diff --git a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/calls/Candidate.kt b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/calls/Candidate.kt
index 3454d1c..9df071d 100644
--- a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/calls/Candidate.kt
+++ b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/calls/Candidate.kt
@@ -74,9 +74,10 @@
     val system: NewConstraintSystemImpl by lazy(LazyThreadSafetyMode.NONE) {
         val system = constraintSystemFactory.createConstraintSystem()
 
-        val outerCs = runUnless(baseSystem.usesOuterCs) { inferenceSession.outerCSForCandidate(this) }
-        if (outerCs != null) {
-            system.addOuterSystem(outerCs)
+        val baseCSFromInferenceSession =
+            runUnless(baseSystem.usesOuterCs) { inferenceSession.baseConstraintStorageForCandidate(this) }
+        if (baseCSFromInferenceSession != null) {
+            system.setBaseSystem(baseCSFromInferenceSession)
             system.addOtherSystem(baseSystem)
         } else {
             system.setBaseSystem(baseSystem)
diff --git a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/inference/FirBuilderInferenceSession2.kt b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/inference/FirBuilderInferenceSession2.kt
index 6e8a1a1..82f9784 100644
--- a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/inference/FirBuilderInferenceSession2.kt
+++ b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/inference/FirBuilderInferenceSession2.kt
@@ -26,9 +26,9 @@
     private val inferenceComponents: InferenceComponents,
 ) : FirInferenceSession() {
 
-    var currentCommonSystem = outerCandidate.system
+    var currentCommonSystem = prepareSharedBaseSystem(outerCandidate.system, inferenceComponents)
+        private set
 
-    private val initialOuterTypeVariables = outerCandidate.system.currentStorage().allTypeVariables.keys
     private val qualifiedAccessesToProcess = mutableSetOf<FirExpression>()
 
     override fun <T> shouldAvoidFullCompletion(call: T): Boolean where T : FirResolvable, T : FirStatement {
@@ -89,6 +89,10 @@
         (resolutionMode as? ResolutionMode.ContextIndependent.ForDeclaration)?.declaration?.let(outerCandidate.updateDeclarations::add)
     }
 
+    fun applyResultsToMainCandidate() {
+        outerCandidate.system.replaceContentWith(currentCommonSystem.currentStorage())
+    }
+
     fun integrateChildSession(
         childCalls: Collection<FirStatement>,
         childStorage: ConstraintStorage,
@@ -170,7 +174,7 @@
     private fun Candidate.isNotTrivial(): Boolean =
         usedOuterCs
 
-    override fun outerCSForCandidate(candidate: Candidate): ConstraintStorage? {
+    override fun baseConstraintStorageForCandidate(candidate: Candidate): ConstraintStorage? {
         if (candidate.needsToBePostponed()) return currentCommonSystem.currentStorage()
         if (candidate.callInfo.arguments.any { it.isLambda() }) return currentCommonSystem.currentStorage()
         // TODO: context receivers
diff --git a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/inference/FirCallCompleter.kt b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/inference/FirCallCompleter.kt
index 5e4525c..e38c534 100644
--- a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/inference/FirCallCompleter.kt
+++ b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/inference/FirCallCompleter.kt
@@ -388,6 +388,8 @@
                         transformer.context.withOuterConstraintStorage(candidate.system.currentStorage()) {
                             lambdaArgument.transformSingle(transformer, ResolutionMode.LambdaResolution(expectedReturnTypeRef))
                         }
+
+                        applyResultsToMainCandidate()
                     }
                 } else {
                     transformer.context.inferenceSession.runLambdaCompletion(candidate) {
diff --git a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/inference/FirDelegatedPropertyInferenceSession.kt b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/inference/FirDelegatedPropertyInferenceSession.kt
index 50cc629..19120f9 100644
--- a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/inference/FirDelegatedPropertyInferenceSession.kt
+++ b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/inference/FirDelegatedPropertyInferenceSession.kt
@@ -42,10 +42,12 @@
     private val nonTrivialParentSession: FirInferenceSession? =
         resolutionContext.bodyResolveContext.inferenceSession.takeIf { it !== DEFAULT }
 
