FIR. Fix smart-cast expression unwrapping in plus with assign resolve

analysis/low-level-api-fir/tests/org/jetbrains/kotlin/analysis/low/level/api/fir/diagnostic/compiler/based/DiagnosisCompilerFirTestdataTestGenerated.java

@@ -2117,6 +2117,12 @@ public class DiagnosisCompilerFirTestdataTestGenerated extends AbstractDiagnosis
                 runTest("compiler/fir/analysis-tests/testData/resolve/expresssions/overriddenJavaGetter.kt");
             }
 
+            @Test
+            @TestMetadata("plusWithAssignSmartCast.kt")
+            public void testPlusWithAssignSmartCast() throws Exception {
+                runTest("compiler/fir/analysis-tests/testData/resolve/expresssions/plusWithAssignSmartCast.kt");
+            }
+
             @Test
             @TestMetadata("privateObjectLiteral.kt")
             public void testPrivateObjectLiteral() throws Exception {

compiler/fir/analysis-tests/legacy-fir-tests/tests-gen/org/jetbrains/kotlin/fir/LazyBodyIsNotTouchedTilContractsPhaseTestGenerated.java

@@ -1846,6 +1846,11 @@ public class LazyBodyIsNotTouchedTilContractsPhaseTestGenerated extends Abstract
             runTest("compiler/fir/analysis-tests/testData/resolve/expresssions/overriddenJavaGetter.kt");
         }
 
+        @TestMetadata("plusWithAssignSmartCast.kt")
+        public void testPlusWithAssignSmartCast() throws Exception {
+            runTest("compiler/fir/analysis-tests/testData/resolve/expresssions/plusWithAssignSmartCast.kt");
+        }
+
         @TestMetadata("privateObjectLiteral.kt")
         public void testPrivateObjectLiteral() throws Exception {
             runTest("compiler/fir/analysis-tests/testData/resolve/expresssions/privateObjectLiteral.kt");

compiler/fir/analysis-tests/testData/resolve/expresssions/plusWithAssignSmartCast.fir.txt

@@ -0,0 +1,23 @@
+FILE: plusWithAssignSmartCast.kt
+    public final class Foo : R|kotlin/Any| {
+        public constructor(): R|Foo| {
+            super<R|kotlin/Any|>()
+        }
+
+        public final var bar: R|kotlin/String?| = Null(null)
+            public get(): R|kotlin/String?|
+            public set(value: R|kotlin/String?|): R|kotlin/Unit|
+
+        public final fun addToBar(other: R|kotlin/String|): R|kotlin/Unit| {
+            when () {
+                ==(this@R|/Foo|.R|/Foo.bar|, Null(null)) ->  {
+                    this@R|/Foo|.R|/Foo.bar| = R|<local>/other|
+                }
+                else ->  {
+                    this@R|/Foo|.R|/Foo.bar| = this@R|/Foo|.R|/Foo.bar|.R|kotlin/plus|(<strcat>(String( ), R|<local>/other|))
+                }
+            }
+
+        }
+
+    }

compiler/fir/analysis-tests/testData/resolve/expresssions/plusWithAssignSmartCast.kt

@@ -0,0 +1,11 @@
+class Foo {
+    var bar: String? = null
+
+    fun addToBar(other: String) {
+        if (bar == null) {
+            bar = other
+        } else {
+            bar += " $other"
+        }
+    }
+}

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

@@ -2117,6 +2117,12 @@ public class FirDiagnosticTestGenerated extends AbstractFirDiagnosticTest {
                 runTest("compiler/fir/analysis-tests/testData/resolve/expresssions/overriddenJavaGetter.kt");
             }
 
+            @Test
+            @TestMetadata("plusWithAssignSmartCast.kt")
+            public void testPlusWithAssignSmartCast() throws Exception {
+                runTest("compiler/fir/analysis-tests/testData/resolve/expresssions/plusWithAssignSmartCast.kt");
+            }
+
             @Test
             @TestMetadata("privateObjectLiteral.kt")
             public void testPrivateObjectLiteral() throws Exception {

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

@@ -2117,6 +2117,12 @@ public class FirDiagnosticsWithLightTreeTestGenerated extends AbstractFirDiagnos
                 runTest("compiler/fir/analysis-tests/testData/resolve/expresssions/overriddenJavaGetter.kt");
             }
 
+            @Test
+            @TestMetadata("plusWithAssignSmartCast.kt")
+            public void testPlusWithAssignSmartCast() throws Exception {
+                runTest("compiler/fir/analysis-tests/testData/resolve/expresssions/plusWithAssignSmartCast.kt");
+            }
+
             @Test
             @TestMetadata("privateObjectLiteral.kt")
             public void testPrivateObjectLiteral() throws Exception {

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

@@ -605,7 +605,7 @@ open class FirExpressionsResolveTransformer(transformer: FirBodyResolveTransform
                             }
                         }
                     }
-                    (leftArgument as? FirQualifiedAccess)?.let {
+                    (leftArgument.unwrapSmartcastExpression() as? FirQualifiedAccess)?.let {
                         dispatchReceiver = it.dispatchReceiver
                         extensionReceiver = it.extensionReceiver
                         contextReceiverArguments.addAll(it.contextReceiverArguments)