FIR2IR: don't generate unnecessary 'return throw'

#KT-60245 Fixed

compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/Fir2IrImplicitCastInserter.kt

@@ -190,7 +190,7 @@ class Fir2IrImplicitCastInserter(
         (data as? IrContainerExpression)?.insertImplicitCasts() ?: data
 
     override fun visitReturnExpression(returnExpression: FirReturnExpression, data: IrElement): IrElement {
-        val irReturn = data as IrReturn
+        val irReturn = data as? IrReturn ?: return data
         val expectedType = returnExpression.target.labeledElement.returnTypeRef
         irReturn.value = irReturn.value.cast(returnExpression.result, returnExpression.result.typeRef, expectedType)
         return data

compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/Fir2IrVisitor.kt

@@ -498,9 +498,13 @@ class Fir2IrVisitor(
     // ==================================================================================
 
     override fun visitReturnExpression(returnExpression: FirReturnExpression, data: Any?): IrElement {
+        val result = returnExpression.result
+        if (result is FirThrowExpression) {
+            // Note: in FIR we must have 'return' as the last statement
+            return convertToIrExpression(result)
+        }
         val irTarget = conversionScope.returnTarget(returnExpression, declarationStorage)
         return returnExpression.convertWithOffsets { startOffset, endOffset ->
-            val result = returnExpression.result
             // For implicit returns, use the expression endOffset to generate the expected line number for debugging.
             val returnStartOffset = if (returnExpression.source?.kind is KtFakeSourceElementKind.ImplicitReturn) endOffset else startOffset
             IrReturnImpl(

compiler/testData/ir/irText/expressions/kt30796.fir.ir.txt

@@ -2,9 +2,8 @@ FILE fqName:<root> fileName:/kt30796.kt
   FUN name:magic visibility:public modality:FINAL <T> () returnType:T of <root>.magic
     TYPE_PARAMETER name:T index:0 variance: superTypes:[kotlin.Any?] reified:false
     BLOCK_BODY
-      RETURN type=kotlin.Nothing from='public final fun magic <T> (): T of <root>.magic declared in <root>'
-        THROW type=kotlin.Nothing
-          CONSTRUCTOR_CALL 'public constructor <init> () declared in java.lang.Exception' type=java.lang.Exception origin=null
+      THROW type=kotlin.Nothing
+        CONSTRUCTOR_CALL 'public constructor <init> () declared in java.lang.Exception' type=java.lang.Exception origin=null
   FUN name:test visibility:public modality:FINAL <T> (value:T of <root>.test, value2:T of <root>.test) returnType:kotlin.Unit
     TYPE_PARAMETER name:T index:0 variance: superTypes:[kotlin.Any?] reified:false
     VALUE_PARAMETER name:value index:0 type:T of <root>.test

compiler/testData/ir/irText/expressions/kt30796.fir.kt.txt

@@ -1,5 +1,5 @@
 fun <T : Any?> magic(): T {
-  return throw Exception()
+  throw Exception()
 }
 
 fun <T : Any?> test(value: T, value2: T) {

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

@@ -124,9 +124,8 @@ FILE fqName:<root> fileName:/ArrayMap.kt
                   public abstract fun next (): T of kotlin.collections.Iterator declared in kotlin.collections.Iterator
                 $this: VALUE_PARAMETER name:<this> type:<root>.EmptyArrayMap.iterator.<no name provided>
                 BLOCK_BODY
-                  RETURN type=kotlin.Nothing from='public open fun next (): kotlin.Nothing declared in <root>.EmptyArrayMap.iterator.<no name provided>'
-                    THROW type=kotlin.Nothing
-                      CONSTRUCTOR_CALL 'public constructor <init> () declared in java.util.NoSuchElementException' type=java.util.NoSuchElementException origin=null
+                  THROW type=kotlin.Nothing
+                    CONSTRUCTOR_CALL 'public constructor <init> () declared in java.util.NoSuchElementException' type=java.util.NoSuchElementException origin=null
               FUN FAKE_OVERRIDE name:equals visibility:public modality:OPEN <> ($this:kotlin.Any, other:kotlin.Any?) returnType:kotlin.Boolean [fake_override,operator]
                 overridden:
                   public open fun equals (other: kotlin.Any?): kotlin.Boolean declared in kotlin.collections.Iterator

compiler/testData/ir/irText/firProblems/ArrayMap.fir.kt.txt

@@ -62,7 +62,7 @@ internal object EmptyArrayMap : ArrayMap<Nothing> {
         }
 
         override operator fun next(): Nothing {
-          return throw NoSuchElementException()
+          throw NoSuchElementException()
         }
 
       }
@@ -328,4 +328,3 @@ internal class ArrayMapImpl<T : Any> : ArrayMap<T> {
   }
 
 }
-

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

@@ -94,9 +94,8 @@ FILE fqName:<root> fileName:/typeAliasWithUnsafeVariance.kt
           CONST Boolean type=kotlin.Boolean value=true
   FUN name:getTag visibility:public modality:FINAL <> () returnType:<root>.Tag<*>
     BLOCK_BODY
-      RETURN type=kotlin.Nothing from='public final fun getTag (): <root>.Tag<*> declared in <root>'
-        THROW type=kotlin.Nothing
-          CONSTRUCTOR_CALL 'public constructor <init> () declared in java.lang.Exception' type=java.lang.Exception origin=null
+      THROW type=kotlin.Nothing
+        CONSTRUCTOR_CALL 'public constructor <init> () declared in java.lang.Exception' type=java.lang.Exception origin=null
   FUN name:doAction visibility:public modality:FINAL <> () returnType:kotlin.Unit
     BLOCK_BODY
       TYPE_OP type=kotlin.Unit origin=IMPLICIT_COERCION_TO_UNIT typeOperand=kotlin.Unit

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

@@ -43,10 +43,9 @@ data class Tag<out RenderingT : Any?> {
 }
 
 fun getTag(): Tag<*> {
-  return throw Exception()
+  throw Exception()
 }
 
 fun doAction() {
   getTag().<get-action>() /*~> Unit */
 }
-