `this` references the same `this` made available from `declaration`. This is because
+                        (dispatchReceiver is FirThisReceiverExpression && dispatchReceiver.calleeReference.boundSymbol == dispatchReceiverOwner) ||
-                // 1. if `this` is not labeled, then it references the innermost `this` receiver. If the innermost scope is not the
+                        dispatchReceiverOwner?.classKind?.isSingleton == true
-                //    `declaration` body, then follow-up checks on following nodes would report there to be more instructions, which would
+                if (!sameReceiver) {
-                //    then make this call non-tailrec.
+                    // A call on a different receiver might get dispatched to a different method, so it can't be optimized.
-                // 2. If `this` is labeled, then one of the following is possible.
-                //    a. the call is in some context that has additional implicit `this` declared. But this can only happen if the call is
-                //       placed inside some extension lambda, which would be covered by the later check on exiting node.
-                //    b. `declaration` is a member function in a local class and the receiver is a labeled `this` pointing to the outer
-                //       non-local class. In this case, the resolved symbol cannot be the same as the symbol of `declaration`, and this case
-                //       is already bailed out earlier. So there is no need to report anything.
-                //    c. `declaration` is a member function of an inner class and the receiver is a labeled `this` pointing to the outer
-                //       class. The reasoning is the same with b.
-                if (dispatchReceiver !is FirThisReceiverExpression &&
-                    dispatchReceiver !is FirNoReceiverExpression &&
-                    (declaration.dispatchReceiverType?.toSymbol(context.session) as? FirClassSymbol<*>)?.classKind?.isSingleton != true
-                ) {
                     reporter.reportOn(functionCall.source, FirErrors.NON_TAIL_RECURSIVE_CALL, context)
-                    return
+                } else if (tryScopeCount > 0 || catchScopeCount > 0 || finallyScopeCount > 0) {
-                }
-                if (tryScopeCount > 0 || catchScopeCount > 0 || finallyScopeCount > 0) {
                     reporter.reportOn(functionCall.source, FirErrors.TAIL_RECURSION_IN_TRY_IS_NOT_SUPPORTED, context)
-                } else if (lambdaScopeCount > 0 || node.hasMoreFollowingInstructions(declaration)) {
+                } else if (node.hasMoreFollowingInstructions(declaration)) {
                     reporter.reportOn(functionCall.source, FirErrors.NON_TAIL_RECURSIVE_CALL, context)
                 } else if (!node.isDead) {
                     tailrecCount++

compiler/testData/codegen/box/diagnostics/functions/tailRecursion/recursiveCallInInlineLambda.kt

@@ -1,6 +1,5 @@
 // KT-16549
 // IGNORE_BACKEND: JVM, JS
-// IGNORE_FIR_DIAGNOSTICS_DIFF
 
 class TailInline {
     private inline fun act(action: () -> Unit) {

compiler/testData/codegen/box/diagnostics/functions/tailRecursion/recursiveCallInInlineLambdaWithCapture.kt

@@ -1,7 +1,6 @@
 // KT-14961
 // IGNORE_BACKEND: JVM, JS_IR, WASM
 // WITH_RUNTIME
-// IGNORE_FIR_DIAGNOSTICS_DIFF
 
 fun listOfFactor(number: Int): List<Int> {
     tailrec fun listOfFactor(number: Int, acc: List<Int>): List<Int> {

compiler/testData/diagnostics/tests/tailRecInNestedScopes.fir.kt

@@ -1,37 +0,0 @@
-// WITH_STDLIB
-
-<!NO_TAIL_CALLS_FOUND!>tailrec<!> fun foo1() {
-    run {
-        <!NON_TAIL_RECURSIVE_CALL!>foo1<!>()
-    }
-}
-
-fun myRun(f: () -> Unit) = f()
-
-
-<!NO_TAIL_CALLS_FOUND!>tailrec<!> fun foo2() {
-    myRun {
-        <!NON_TAIL_RECURSIVE_CALL!>foo2<!>()
-    }
-}
-
-<!NO_TAIL_CALLS_FOUND!>tailrec<!> fun foo3() {
-    fun bar() {
-        <!NON_TAIL_RECURSIVE_CALL!>foo3<!>()
-    }
-    bar()
-}
-
-class A {
-    <!NO_TAIL_CALLS_FOUND!>tailrec<!> fun foo4() {
-        with(this) {
-            <!NON_TAIL_RECURSIVE_CALL!>foo4<!>()
-        }
-    }
-}
-
-<!NO_TAIL_CALLS_FOUND!>tailrec<!> fun foo5() {
-    run {
-        return <!NON_TAIL_RECURSIVE_CALL!>foo5<!>()
-    }
-}

compiler/testData/diagnostics/tests/tailRecInNestedScopes.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 // WITH_STDLIB
 
 <!NO_TAIL_CALLS_FOUND!>tailrec<!> fun foo1() {