FIR: Support nested light classes

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

@@ -17159,6 +17159,12 @@ public class DiagnosisCompilerTestFE10TestdataTestGenerated extends AbstractDiag
                 runTest("compiler/testData/diagnostics/tests/j+k/InnerClassFromJava.kt");
             }
 
+            @Test
+            @TestMetadata("innerLightClass.kt")
+            public void testInnerLightClass() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/j+k/innerLightClass.kt");
+            }
+
             @Test
             @TestMetadata("innerNestedClassFromJava.kt")
             public void testInnerNestedClassFromJava() throws Exception {

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

@@ -17159,6 +17159,12 @@ public class FirOldFrontendDiagnosticsTestGenerated extends AbstractFirDiagnosti
                 runTest("compiler/testData/diagnostics/tests/j+k/InnerClassFromJava.kt");
             }
 
+            @Test
+            @TestMetadata("innerLightClass.kt")
+            public void testInnerLightClass() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/j+k/innerLightClass.kt");
+            }
+
             @Test
             @TestMetadata("innerNestedClassFromJava.kt")
             public void testInnerNestedClassFromJava() throws Exception {

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

@@ -17159,6 +17159,12 @@ public class FirOldFrontendDiagnosticsWithLightTreeTestGenerated extends Abstrac
                 runTest("compiler/testData/diagnostics/tests/j+k/InnerClassFromJava.kt");
             }
 
+            @Test
+            @TestMetadata("innerLightClass.kt")
+            public void testInnerLightClass() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/j+k/innerLightClass.kt");
+            }
+
             @Test
             @TestMetadata("innerNestedClassFromJava.kt")
             public void testInnerNestedClassFromJava() throws Exception {

compiler/fir/java/src/org/jetbrains/kotlin/fir/java/FirJavaElementFinder.kt

@@ -37,9 +37,7 @@ import org.jetbrains.kotlin.fir.resolve.toSymbol
 import org.jetbrains.kotlin.fir.resolve.transformers.resolveSupertypesInTheAir
 import org.jetbrains.kotlin.fir.symbols.impl.FirClassSymbol
 import org.jetbrains.kotlin.fir.types.*
-import org.jetbrains.kotlin.name.ClassId
+import org.jetbrains.kotlin.name.*
-import org.jetbrains.kotlin.name.FqName
-import org.jetbrains.kotlin.name.StandardClassIds
 import org.jetbrains.kotlin.resolve.jvm.JvmPrimitiveType
 import org.jetbrains.kotlin.resolve.jvm.KotlinFinderMarker
 
@@ -73,13 +71,25 @@ class FirJavaElementFinder(
 
     override fun findClass(qualifiedName: String, scope: GlobalSearchScope): PsiClass? {
         if (qualifiedName.endsWith(".")) return null
-        val classId = ClassId.topLevel(FqName(qualifiedName))
 
-        val firClass = firProviders.firstNotNullOfOrNull { it.getFirClassifierByFqName(classId) as? FirRegularClass } ?: return null
+        val fqName = FqName(qualifiedName)
 
-        val fileStub = createJavaFileStub(classId.packageFqName, psiManager)
+        for (topLevelClass in generateSequence(fqName) { it.parentOrNull() }) {
+            if (topLevelClass.isRoot) break
+            val classId = ClassId.topLevel(topLevelClass)
 
-        return buildStub(firClass, fileStub).psi
+            val firClass = firProviders.firstNotNullOfOrNull { it.getFirClassifierByFqName(classId) as? FirRegularClass } ?: continue
+
+            val fileStub = createJavaFileStub(classId.packageFqName, psiManager)
+            val topLevelResult = buildStub(firClass, fileStub).psi
+            val tail = fqName.tail(topLevelClass).pathSegments()
+
+            return tail.fold(topLevelResult) { psiClass, segment ->
+                psiClass.findInnerClassByName(segment.identifier, false) ?: return null
+            }
+        }
+
+        return null
     }
 
     private fun buildStub(firClass: FirRegularClass, parent: StubElement<*>): PsiClassStub<*> {

compiler/testData/diagnostics/tests/j+k/innerLightClass.kt

@@ -0,0 +1,23 @@
+// SKIP_TXT
+// FIR_IDENTICAL
+// FILE: A.java
+import Outer.Middle;
+
+public class A {
+    public static void foo(Middle x) {}
+    public static void bar(Middle.Innermost x) {}
+    public static void baz(Outer x) {}
+}
+
+// FILE: main.kt
+
+class Outer {
+    class Middle {
+        class Innermost
+        fun main(r: Middle, i: Innermost, o: Outer) {
+            A.foo(r)
+            A.bar(i)
+            A.baz(o)
+        }
+    }
+}

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

@@ -17165,6 +17165,12 @@ public class DiagnosticTestGenerated extends AbstractDiagnosticTest {
                 runTest("compiler/testData/diagnostics/tests/j+k/InnerClassFromJava.kt");
             }
 
+            @Test
+            @TestMetadata("innerLightClass.kt")
+            public void testInnerLightClass() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/j+k/innerLightClass.kt");
+            }
+
             @Test
             @TestMetadata("innerNestedClassFromJava.kt")
             public void testInnerNestedClassFromJava() throws Exception {