[LC] FakeFileForLightClass: align modification stamp with underlying file

Even though light classes don't support modification, it is still
important to provide realistic modification timestamps.
In this particular case, the previous implementation used a virtual
file of the original file instead of the file itself to provide
the result

^KT-82546 Fixed

Author: dimonchik0036

analysis/light-classes-base/src/org/jetbrains/kotlin/asJava/elements/FakeFileForLightClass.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2010-2024 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Copyright 2010-2025 JetBrains s.r.o. and Kotlin Programming Language contributors.
  * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
  */
 
@@ -45,6 +45,8 @@ open class FakeFileForLightClass(
         return javaFileStub
     }
 
+    override fun getModificationStamp(): Long = ktFile.modificationStamp
+
     override fun getStub() = createFakeJavaFileStub()
 
     override fun getClasses() = arrayOf(lightClass)

analysis/symbol-light-classes/build.gradle.kts

@@ -33,7 +33,10 @@ dependencies {
 
 sourceSets {
     "main" { projectDefault() }
-    "test" { generatedTestDir() }
+    "test" {
+        projectDefault()
+        generatedTestDir()
+    }
     "testFixtures" { projectDefault() }
 }
 

analysis/symbol-light-classes/testData/custom/fileModificationTracker.kt

@@ -0,0 +1,9 @@
+package foo
+
+// A class to have a light class class
+class SimpleClass
+
+// A function to have a facade light class
+fun simpleFunction() {
+
+}

analysis/symbol-light-classes/tests/org.jetbrains.kotlin.light.classes.symbol/SymbolLightClassesCustomTest.kt

@@ -0,0 +1,41 @@
+/*
+ * Copyright 2010-2025 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.light.classes.symbol
+
+import org.jetbrains.kotlin.analysis.low.level.api.fir.test.configurators.AnalysisApiFirSourceTestConfigurator
+import org.jetbrains.kotlin.analysis.test.framework.base.AbstractAnalysisApiExecutionTest
+import org.jetbrains.kotlin.asJava.findFacadeClass
+import org.jetbrains.kotlin.asJava.toLightClass
+import org.jetbrains.kotlin.psi.KtClassOrObject
+import org.jetbrains.kotlin.psi.KtFile
+import org.jetbrains.kotlin.test.services.TestServices
+import org.jetbrains.kotlin.test.services.assertions
+import org.junit.jupiter.api.Test
+
+class SymbolLightClassesCustomTest : AbstractAnalysisApiExecutionTest(
+    testDirPathString = "analysis/symbol-light-classes/testData/custom",
+) {
+    override val configurator = AnalysisApiFirSourceTestConfigurator(analyseInDependentSession = false)
+
+    @Test
+    fun fileModificationTracker(file: KtFile, testServices: TestServices) {
+        val facadeLightClass = file.findFacadeClass() ?: error("Facade light class was not found")
+        val classLightClass = (file.declarations.first() as KtClassOrObject).toLightClass() ?: error("Light class was not found")
+        val fakeFilesWithModificationStamp = listOf(facadeLightClass, classLightClass).map { lightClass ->
+            lightClass.containingFile to lightClass.containingFile.modificationStamp
+        }
+
+        // Emulate file modification
+        file.clearCaches()
+
+        for ((fakeFile, originalStamp) in fakeFilesWithModificationStamp) {
+            val newStamp = fakeFile.modificationStamp
+            testServices.assertions.assertTrue(originalStamp < newStamp) {
+                "Expected that $fakeFile will have a modification stamp greater than $originalStamp, but $newStamp was found"
+            }
+        }
+    }
+}