IrDeclaration.transform(transformer, data) returns IrStatement.

Declaration containers assume that transformers maintain declaration structure.

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/declarations/IrDeclaration.kt

@@ -25,8 +25,8 @@ interface IrDeclaration : IrStatement {
     val declarationKind: IrDeclarationKind
     val origin: IrDeclarationOrigin
 
-    override fun <D> transform(transformer: IrElementTransformer<D>, data: D): IrDeclaration =
+    override fun <D> transform(transformer: IrElementTransformer<D>, data: D): IrStatement =
-            accept(transformer, data) as IrDeclaration
+            accept(transformer, data) as IrStatement
 }
 
 enum class IrDeclarationKind {

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/declarations/impl/IrClassImpl.kt

@@ -57,7 +57,7 @@ class IrClassImpl(
 
     override fun <D> transformChildren(transformer: IrElementTransformer<D>, data: D) {
         declarations.forEachIndexed { i, irDeclaration ->
-            declarations[i] = irDeclaration.transform(transformer, data)
+            declarations[i] = irDeclaration.transform(transformer, data) as IrDeclaration
         }
     }
 }

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/declarations/impl/IrFileImpl.kt

@@ -51,7 +51,7 @@ class IrFileImpl(
 
     override fun <D> transformChildren(transformer: IrElementTransformer<D>, data: D) {
         declarations.forEachIndexed { i, irDeclaration ->
-            declarations[i] = irDeclaration.transform(transformer, data)
+            declarations[i] = irDeclaration.transform(transformer, data) as IrDeclaration
         }
     }
 }

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/visitors/IrElementTransformer.kt

@@ -17,6 +17,7 @@
 package org.jetbrains.kotlin.ir.visitors
 
 import org.jetbrains.kotlin.ir.IrElement
+import org.jetbrains.kotlin.ir.IrStatement
 import org.jetbrains.kotlin.ir.declarations.*
 import org.jetbrains.kotlin.ir.expressions.*
 
@@ -36,7 +37,7 @@ interface IrElementTransformer<in D> : IrElementVisitor<IrElement, D> {
         return declaration
     }
 
-    override fun visitDeclaration(declaration: IrDeclaration, data: D): IrDeclaration {
+    override fun visitDeclaration(declaration: IrDeclaration, data: D): IrStatement {
         declaration.transformChildren(this, data)
         return declaration
     }