+    private val parentConstraintSystem = ((delegateExpression as? FirResolvable)?.candidate()?.system
+        ?: (resolutionContext.bodyResolveContext.inferenceSession as? FirBuilderInferenceSession2)?.currentCommonSystem)
+        ?: components.session.inferenceComponents.createConstraintSystem()
     private val currentConstraintSystem =
-        (delegateExpression as? FirResolvable)?.candidate()?.system
-            ?: (resolutionContext.bodyResolveContext.inferenceSession as? FirBuilderInferenceSession2)?.currentCommonSystem
-            ?: components.session.inferenceComponents.createConstraintSystem()
+        prepareSharedBaseSystem(parentConstraintSystem, components.session.inferenceComponents)
+
     val currentConstraintStorage: ConstraintStorage get() = currentConstraintSystem.currentStorage()
 
     private val unitType: ConeClassLikeType = components.session.builtinTypes.unitType.type
@@ -100,17 +102,19 @@
     private fun <T> T.isProvideDelegate() where T : FirResolvable, T : FirStatement =
         isAnyOfDelegateOperators() && (this as FirResolvable).candidate()?.callInfo?.name == OperatorNameConventions.PROVIDE_DELEGATE
 
-    override fun outerCSForCandidate(candidate: Candidate): ConstraintStorage? =
+    override fun baseConstraintStorageForCandidate(candidate: Candidate): ConstraintStorage? =
         resolutionContext.bodyResolveContext.outerConstraintStorage.takeIf { it !== ConstraintStorage.Empty }
 
     fun completeSessionOrPostponeIfNonRoot(afterCompletion: (ConeSubstitutor) -> Unit) {
         check(!wasCompletionRun)
         wasCompletionRun = true
 
+        parentConstraintSystem.addOtherSystem(currentConstraintStorage)
+
         (nonTrivialParentSession as? FirBuilderInferenceSession2)?.apply {
             integrateChildSession(
                 partiallyResolvedCalls.map { it.first as FirStatement },
-                currentConstraintStorage,
+                parentConstraintSystem.currentStorage(),
                 afterCompletion,
             )
             return
@@ -118,7 +122,7 @@
 
         val completedCalls = completeCandidatesForRootSession()
 
-        val finalSubstitutor = currentConstraintSystem.asReadOnlyStorage()
+        val finalSubstitutor = parentConstraintSystem.asReadOnlyStorage()
             .buildAbstractResultingSubstitutor(components.session.typeContext) as ConeSubstitutor
 
         val callCompletionResultsWriter = callCompleter.createCompletionResultsWriter(
@@ -134,7 +138,7 @@
     }
 
     private fun completeCandidatesForRootSession(): List<FirResolvable> {
-        val commonSystem = currentConstraintSystem.apply { prepareForGlobalCompletion() }
+        val parentSystem = parentConstraintSystem.apply { prepareForGlobalCompletion() }
 
         val notCompletedCalls =
             buildList {
@@ -149,7 +153,7 @@
         resolutionContext.bodyResolveContext.withInferenceSession(DEFAULT) {
             @Suppress("UNCHECKED_CAST")
             components.callCompleter.completer.complete(
-                commonSystem.asConstraintSystemCompleterContext(),
+                parentSystem.asConstraintSystemCompleterContext(),
                 ConstraintSystemCompletionMode.FULL,
                 notCompletedCalls as List<FirStatement>,
                 unitType, resolutionContext
@@ -165,7 +169,7 @@
                     found
                 }.candidate
                 callCompleter.createPostponedArgumentsAnalyzer(resolutionContext).analyze(
-                    commonSystem,
+                    parentSystem,
                     lambdaAtom,
                     containingCandidateForLambda,
                 )
@@ -173,7 +177,7 @@
         }
 
         for (candidate in notCompletedCalls.mapNotNull { it.candidate() }) {
-            for (error in commonSystem.errors) {
+            for (error in parentSystem.errors) {
                 candidate.addDiagnostic(InferenceError(error))
             }
         }
diff --git a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/inference/FirInferenceSession.kt b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/inference/FirInferenceSession.kt
index 48189a3..5366b82 100644
--- a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/inference/FirInferenceSession.kt
+++ b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/inference/FirInferenceSession.kt
@@ -13,6 +13,7 @@
 import org.jetbrains.kotlin.fir.resolve.calls.Candidate
 import org.jetbrains.kotlin.resolve.calls.inference.components.ConstraintSystemCompletionMode
 import org.jetbrains.kotlin.resolve.calls.inference.model.ConstraintStorage
+import org.jetbrains.kotlin.resolve.calls.inference.model.NewConstraintSystemImpl
 
