FIR: revert a hack that allows overriding T!! with T

1. this should've been only done if the language feature for validating
    that is disabled;

 2. that feature probably won't matter by the time FIR is stable;

 3. it only worked because type enhancement of type arguments is broken
    anyway - a more correct hack would be to provide a custom
    ConeTypePreparator.

compiler/testData/ir/irText/firProblems/AbstractMutableMap.fir.txt

@@ -82,6 +82,13 @@ FILE fqName:<root> fileName:/AbstractMutableMap.kt
       VALUE_PARAMETER name:p0 index:0 type:K of <root>.MyMap
       VALUE_PARAMETER name:p1 index:1 type:@[EnhancedNullability] V of <root>.MyMap
       VALUE_PARAMETER name:p2 index:2 type:@[EnhancedNullability] java.util.function.BiFunction<in @[FlexibleNullability] V of <root>.MyMap?, in @[FlexibleNullability] V of <root>.MyMap?, out @[FlexibleNullability] V of <root>.MyMap?>
+    FUN FAKE_OVERRIDE name:merge visibility:public modality:OPEN <> ($this:kotlin.collections.MutableMap<K of kotlin.collections.MutableMap, V of kotlin.collections.MutableMap>, p0:@[FlexibleNullability] K of <root>.MyMap?, p1:V of <root>.MyMap, p2:@[EnhancedNullability] java.util.function.BiFunction<in @[FlexibleNullability] V of <root>.MyMap?, in @[FlexibleNullability] V of <root>.MyMap?, out @[FlexibleNullability] V of <root>.MyMap?>) returnType:V of <root>.MyMap? [fake_override]
+      overridden:
+        public open fun merge (p0: @[FlexibleNullability] K of kotlin.collections.AbstractMutableMap?, p1: V of kotlin.collections.AbstractMutableMap, p2: @[EnhancedNullability] java.util.function.BiFunction<in @[FlexibleNullability] V of kotlin.collections.AbstractMutableMap?, in @[FlexibleNullability] V of kotlin.collections.AbstractMutableMap?, out @[FlexibleNullability] V of kotlin.collections.AbstractMutableMap?>): V of kotlin.collections.AbstractMutableMap? [fake_override] declared in kotlin.collections.AbstractMutableMap
+      $this: VALUE_PARAMETER name:<this> type:kotlin.collections.MutableMap<K of kotlin.collections.MutableMap, V of kotlin.collections.MutableMap>
+      VALUE_PARAMETER name:p0 index:0 type:@[FlexibleNullability] K of <root>.MyMap?
+      VALUE_PARAMETER name:p1 index:1 type:V of <root>.MyMap
+      VALUE_PARAMETER name:p2 index:2 type:@[EnhancedNullability] java.util.function.BiFunction<in @[FlexibleNullability] V of <root>.MyMap?, in @[FlexibleNullability] V of <root>.MyMap?, out @[FlexibleNullability] V of <root>.MyMap?>
     FUN FAKE_OVERRIDE name:computeIfPresent visibility:public modality:OPEN <> ($this:kotlin.collections.MutableMap<K of kotlin.collections.MutableMap, V of kotlin.collections.MutableMap>, p0:K of <root>.MyMap, p1:@[EnhancedNullability] java.util.function.BiFunction<in @[FlexibleNullability] K of <root>.MyMap?, in @[FlexibleNullability] V of <root>.MyMap?, out @[FlexibleNullability] V of <root>.MyMap?>) returnType:V of <root>.MyMap? [fake_override]
       overridden:
         public open fun computeIfPresent (p0: K of kotlin.collections.AbstractMutableMap, p1: @[EnhancedNullability] java.util.function.BiFunction<in @[FlexibleNullability] K of kotlin.collections.AbstractMutableMap?, in @[FlexibleNullability] V of kotlin.collections.AbstractMutableMap?, out @[FlexibleNullability] V of kotlin.collections.AbstractMutableMap?>): V of kotlin.collections.AbstractMutableMap? [fake_override] declared in kotlin.collections.AbstractMutableMap

