[tests] Add test data for KT-43710

compiler/testData/codegen/box/builderInference/issues/kt43710.kt

@@ -0,0 +1,33 @@
+// ISSUE: KT-43710
+
+// IGNORE_LIGHT_ANALYSIS
+// IGNORE_BACKEND: ANY
+// REASON: red code (see corresponding diagnostic test)
+
+fun box(): String {
+    build {
+        setTypeVariable(TargetType())
+        letForTypeVariable { it.extensionProduceConcreteType() }
+    }
+    return "OK"
+}
+
+
+
+
+class ConcreteType
+class TargetType
+class DifferentType
+
+fun TargetType.extensionProduceConcreteType(): ConcreteType = ConcreteType()
+fun DifferentType.extensionProduceConcreteType(): ConcreteType = ConcreteType()
+
+class Buildee<TV> {
+    fun setTypeVariable(value: TV) { storage = value }
+    fun letForTypeVariable(action: (TV) -> ConcreteType): ConcreteType = storage.let(action)
+    private var storage: TV = TargetType() as TV
+}
+
+fun <PTV> build(instructions: Buildee<PTV>.() -> Unit): Buildee<PTV> {
+    return Buildee<PTV>().apply(instructions)
+}

compiler/testData/diagnostics/tests/builderInference/issues/kt43710.fir.kt

@@ -0,0 +1,32 @@
+// ISSUE: KT-43710
+// CHECK_TYPE_WITH_EXACT
+
+fun test() {
+    val buildee = build {
+        setTypeVariable(TargetType())
+        letForTypeVariable { it.<!OVERLOAD_RESOLUTION_AMBIGUITY!>extensionProduceConcreteType<!>() }
+    }
+    // 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 ConcreteType
+class TargetType
+class DifferentType
+
+fun TargetType.extensionProduceConcreteType(): ConcreteType = ConcreteType()
+fun DifferentType.extensionProduceConcreteType(): ConcreteType = ConcreteType()
+
+class Buildee<TV> {
+    fun setTypeVariable(value: TV) { storage = value }
+    fun letForTypeVariable(action: (TV) -> ConcreteType): ConcreteType = storage.let(action)
+    private var storage: TV = null!!
+}
+
+fun <PTV> build(instructions: Buildee<PTV>.() -> Unit): Buildee<PTV> {
+    return Buildee<PTV>().apply(instructions)
+}

compiler/testData/diagnostics/tests/builderInference/issues/kt43710.kt

@@ -0,0 +1,32 @@
+// ISSUE: KT-43710
+// CHECK_TYPE_WITH_EXACT
+
+fun test() {
+    val buildee = build {
+        setTypeVariable(TargetType())
+        letForTypeVariable <!TYPE_MISMATCH("ConcreteType; Unit")!>{ <!STUB_TYPE_IN_RECEIVER_CAUSES_AMBIGUITY!>it<!>.<!DEBUG_INFO_ELEMENT_WITH_ERROR_TYPE, OVERLOAD_RESOLUTION_AMBIGUITY, OVERLOAD_RESOLUTION_AMBIGUITY_BECAUSE_OF_STUB_TYPES!>extensionProduceConcreteType<!>() }<!>
+    }
+    // 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 ConcreteType
+class TargetType
+class DifferentType
+
+fun TargetType.extensionProduceConcreteType(): ConcreteType = ConcreteType()
+fun DifferentType.extensionProduceConcreteType(): ConcreteType = ConcreteType()
+
+class Buildee<TV> {
+    fun setTypeVariable(value: TV) { storage = value }
+    fun letForTypeVariable(action: (TV) -> ConcreteType): ConcreteType = storage.let(action)
+    private var storage: TV = null!!
+}
+
+fun <PTV> build(instructions: Buildee<PTV>.() -> Unit): Buildee<PTV> {
+    return Buildee<PTV>().apply(instructions)
+}