[FIR] Support for EXPOSED_TYPEALIAS_EXPANDED_TYPE

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

@@ -14,7 +14,8 @@ object DeclarationCheckers {
         FirModifierChecker
     )
     val MEMBER_DECLARATIONS: List<FirDeclarationChecker<FirMemberDeclaration>> = DECLARATIONS + listOf(
-        FirInfixFunctionDeclarationChecker
+        FirInfixFunctionDeclarationChecker,
+        FirExposedVisibilityChecker
     )
     val CONSTRUCTORS: List<FirDeclarationChecker<FirConstructor>> = MEMBER_DECLARATIONS + listOf(
         FirConstructorChecker

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

@@ -0,0 +1,54 @@
+/*
+ * Copyright 2010-2020 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.checkers.declaration
+
+import com.intellij.psi.PsiElement
+import org.jetbrains.kotlin.diagnostics.DiagnosticFactory2
+import org.jetbrains.kotlin.diagnostics.DiagnosticFactory3
+import org.jetbrains.kotlin.diagnostics.Errors
+import org.jetbrains.kotlin.fir.*
+import org.jetbrains.kotlin.fir.analysis.checkers.context.CheckerContext
+import org.jetbrains.kotlin.fir.analysis.diagnostics.DiagnosticReporter
+import org.jetbrains.kotlin.fir.analysis.diagnostics.onSource
+import org.jetbrains.kotlin.fir.declarations.*
+import org.jetbrains.kotlin.fir.resolve.toSymbol
+import org.jetbrains.kotlin.fir.resolve.transformers.firClassLike
+import org.jetbrains.kotlin.fir.types.ConeKotlinType
+import org.jetbrains.kotlin.fir.types.coneTypeSafe
+
+object FirExposedVisibilityChecker : FirDeclarationChecker<FirMemberDeclaration>() {
+    override fun check(declaration: FirMemberDeclaration, context: CheckerContext, reporter: DiagnosticReporter) {
+        if (declaration is FirTypeAlias) {
+            checkTypeAlias(declaration, context, reporter)
+        }
+    }
+
+    private fun checkTypeAlias(declaration: FirTypeAlias, context: CheckerContext, reporter: DiagnosticReporter) {
+        val expandedType = declaration.expandedConeType
+
+        val restricting = expandedType?.leastPermissiveDescriptor(declaration.session, declaration.effectiveVisibility)
+        if (restricting != null) {
+            reporter.reportExposure(Error.EXPOSED_TYPEALIAS_EXPANDED_TYPE, declaration.source)
+        }
+    }
+
+    private enum class Error {
+        EXPOSED_TYPEALIAS_EXPANDED_TYPE
+    }
+
+    private fun DiagnosticReporter.reportExposure(
+        error: Error,
+        source: FirSourceElement?
+    ) {
+        when (error) {
+            Error.EXPOSED_TYPEALIAS_EXPANDED_TYPE -> {
+                source?.let {
+                    report(Errors.FIR_EXPOSED_TYPEALIAS_EXPANDED_TYPE.onSource(it))
+                }
+            }
+        }
+    }
+}