IR BE Common: Provisional IrFunctionExpression lowering

Transforms IrFunctionExpression elements to local function references,
as it was done before IrFunctionExpression introduction.

compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/lower/ProvisionalFunctionExpressionLowering.kt

@@ -0,0 +1,46 @@
+/*
+ * Copyright 2010-2019 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.backend.common.lower
+
+import org.jetbrains.kotlin.backend.common.FileLoweringPass
+import org.jetbrains.kotlin.ir.declarations.IrFile
+import org.jetbrains.kotlin.ir.expressions.IrExpression
+import org.jetbrains.kotlin.ir.expressions.IrFunctionExpression
+import org.jetbrains.kotlin.ir.expressions.impl.IrBlockImpl
+import org.jetbrains.kotlin.ir.expressions.impl.IrFunctionReferenceImpl
+import org.jetbrains.kotlin.ir.visitors.IrElementTransformerVoid
+import org.jetbrains.kotlin.ir.visitors.transformChildrenVoid
+
+class ProvisionalFunctionExpressionLowering :
+    IrElementTransformerVoid(),
+    FileLoweringPass {
+
+    override fun lower(irFile: IrFile) {
+        irFile.transformChildrenVoid(this)
+    }
+
+    override fun visitFunctionExpression(expression: IrFunctionExpression): IrExpression {
+        expression.transformChildrenVoid(this)
+
+        val startOffset = expression.startOffset
+        val endOffset = expression.endOffset
+        val type = expression.type
+        val origin = expression.origin
+        val function = expression.function
+
+        return IrBlockImpl(
+            startOffset, endOffset, type, origin,
+            listOf(
+                function,
+                IrFunctionReferenceImpl(
+                    startOffset, endOffset, type,
+                    function.symbol, function.descriptor, 0,
+                    origin
+                )
+            )
+        )
+    }
+}

compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/JsLoweringPhases.kt

@@ -98,6 +98,13 @@ private val lateinitLoweringPhase = makeJsModulePhase(
     description = "Insert checks for lateinit field references"
 )
 
+// TODO make all lambda-related stuff work with IrFunctionExpression and drop this phase
+private val provisionalFunctionExpressionPhase = makeJsModulePhase(
+    { ProvisionalFunctionExpressionLowering() },
+    name = "FunctionExpression",
+    description = "Transform IrFunctionExpression to a local function reference"
+)
+
 private val arrayConstructorPhase = makeJsModulePhase(
     ::ArrayConstructorLowering,
     name = "ArrayConstructor",
@@ -389,6 +396,7 @@ val jsPhases = namedIrModulePhase(
     lower = validateIrBeforeLowering then
             testGenerationPhase then
             expectDeclarationsRemovingPhase then
+            provisionalFunctionExpressionPhase then
             arrayConstructorPhase then
             functionInliningPhase then
             lateinitLoweringPhase then

compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/JvmLower.kt

@@ -36,6 +36,13 @@ private fun makePatchParentsPhase(number: Int) = namedIrFilePhase(
     nlevels = 0
 )
 
+// TODO make all lambda-related stuff work with IrFunctionExpression and drop this phase
+private val provisionalFunctionExpressionPhase = makeIrFilePhase<CommonBackendContext>(
+    { ProvisionalFunctionExpressionLowering() },
+    name = "FunctionExpression",
+    description = "Transform IrFunctionExpression to a local function reference"
+)
+
 private val arrayConstructorPhase = makeIrFilePhase(
     ::ArrayConstructorLowering,
     name = "ArrayConstructor",
@@ -91,8 +98,9 @@ private val innerClassesPhase = makeIrFilePhase(
     prerequisite = setOf(localDeclarationsPhase)
 )
 
-private val jvmFilePhases = inventNamesForLocalClassesPhase then
+private val jvmFilePhases =
-
+        provisionalFunctionExpressionPhase then
+        inventNamesForLocalClassesPhase then
         kCallableNamePropertyPhase then
         arrayConstructorPhase then