FirSyntheticProperty: take returnTypeRef directly from delegate

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/body/resolve/FirDeclarationsResolveTransformer.kt

@@ -98,7 +98,6 @@ open class FirDeclarationsResolveTransformer(transformer: FirBodyResolveTransfor
     override fun transformProperty(property: FirProperty, data: ResolutionMode): CompositeTransformResult<FirProperty> {
         if (property is FirSyntheticProperty) {
             transformSimpleFunction(property.getter.delegate, data)
-            property.replaceReturnTypeRef(property.getter.delegate.returnTypeRef)
             return property.compose()
         }
         return withTypeParametersOf(property) {

compiler/fir/tree/src/org/jetbrains/kotlin/fir/declarations/synthetic/FirSyntheticProperty.kt

@@ -23,7 +23,6 @@ import org.jetbrains.kotlin.serialization.deserialization.descriptors.Deserializ
 
 class FirSyntheticProperty(
     override val session: FirSession,
-    override var returnTypeRef: FirTypeRef,
     override val name: Name,
     override val isVar: Boolean,
     override val symbol: FirAccessorSymbol,
@@ -36,6 +35,9 @@ class FirSyntheticProperty(
         symbol.bind(this)
     }
 
+    override val returnTypeRef: FirTypeRef
+        get() = getter.returnTypeRef
+
     override val source: FirSourceElement?
         get() = null
 
@@ -130,7 +132,7 @@ class FirSyntheticProperty(
     }
 
     override fun replaceReturnTypeRef(newReturnTypeRef: FirTypeRef) {
-        returnTypeRef = newReturnTypeRef
+        throw AssertionError("Mutation of synthetic property isn't supported")
     }
 
     override fun replaceReceiverTypeRef(newReceiverTypeRef: FirTypeRef?) {

compiler/fir/tree/src/org/jetbrains/kotlin/fir/declarations/synthetic/FirSyntheticPropertyBuilder.kt

@@ -21,7 +21,7 @@ class FirSyntheticPropertyBuilder {
     var delegateSetter: FirSimpleFunction? = null
 
     fun build(): FirSyntheticProperty = FirSyntheticProperty(
-        session, delegateGetter.returnTypeRef, name, isVar = delegateSetter != null, symbol = symbol,
+        session, name, isVar = delegateSetter != null, symbol = symbol,
         status = FirDeclarationStatusImpl(delegateGetter.visibility, delegateGetter.modality),
         resolvePhase = delegateGetter.resolvePhase,
         getter = FirSyntheticPropertyAccessor(delegateGetter, isGetter = true),