compiler/testData/ir/irText/types/castsInsideCoroutineInference.fir.kt.txt

@@ -1,7 +1,7 @@
 @OptIn(markerClass = [ExperimentalTypeInference::class])
 fun <R : Any?> scopedFlow(@BuilderInference block: @ExtensionFunctionType SuspendFunction2<CoroutineScope, FlowCollector<R>, Unit>): Flow<R> {
   return flow<R>(block = local suspend fun FlowCollector<R>.<anonymous>() {
-    val collector: FlowCollector<Nothing> = <this>
+    val collector: FlowCollector<R> = <this>
     flowScope<Unit>(block = local suspend fun CoroutineScope.<anonymous>() {
       block.invoke(p1 = <this>, p2 = collector)
     }
@@ -12,7 +12,7 @@ fun <R : Any?> scopedFlow(@BuilderInference block: @ExtensionFunctionType Suspen
 
 fun <T : Any?> Flow<T>.onCompletion(action: @ExtensionFunctionType SuspendFunction2<FlowCollector<T>, Throwable?, Unit>): Flow<T> {
   return unsafeFlow<T>(block = local suspend fun FlowCollector<T>.<anonymous>() {
-    val safeCollector: SafeCollector<Any?> = SafeCollector<T>(collector = <this>)
+    val safeCollector: SafeCollector<T> = SafeCollector<T>(collector = <this>)
     safeCollector.invokeSafely<T>(action = action)
   }
 )
@@ -27,11 +27,11 @@ inline fun <T : Any?> unsafeFlow(@BuilderInference crossinline block: @Extension
 }
 
 @Deprecated(message = "binary compatibility with a version w/o FlowCollector receiver", level = DeprecationLevel.HIDDEN)
-fun <T : Any?> Flow<T>.onCompletion(action: SuspendFunction1<Throwable?, Unit>): ErrorType {
-  return error("") /* ErrorCallExpression */local fun <anonymous>() {
-    action.invoke(p1 = error("") /* ErrorCallExpression */)
+fun <T : Any?> Flow<T>.onCompletion(action: SuspendFunction1<Throwable?, Unit>): Flow<T> {
+  return <this>.onCompletion<T>(action = local suspend fun FlowCollector<T>.<anonymous>(it: Throwable?) {
+    action.invoke(p1 = it)
   }
-;
+)
 }
 
 private fun CoroutineScope.asFairChannel(flow: Flow<*>): ReceiveChannel<Any> {
@@ -113,12 +113,12 @@ interface CoroutineScope {
 }
 
 interface Flow<out T : Any?> {
-  abstract suspend fun collect(collector: FlowCollector<T>)
+  abstract suspend fun collect(collector: FlowCollector<T>) 
 
 }
 
 interface FlowCollector<in T : Any?> {
-  abstract suspend fun emit(value: T)
+  abstract suspend fun emit(value: T) 
 
 }
 
@@ -138,7 +138,7 @@ interface ProducerScope<in E : Any?> : CoroutineScope, SendChannel<E> {
 }
 
 interface SendChannel<in E : Any?> {
-  abstract suspend fun send(e: E)
+  abstract suspend fun send(e: E) 
 