 abstract class FirInferenceSession {
     companion object {
@@ -31,6 +32,16 @@
                 // Do nothing
             }
         }
+
+        @JvmStatic
+        protected fun prepareSharedBaseSystem(
+            outerSystem: NewConstraintSystemImpl,
+            components: InferenceComponents,
+        ): NewConstraintSystemImpl {
+            return components.createConstraintSystem().apply {
+                addOuterSystem(outerSystem.currentStorage())
+            }
+        }
     }
 
     open fun <T> runLambdaCompletion(candidate: Candidate, block: () -> T): T = block()
@@ -54,5 +65,5 @@
 
     open fun <R> onCandidatesResolution(call: FirFunctionCall, candidatesResolutionCallback: () -> R) = candidatesResolutionCallback()
 
-    open fun outerCSForCandidate(candidate: Candidate): ConstraintStorage? = null
+    open fun baseConstraintStorageForCandidate(candidate: Candidate): ConstraintStorage? = null
 }
diff --git a/compiler/resolution.common/src/org/jetbrains/kotlin/resolve/calls/inference/model/NewConstraintSystemImpl.kt b/compiler/resolution.common/src/org/jetbrains/kotlin/resolve/calls/inference/model/NewConstraintSystemImpl.kt
index 0837d85..7bdff81 100644
--- a/compiler/resolution.common/src/org/jetbrains/kotlin/resolve/calls/inference/model/NewConstraintSystemImpl.kt
+++ b/compiler/resolution.common/src/org/jetbrains/kotlin/resolve/calls/inference/model/NewConstraintSystemImpl.kt
@@ -338,6 +338,8 @@
     }
 
     private fun addOtherSystem(otherSystem: ConstraintStorage, isAddingOuter: Boolean, clearNotFixedTypeVariables: Boolean = false) {
+        runOuterCSRelatedAssertions(otherSystem, isAddingOuter)
+
         if (otherSystem.allTypeVariables.isNotEmpty()) {
             otherSystem.allTypeVariables.forEach {
                 transactionRegisterVariable(it.value)
@@ -369,7 +371,6 @@
         storage.postponedTypeVariables.addAll(otherSystem.postponedTypeVariables)
         storage.constraintsFromAllForkPoints.addAll(otherSystem.constraintsFromAllForkPoints)
 
-        runOuterCSRelatedAssertions(otherSystem, isAddingOuter)
     }
 
 
@@ -383,9 +384,9 @@
         require(storage.usesOuterCs)
 
         if (!isAddingOuter) {
-//            require(storage.outerSystemVariablesPrefixSize == otherSystem.outerSystemVariablesPrefixSize) {
-//                "Expected to be ${otherSystem.outerSystemVariablesPrefixSize}, but ${storage.outerSystemVariablesPrefixSize} found"
-//            }
+            require(storage.outerSystemVariablesPrefixSize == otherSystem.outerSystemVariablesPrefixSize) {
+                "Expected to be ${otherSystem.outerSystemVariablesPrefixSize}, but ${storage.outerSystemVariablesPrefixSize} found"
+            }
         }
     }
 
