[IR] Make IrFunction.returnType lateinit

to replace custom logic that would throw in case of IrUninitialized.

This is to remove custom logic from those classes,
so that source generator will be able to generate
them in a straight-forward way.
Otherwise, it would need to be thought about those special cases, which
is better to be avoided.

There are two consequences:
- The error message will be more generic, and won't contain reference
to the element.
- It becomes possible to reassign returnType
property with IrUninitializedType
and read it afterward.

^KT-65773 In Progress

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

@@ -26,7 +26,6 @@ class IrConstructorImpl @IrImplementationDetail constructor(
     override val symbol: IrConstructorSymbol,
     override var name: Name,
     override var visibility: DescriptorVisibility,
-    returnType: IrType,
     override var isInline: Boolean,
     override var isExternal: Boolean,
     override var isPrimary: Boolean,
@@ -41,13 +40,6 @@ class IrConstructorImpl @IrImplementationDetail constructor(
     override lateinit var parent: IrDeclarationParent
     override var annotations: List<IrConstructorCall> = emptyList()
 
-    override var returnType: IrType = returnType
-        get() = if (field === IrUninitializedType) {
-            throw ReturnTypeIsNotInitializedException(this)
-        } else {
-            field
-        }
-
     override var typeParameters: List<IrTypeParameter> = emptyList()
 
     override var dispatchReceiverParameter: IrValueParameter? = null
@@ -55,6 +47,8 @@ class IrConstructorImpl @IrImplementationDetail constructor(
     override var contextReceiverParametersCount: Int = 0
     override var valueParameters: List<IrValueParameter> = emptyList()
 
+    override lateinit var returnType: IrType
+
     override var body: IrBody? = null
 
     override var metadata: MetadataSource? = null

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

@@ -15,6 +15,7 @@ import org.jetbrains.kotlin.ir.expressions.impl.IrBlockBodyImpl
 import org.jetbrains.kotlin.ir.expressions.impl.IrExpressionBodyImpl
 import org.jetbrains.kotlin.ir.symbols.*
 import org.jetbrains.kotlin.ir.types.IrType
+import org.jetbrains.kotlin.ir.types.impl.IrUninitializedType
 import org.jetbrains.kotlin.name.Name
 import org.jetbrains.kotlin.serialization.deserialization.descriptors.DeserializedContainerSource
 import org.jetbrains.kotlin.types.Variance
@@ -83,9 +84,13 @@ abstract class AbstractIrFactoryImpl : IrFactory {
         containerSource: DeserializedContainerSource?,
     ): IrConstructor =
         IrConstructorImpl(
-            startOffset, endOffset, origin, symbol, name, visibility, returnType, isInline, isExternal, isPrimary, isExpect,
+            startOffset, endOffset, origin, symbol, name, visibility, isInline, isExternal, isPrimary, isExpect,
             containerSource, factory = this
-        )
+        ).apply {
+            if (returnType != IrUninitializedType) {
+                this.returnType = returnType
+            }
+        }
 
     override fun createEnumEntry(
         startOffset: Int,
@@ -137,10 +142,14 @@ abstract class AbstractIrFactoryImpl : IrFactory {
         isFakeOverride: Boolean,
     ): IrSimpleFunction =
         IrFunctionImpl(
-            startOffset, endOffset, origin, symbol, name, visibility, modality, returnType,
+            startOffset, endOffset, origin, symbol, name, visibility, modality,
             isInline, isExternal, isTailrec, isSuspend, isOperator, isInfix, isExpect, isFakeOverride,
             containerSource, factory = this
-        )
+        ).apply {
+            if (returnType != IrUninitializedType) {
+                this.returnType = returnType
+            }
+        }
 
     override fun createFunctionWithLateBinding(
         startOffset: Int,
@@ -166,7 +175,6 @@ abstract class AbstractIrFactoryImpl : IrFactory {
             name = name,
             visibility = visibility,
             modality = modality,
-            returnType = returnType,
             isInline = isInline,
             isExternal = isExternal,
             isTailrec = isTailrec,
@@ -176,7 +184,11 @@ abstract class AbstractIrFactoryImpl : IrFactory {
             isExpect = isExpect,
             isFakeOverride = isFakeOverride,
             factory = this
-        )
+        ).apply {
+            if (returnType != IrUninitializedType) {
+                this.returnType = returnType
+            }
+        }
 
     override fun createLocalDelegatedProperty(
         startOffset: Int,

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

@@ -32,7 +32,6 @@ class IrFunctionImpl @IrImplementationDetail constructor(
     override var name: Name,
     override var visibility: DescriptorVisibility,
     override var modality: Modality,
-    returnType: IrType,
     override var isInline: Boolean,
     override var isExternal: Boolean,
     override var isTailrec: Boolean,
@@ -55,13 +54,6 @@ class IrFunctionImpl @IrImplementationDetail constructor(
     override lateinit var parent: IrDeclarationParent
     override var annotations: List<IrConstructorCall> = emptyList()
 
-    override var returnType: IrType = returnType
-        get() = if (field === IrUninitializedType) {
-            throw ReturnTypeIsNotInitializedException(this)
-        } else {
-            field
-        }
-
     override var typeParameters: List<IrTypeParameter> = emptyList()
 
     override var dispatchReceiverParameter: IrValueParameter? = null
@@ -70,6 +62,8 @@ class IrFunctionImpl @IrImplementationDetail constructor(
 
     override var contextReceiverParametersCount: Int = 0
 
+    override lateinit var returnType: IrType
+
     override var body: IrBody? = null
 
     override var metadata: MetadataSource? = null

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

@@ -28,7 +28,6 @@ class IrFunctionWithLateBindingImpl @IrImplementationDetail constructor(
     override var name: Name,
     override var visibility: DescriptorVisibility,
     override var modality: Modality,
-    returnType: IrType,
     override var isInline: Boolean,
     override var isExternal: Boolean,
     override var isTailrec: Boolean,
@@ -66,13 +65,6 @@ class IrFunctionWithLateBindingImpl @IrImplementationDetail constructor(
 
     override var annotations: List<IrConstructorCall> = emptyList()
 
-    override var returnType: IrType = returnType
-        get() = if (field === IrUninitializedType) {
-            throw ReturnTypeIsNotInitializedException(this)
-        } else {
-            field
-        }
-
     override var typeParameters: List<IrTypeParameter> = emptyList()
 
     override var dispatchReceiverParameter: IrValueParameter? = null
@@ -81,6 +73,8 @@ class IrFunctionWithLateBindingImpl @IrImplementationDetail constructor(
 
     override var contextReceiverParametersCount: Int = 0
 
+    override lateinit var returnType: IrType
+
     override var body: IrBody? = null
 
     override var metadata: MetadataSource? = null