FIR: introduce Java vs Kotlin class conflicts checker #KT-44441 Fixed

compiler/fir/checkers/checkers.jvm/src/org/jetbrains/kotlin/fir/analysis/jvm/checkers/JvmDeclarationCheckers.kt

@@ -28,7 +28,8 @@ object JvmDeclarationCheckers : DeclarationCheckers() {
     override val regularClassCheckers: Set<FirRegularClassChecker>
         get() = setOf(
             FirJvmRecordChecker,
-            FirJvmInlineApplicabilityChecker
+            FirJvmInlineApplicabilityChecker,
+            FirJvmConflictsChecker,
         )
 
     override val propertyCheckers: Set<FirPropertyChecker>

compiler/fir/checkers/checkers.jvm/src/org/jetbrains/kotlin/fir/analysis/jvm/checkers/declaration/FirJvmConflictsChecker.kt

@@ -0,0 +1,24 @@
+/*
+ * Copyright 2010-2022 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.fir.analysis.jvm.checkers.declaration
+
+import org.jetbrains.kotlin.diagnostics.DiagnosticReporter
+import org.jetbrains.kotlin.diagnostics.reportOn
+import org.jetbrains.kotlin.fir.analysis.checkers.context.CheckerContext
+import org.jetbrains.kotlin.fir.analysis.checkers.declaration.FirRegularClassChecker
+import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors
+import org.jetbrains.kotlin.fir.declarations.FirRegularClass
+import org.jetbrains.kotlin.fir.declarations.utils.classId
+import org.jetbrains.kotlin.fir.java.javaSymbolProvider
+
+object FirJvmConflictsChecker : FirRegularClassChecker() {
+    override fun check(declaration: FirRegularClass, context: CheckerContext, reporter: DiagnosticReporter) {
+        val javaSymbol = context.session.javaSymbolProvider.getClassLikeSymbolByClassId(declaration.classId) ?: return
+        reporter.reportOn(
+            declaration.source, FirErrors.PACKAGE_OR_CLASSIFIER_REDECLARATION, listOf(declaration.symbol, javaSymbol), context
+        )
+    }
+}