 }
 

compiler/testData/ir/irText/types/castsInsideCoroutineInference.fir.txt

@@ -14,8 +14,8 @@ FILE fqName:<root> fileName:/castsInsideCoroutineInference.kt
             FUN LOCAL_FUNCTION_FOR_LAMBDA name:<anonymous> visibility:local modality:FINAL <> ($receiver:<root>.FlowCollector<R of <root>.scopedFlow>) returnType:kotlin.Unit [suspend]
               $receiver: VALUE_PARAMETER name:<this> type:<root>.FlowCollector<R of <root>.scopedFlow>
               BLOCK_BODY
-                VAR name:collector type:<root>.FlowCollector<kotlin.Nothing> [val]
-                  GET_VAR '<this>: <root>.FlowCollector<R of <root>.scopedFlow> declared in <root>.scopedFlow.<anonymous>' type=<root>.FlowCollector<kotlin.Nothing> origin=null
+                VAR name:collector type:<root>.FlowCollector<R of <root>.scopedFlow> [val]
+                  GET_VAR '<this>: <root>.FlowCollector<R of <root>.scopedFlow> declared in <root>.scopedFlow.<anonymous>' type=<root>.FlowCollector<R of <root>.scopedFlow> origin=null
                 CALL 'public final fun flowScope <R> (block: @[ExtensionFunctionType] kotlin.coroutines.SuspendFunction1<<root>.CoroutineScope, R of <root>.flowScope>): R of <root>.flowScope [suspend] declared in <root>' type=kotlin.Unit origin=null
                   <R>: kotlin.Unit
                   block: FUN_EXPR type=kotlin.coroutines.SuspendFunction1<<root>.CoroutineScope, kotlin.Unit> origin=LAMBDA
@@ -25,7 +25,7 @@ FILE fqName:<root> fileName:/castsInsideCoroutineInference.kt
                         CALL 'public abstract fun invoke (p1: P1 of kotlin.coroutines.SuspendFunction2, p2: P2 of kotlin.coroutines.SuspendFunction2): R of kotlin.coroutines.SuspendFunction2 [suspend,operator] declared in kotlin.coroutines.SuspendFunction2' type=kotlin.Unit origin=null
                           $this: GET_VAR 'block: @[ExtensionFunctionType] kotlin.coroutines.SuspendFunction2<<root>.CoroutineScope, <root>.FlowCollector<R of <root>.scopedFlow>, kotlin.Unit> declared in <root>.scopedFlow' type=@[ExtensionFunctionType] kotlin.coroutines.SuspendFunction2<<root>.CoroutineScope, <root>.FlowCollector<R of <root>.scopedFlow>, kotlin.Unit> origin=VARIABLE_AS_FUNCTION
                           p1: GET_VAR '<this>: <root>.CoroutineScope declared in <root>.scopedFlow.<anonymous>.<anonymous>' type=<root>.CoroutineScope origin=null
-                          p2: GET_VAR 'val collector: <root>.FlowCollector<kotlin.Nothing> [val] declared in <root>.scopedFlow.<anonymous>' type=<root>.FlowCollector<R of <root>.scopedFlow> origin=null
+                          p2: GET_VAR 'val collector: <root>.FlowCollector<R of <root>.scopedFlow> [val] declared in <root>.scopedFlow.<anonymous>' type=<root>.FlowCollector<R of <root>.scopedFlow> origin=null
   FUN name:onCompletion visibility:public modality:FINAL <T> ($receiver:<root>.Flow<T of <root>.onCompletion>, action:@[ExtensionFunctionType] kotlin.coroutines.SuspendFunction2<<root>.FlowCollector<T of <root>.onCompletion>, kotlin.Throwable?, kotlin.Unit>) returnType:<root>.Flow<T of <root>.onCompletion>
     TYPE_PARAMETER name:T index:0 variance: superTypes:[kotlin.Any?]
     $receiver: VALUE_PARAMETER name:<this> type:<root>.Flow<T of <root>.onCompletion>
@@ -38,13 +38,13 @@ FILE fqName:<root> fileName:/castsInsideCoroutineInference.kt
             FUN LOCAL_FUNCTION_FOR_LAMBDA name:<anonymous> visibility:local modality:FINAL <> ($receiver:<root>.FlowCollector<T of <root>.onCompletion>) returnType:kotlin.Unit [suspend]