diff --git a/compiler/testData/diagnostics/tests/builderInference/issues/kt63841.fir.kt b/compiler/testData/diagnostics/tests/builderInference/issues/kt63841.fir.kt
index 063db80..90a3e34 100644
--- a/compiler/testData/diagnostics/tests/builderInference/issues/kt63841.fir.kt
+++ b/compiler/testData/diagnostics/tests/builderInference/issues/kt63841.fir.kt
@@ -28,7 +28,7 @@
         variable = TargetType()
         <!BUILDER_INFERENCE_STUB_RECEIVER!>variable<!>.targetTypeMemberFunction()
         variable = <!ASSIGNMENT_TYPE_MISMATCH!>DifferentType()<!>
-        <!BUILDER_INFERENCE_STUB_RECEIVER!>variable<!>.targetTypeMemberFunction()
+        variable.<!UNRESOLVED_REFERENCE!>targetTypeMemberFunction<!>()
         variable = TargetType()
         <!BUILDER_INFERENCE_STUB_RECEIVER!>variable<!>.targetTypeMemberFunction()
     }
diff --git a/compiler/testData/diagnostics/tests/inference/builderInference/nestedLambdasWithOverloadResolutionByReturnType.kt b/compiler/testData/diagnostics/tests/inference/builderInference/nestedLambdasWithOverloadResolutionByReturnType.kt
new file mode 100644
index 0000000..22e090e
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/inference/builderInference/nestedLambdasWithOverloadResolutionByReturnType.kt
@@ -0,0 +1,27 @@
+// FIR_IDENTICAL
+// WITH_STDLIB
+
+class Controller<T> {
+    fun yield(t: T): Boolean = true
+}
+
+fun <S> generate(g: suspend Controller<S>.() -> Unit): S = TODO()
+
+fun foo(x1: List<String>, x2: List<String>) {
+    generate {
+        yield(
+            x1.flatMap1 {
+                x2.map2 { "" }
+            }
+        )
+    }.length
+}
+
+fun <T1> Iterable<T1>.flatMap1(transform: (T1) -> String): String = TODO()
+
+@OptIn(kotlin.experimental.ExperimentalTypeInference::class)
+@OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName("flatMapSequence")
+fun <T2> Iterable<T2>.flatMap1(transform: (T2) -> Int): Int = TODO()
+
+fun <T3, R3> T3.map2(transform: (T3) -> R3): R3 = TODO()
\ No newline at end of file
diff --git a/compiler/testData/diagnostics/tests/inference/builderInference/stubTypes/memberScope.fir.kt b/compiler/testData/diagnostics/tests/inference/builderInference/stubTypes/memberScope.fir.kt
index ddcd09e..e3751ff 100644
--- a/compiler/testData/diagnostics/tests/inference/builderInference/stubTypes/memberScope.fir.kt
+++ b/compiler/testData/diagnostics/tests/inference/builderInference/stubTypes/memberScope.fir.kt
@@ -39,7 +39,7 @@
         val x = get()
         <!BUILDER_INFERENCE_STUB_RECEIVER!>x<!>?.hashCode()
         x?.<!NONE_APPLICABLE!>equals<!>(1)
-        x<!UNSAFE_CALL!>.<!>equals("")
+        x.<!NONE_APPLICABLE!>equals<!>("")
     }
     val ret3 = build {
         emit(1)
@@ -59,12 +59,12 @@
         if (x != null) {
             <!BUILDER_INFERENCE_STUB_RECEIVER!>x<!><!UNNECESSARY_SAFE_CALL!>?.<!>hashCode()
             x<!UNNECESSARY_SAFE_CALL!>?.<!><!NONE_APPLICABLE!>equals<!>(1)
-            x.<!UNRESOLVED_REFERENCE!>equals<!>("")
-            x.hashCode()
-            x.toString()
-            x.test()
+            x.<!NONE_APPLICABLE!>equals<!>("")
+            <!BUILDER_INFERENCE_STUB_RECEIVER!>x<!>.hashCode()
+            <!BUILDER_INFERENCE_STUB_RECEIVER!>x<!>.toString()
+            <!BUILDER_INFERENCE_STUB_RECEIVER!>x<!>.test()
             <!BUILDER_INFERENCE_STUB_RECEIVER!>x<!><!UNNECESSARY_SAFE_CALL!>?.<!>test2()
-            x.test2()
+            <!BUILDER_INFERENCE_STUB_RECEIVER!>x<!>.test2()
         }
 
         ""
diff --git a/compiler/testData/diagnostics/tests/resolve/varInsideLambdaThatPassedToExtensionFunctionWithTypeParametersThatDefinedInsideThisLambda.fir.kt b/compiler/testData/diagnostics/tests/resolve/varInsideLambdaThatPassedToExtensionFunctionWithTypeParametersThatDefinedInsideThisLambda.fir.kt
index db81fc4..61acbba 100644
--- a/compiler/testData/diagnostics/tests/resolve/varInsideLambdaThatPassedToExtensionFunctionWithTypeParametersThatDefinedInsideThisLambda.fir.kt
+++ b/compiler/testData/diagnostics/tests/resolve/varInsideLambdaThatPassedToExtensionFunctionWithTypeParametersThatDefinedInsideThisLambda.fir.kt
@@ -12,6 +12,6 @@
             "Error: $x"
             x.<!UNRESOLVED_REFERENCE!>length<!>
         }
-        x<!UNSAFE_CALL!>.<!>length
+        x.<!UNRESOLVED_REFERENCE!>length<!>
     }
 }