diff --git a/compiler/testData/codegen/box/builderInference/issues/kt55056.kt b/compiler/testData/codegen/box/builderInference/issues/kt55056.kt
new file mode 100644
index 00000000000..5c305986b4c
--- /dev/null
+++ b/compiler/testData/codegen/box/builderInference/issues/kt55056.kt
@@ -0,0 +1,41 @@
+// ISSUE: KT-55056
+
+// IGNORE_LIGHT_ANALYSIS
+// IGNORE_BACKEND_K1: ANY
+// REASON: red code (see corresponding diagnostic test)
+
+// IGNORE_BACKEND_K2: JVM_IR, WASM
+// REASON: run-time failure (java.lang.ClassCastException: TargetType cannot be cast to DifferentType @ Kt55056Kt$box$1.invoke)
+
+fun box(): String {
+    build {
+        setTypeVariable(TargetType())
+        consumeDifferentType(extensionReplaceOutProjectedTypeVariable(DifferentType()))
+    }
+    return "OK"
+}
+
+
+
+
+class TargetType
+class DifferentType
+
+fun consumeDifferentType(value: DifferentType) {}
+
+class Buildee<TV> {
+    fun setTypeVariable(value: TV) { storage = value }
+    fun getTypeVariable(): TV = storage
+    private var storage: TV = TargetType() as TV
+}
+
+fun <ETV> Buildee<out ETV>.extensionReplaceOutProjectedTypeVariable(value: ETV): ETV {
+    this as Buildee<ETV>
+    val temp = getTypeVariable()
+    setTypeVariable(value)
+    return temp
+}
+
+fun <PTV> build(instructions: Buildee<PTV>.() -> Unit): Buildee<PTV> {
+    return Buildee<PTV>().apply(instructions)
+}
diff --git a/compiler/testData/diagnostics/tests/builderInference/issues/kt55056.fir.kt b/compiler/testData/diagnostics/tests/builderInference/issues/kt55056.fir.kt
new file mode 100644
index 00000000000..3fb787e9a06
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/builderInference/issues/kt55056.fir.kt
@@ -0,0 +1,34 @@
+// ISSUE: KT-55056
+// CHECK_TYPE_WITH_EXACT
+
+fun test() {
+    val buildee = build {
+        setTypeVariable(TargetType())
+        consumeDifferentType(extensionReplaceOutProjectedTypeVariable(DifferentType()))
+    }
+    // exact type equality check — turns unexpected compile-time behavior into red code
+    // considered to be non-user-reproducible code for the purposes of these tests
+    checkExactType<Buildee<TargetType>>(buildee)
+}
+
+
+
+
+class TargetType
+class DifferentType
+
+fun consumeDifferentType(value: DifferentType) {}
+
+class Buildee<TV> {
+    fun setTypeVariable(value: TV) { storage = value }
+    fun getTypeVariable(): TV = storage
+    private var storage: TV = null!!
+}
+
+fun <ETV> Buildee<out ETV>.extensionReplaceOutProjectedTypeVariable(value: ETV): ETV {
+    return null!!
+}
+
+fun <PTV> build(instructions: Buildee<PTV>.() -> Unit): Buildee<PTV> {
+    return Buildee<PTV>().apply(instructions)
+}
diff --git a/compiler/testData/diagnostics/tests/builderInference/issues/kt55056.kt b/compiler/testData/diagnostics/tests/builderInference/issues/kt55056.kt
new file mode 100644
index 00000000000..36d4d6a402b
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/builderInference/issues/kt55056.kt
@@ -0,0 +1,34 @@
+// ISSUE: KT-55056
+// CHECK_TYPE_WITH_EXACT
+
+fun test() {
+    val buildee = build {
+        setTypeVariable(TargetType())
+        consumeDifferentType(<!RECEIVER_TYPE_MISMATCH("DifferentType; TargetType")!>extensionReplaceOutProjectedTypeVariable<!>(DifferentType()))
+    }
+    // exact type equality check — turns unexpected compile-time behavior into red code
+    // considered to be non-user-reproducible code for the purposes of these tests
+    checkExactType<Buildee<TargetType>>(buildee)
+}
+
+
+
+
+class TargetType
+class DifferentType
+
+fun consumeDifferentType(value: DifferentType) {}
+
+class Buildee<TV> {
+    fun setTypeVariable(value: TV) { storage = value }
+    fun getTypeVariable(): TV = storage
+    private var storage: TV = null!!
+}
+
+fun <ETV> Buildee<out ETV>.extensionReplaceOutProjectedTypeVariable(value: ETV): ETV {
+    return null!!
+}
+
+fun <PTV> build(instructions: Buildee<PTV>.() -> Unit): Buildee<PTV> {
+    return Buildee<PTV>().apply(instructions)
+}