               $receiver: VALUE_PARAMETER name:<this> type:<root>.FlowCollector<T of <root>.onCompletion>
               BLOCK_BODY
-                VAR name:safeCollector type:<root>.SafeCollector<kotlin.Any?> [val]
+                VAR name:safeCollector type:<root>.SafeCollector<T of <root>.onCompletion> [val]
                   CONSTRUCTOR_CALL 'public constructor <init> (collector: <root>.FlowCollector<T of <root>.SafeCollector>) [primary] declared in <root>.SafeCollector' type=<root>.SafeCollector<T of <root>.onCompletion> origin=null
                     <class: T>: T of <root>.onCompletion
                     collector: GET_VAR '<this>: <root>.FlowCollector<T of <root>.onCompletion> declared in <root>.onCompletion.<anonymous>' type=<root>.FlowCollector<T of <root>.onCompletion> origin=null
                 CALL 'public final fun invokeSafely <T> (action: @[ExtensionFunctionType] kotlin.coroutines.SuspendFunction2<<root>.FlowCollector<T of <root>.invokeSafely>, kotlin.Throwable?, kotlin.Unit>): kotlin.Unit [suspend] declared in <root>' type=kotlin.Unit origin=null
                   <T>: T of <root>.onCompletion
-                  $receiver: GET_VAR 'val safeCollector: <root>.SafeCollector<kotlin.Any?> [val] declared in <root>.onCompletion.<anonymous>' type=<root>.SafeCollector<T of <root>.onCompletion> origin=null
+                  $receiver: GET_VAR 'val safeCollector: <root>.SafeCollector<T of <root>.onCompletion> [val] declared in <root>.onCompletion.<anonymous>' type=<root>.SafeCollector<T of <root>.onCompletion> origin=null
                   action: GET_VAR 'action: @[ExtensionFunctionType] kotlin.coroutines.SuspendFunction2<<root>.FlowCollector<T of <root>.onCompletion>, kotlin.Throwable?, kotlin.Unit> declared in <root>.onCompletion' type=@[ExtensionFunctionType] kotlin.coroutines.SuspendFunction2<<root>.FlowCollector<T of <root>.onCompletion>, kotlin.Throwable?, kotlin.Unit> origin=null
   FUN name:invokeSafely visibility:public modality:FINAL <T> ($receiver:<root>.FlowCollector<T of <root>.invokeSafely>, action:@[ExtensionFunctionType] kotlin.coroutines.SuspendFunction2<<root>.FlowCollector<T of <root>.invokeSafely>, kotlin.Throwable?, kotlin.Unit>) returnType:kotlin.Unit [suspend]
     TYPE_PARAMETER name:T index:0 variance: superTypes:[kotlin.Any?]
@@ -61,21 +61,25 @@ FILE fqName:<root> fileName:/castsInsideCoroutineInference.kt
     BLOCK_BODY
       RETURN type=kotlin.Nothing from='public final fun unsafeFlow <T> (block: @[ExtensionFunctionType] kotlin.coroutines.SuspendFunction1<<root>.FlowCollector<T of <root>.unsafeFlow>, kotlin.Unit>): <root>.Flow<T of <root>.unsafeFlow> [inline] declared in <root>'
         CALL 'public final fun TODO (): kotlin.Nothing [inline] declared in kotlin.StandardKt' type=kotlin.Nothing origin=null
-  FUN name:onCompletion visibility:public modality:FINAL <T> ($receiver:<root>.Flow<T of <root>.onCompletion>, action:kotlin.coroutines.SuspendFunction1<kotlin.Throwable?, kotlin.Unit>) returnType:IrErrorType(null)
+  FUN name:onCompletion visibility:public modality:FINAL <T> ($receiver:<root>.Flow<T of <root>.onCompletion>, action:kotlin.coroutines.SuspendFunction1<kotlin.Throwable?, kotlin.Unit>) returnType:<root>.Flow<T of <root>.onCompletion>
     annotations:
       Deprecated(message = 'binary compatibility with a version w/o FlowCollector receiver', replaceWith = <null>, level = GET_ENUM 'ENUM_ENTRY IR_EXTERNAL_DECLARATION_STUB name:HIDDEN' type=kotlin.DeprecationLevel)
