Check if the intersecting types aren't empty during finding the result type for variable fixation

^KT-47941 Fixed

compiler/fir/analysis-tests/tests-gen/org/jetbrains/kotlin/test/runners/FirOldFrontendDiagnosticsTestGenerated.java

@@ -15234,6 +15234,12 @@ public class FirOldFrontendDiagnosticsTestGenerated extends AbstractFirDiagnosti
                     runTest("compiler/testData/diagnostics/tests/inference/regressions/kt44440.kt");
                 }
 
+                @Test
+                @TestMetadata("kt47941.kt")
+                public void testKt47941() throws Exception {
+                    runTest("compiler/testData/diagnostics/tests/inference/regressions/kt47941.kt");
+                }
+
                 @Test
                 @TestMetadata("kt702.kt")
                 public void testKt702() throws Exception {

compiler/fir/analysis-tests/tests-gen/org/jetbrains/kotlin/test/runners/FirOldFrontendDiagnosticsWithLightTreeTestGenerated.java

@@ -15234,6 +15234,12 @@ public class FirOldFrontendDiagnosticsWithLightTreeTestGenerated extends Abstrac
                     runTest("compiler/testData/diagnostics/tests/inference/regressions/kt44440.kt");
                 }
 
+                @Test
+                @TestMetadata("kt47941.kt")
+                public void testKt47941() throws Exception {
+                    runTest("compiler/testData/diagnostics/tests/inference/regressions/kt47941.kt");
+                }
+
                 @Test
                 @TestMetadata("kt702.kt")
                 public void testKt702() throws Exception {

compiler/resolution.common/src/org/jetbrains/kotlin/resolve/calls/inference/components/ResultTypeResolver.kt

@@ -265,7 +265,8 @@ class ResultTypeResolver(
                  * fun <T : String> materialize(): T = null as T
                  * val bar: Int = materialize() // no errors, T is inferred into String & Int
                  */
-                intersectTypes(upperConstraints.filterNot { it.isExpectedTypePosition() }.map { it.type })
+                val filteredUpperConstraints = upperConstraints.filterNot { it.isExpectedTypePosition() }.map { it.type }
+                if (filteredUpperConstraints.isNotEmpty()) intersectTypes(filteredUpperConstraints) else intersectionUpperType
             } else intersectionUpperType
 
             return typeApproximator.approximateToSubType(

compiler/testData/diagnostics/tests/inference/regressions/kt47941.kt

@@ -0,0 +1,29 @@
+// FIR_IDENTICAL
+// !DIAGNOSTICS: -ASSIGNED_BUT_NEVER_ACCESSED_VARIABLE -UNCHECKED_CAST
+// WITH_RUNTIME
+
+sealed interface Metric {}
+
+class Counter: Metric
+class Gauge<T>: Metric
+
+fun <K> foo(y: List<K?>, x: Inv<in K?>, p: (Inv<K?>, K?) -> Unit) {}
+fun <M> materialize(): M = null as M
+
+class Inv<L>(var x: L)
+
+fun <T : Metric?> register(name: String, metric: T): T? {
+    when (metric) {
+        is Counter -> {
+            return metric
+        }
+        is Gauge<*> -> {
+            foo(listOf(), Inv(metric)) { x, y ->
+                var a = y
+                a = materialize()
+            }
+            return metric
+        }
+        else -> return null
+    }
+}

compiler/testData/diagnostics/tests/inference/regressions/kt47941.txt

@@ -0,0 +1,33 @@
+package
+
+public fun </*0*/ K> foo(/*0*/ y: kotlin.collections.List<K?>, /*1*/ x: Inv<in K?>, /*2*/ p: (Inv<K?>, K?) -> kotlin.Unit): kotlin.Unit
+public fun </*0*/ M> materialize(): M
+public fun </*0*/ T : Metric?> register(/*0*/ name: kotlin.String, /*1*/ metric: T): T?
+
+public final class Counter : Metric {
+    public constructor Counter()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public final class Gauge</*0*/ T> : Metric {
+    public constructor Gauge</*0*/ T>()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public final class Inv</*0*/ L> {
+    public constructor Inv</*0*/ L>(/*0*/ x: L)
+    public final var x: L
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public sealed interface Metric {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}

compiler/tests-common-new/tests-gen/org/jetbrains/kotlin/test/runners/DiagnosticTestGenerated.java

@@ -15240,6 +15240,12 @@ public class DiagnosticTestGenerated extends AbstractDiagnosticTest {
                     runTest("compiler/testData/diagnostics/tests/inference/regressions/kt44440.kt");
                 }
 
+                @Test
+                @TestMetadata("kt47941.kt")
+                public void testKt47941() throws Exception {
+                    runTest("compiler/testData/diagnostics/tests/inference/regressions/kt47941.kt");
+                }
+
                 @Test
                 @TestMetadata("kt702.kt")
                 public void testKt702() throws Exception {

idea/idea-frontend-fir/idea-fir-low-level-api/tests/org/jetbrains/kotlin/idea/fir/low/level/api/diagnostic/compiler/based/DiagnosisCompilerTestFE10TestdataTestGenerated.java

@@ -15234,6 +15234,12 @@ public class DiagnosisCompilerTestFE10TestdataTestGenerated extends AbstractDiag
                     runTest("compiler/testData/diagnostics/tests/inference/regressions/kt44440.kt");
                 }
 
+                @Test
+                @TestMetadata("kt47941.kt")
+                public void testKt47941() throws Exception {
+                    runTest("compiler/testData/diagnostics/tests/inference/regressions/kt47941.kt");
+                }
+
                 @Test
                 @TestMetadata("kt702.kt")
                 public void testKt702() throws Exception {