     TYPE_PARAMETER name:T index:0 variance: superTypes:[kotlin.Any?]
     $receiver: VALUE_PARAMETER name:<this> type:<root>.Flow<T of <root>.onCompletion>
     VALUE_PARAMETER name:action index:0 type:kotlin.coroutines.SuspendFunction1<kotlin.Throwable?, kotlin.Unit>
     BLOCK_BODY
-      RETURN type=kotlin.Nothing from='public final fun onCompletion <T> (action: kotlin.coroutines.SuspendFunction1<kotlin.Throwable?, kotlin.Unit>): IrErrorType(null) declared in <root>'
-        ERROR_CALL 'Unresolved reference: <Ambiguity: onCompletion, [/onCompletion, /onCompletion]>#' type=IrErrorType(null)
-          FUN_EXPR type=kotlin.Function0<kotlin.Unit> origin=LAMBDA
-            FUN LOCAL_FUNCTION_FOR_LAMBDA name:<anonymous> visibility:local modality:FINAL <> () returnType:kotlin.Unit
+      RETURN type=kotlin.Nothing from='public final fun onCompletion <T> (action: kotlin.coroutines.SuspendFunction1<kotlin.Throwable?, kotlin.Unit>): <root>.Flow<T of <root>.onCompletion> declared in <root>'
+        CALL 'public final fun onCompletion <T> (action: @[ExtensionFunctionType] kotlin.coroutines.SuspendFunction2<<root>.FlowCollector<T of <root>.onCompletion>, kotlin.Throwable?, kotlin.Unit>): <root>.Flow<T of <root>.onCompletion> declared in <root>' type=<root>.Flow<T of <root>.onCompletion> origin=null
+          <T>: T of <root>.onCompletion
+          $receiver: GET_VAR '<this>: <root>.Flow<T of <root>.onCompletion> declared in <root>.onCompletion' type=<root>.Flow<T of <root>.onCompletion> origin=null
+          action: FUN_EXPR type=kotlin.coroutines.SuspendFunction2<<root>.FlowCollector<T of <root>.onCompletion>, kotlin.Throwable?, kotlin.Unit> origin=LAMBDA
+            FUN LOCAL_FUNCTION_FOR_LAMBDA name:<anonymous> visibility:local modality:FINAL <> ($receiver:<root>.FlowCollector<T of <root>.onCompletion>, it:kotlin.Throwable?) returnType:kotlin.Unit [suspend]
+              $receiver: VALUE_PARAMETER name:<this> type:<root>.FlowCollector<T of <root>.onCompletion>
+              VALUE_PARAMETER name:it index:0 type:kotlin.Throwable?
               BLOCK_BODY
                 CALL 'public abstract fun invoke (p1: P1 of kotlin.coroutines.SuspendFunction1): R of kotlin.coroutines.SuspendFunction1 [suspend,operator] declared in kotlin.coroutines.SuspendFunction1' type=kotlin.Unit origin=null
                   $this: GET_VAR 'action: kotlin.coroutines.SuspendFunction1<kotlin.Throwable?, kotlin.Unit> declared in <root>.onCompletion' type=kotlin.coroutines.SuspendFunction1<kotlin.Throwable?, kotlin.Unit> origin=VARIABLE_AS_FUNCTION
-                  p1: ERROR_CALL 'Unresolved reference: <Unresolved name: it>#' type=IrErrorType(null)
+                  p1: GET_VAR 'it: kotlin.Throwable? declared in <root>.onCompletion.<anonymous>' type=kotlin.Throwable? origin=null
   FUN name:asFairChannel visibility:private modality:FINAL <> ($receiver:<root>.CoroutineScope, flow:<root>.Flow<*>) returnType:<root>.ReceiveChannel<kotlin.Any>
     $receiver: VALUE_PARAMETER name:<this> type:<root>.CoroutineScope
     VALUE_PARAMETER name:flow index:0 type:<root>.Flow<*>

compiler/testData/ir/irText/types/castsInsideCoroutineInference.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 // !LANGUAGE: +NewInference
 // WITH_RUNTIME
 
@@ -84,4 +83,4 @@ interface ProducerScope<in E> : CoroutineScope, SendChannel<E> {
 
 interface SendChannel<in E> {
     suspend fun send(e: E)
-}
+}