diff --git a/compiler/testData/diagnostics/tests/typealias/noRHS.kt b/compiler/testData/diagnostics/tests/typealias/noRHS.kt
index 646828f1dbe..0b98d61514c 100644
--- a/compiler/testData/diagnostics/tests/typealias/noRHS.kt
+++ b/compiler/testData/diagnostics/tests/typealias/noRHS.kt
@@ -4,4 +4,5 @@ typealias<!SYNTAX!><!>
 <!SYNTAX!><!>typealias A1<!SYNTAX!><!>
 
 <!SYNTAX!><!>typealias A2 =
-<!SYNTAX!><!>
+
+<!SYNTAX!><!>typealias Valid = String
diff --git a/compiler/tests-common-new/tests/org/jetbrains/kotlin/test/frontend/fir/handlers/FirDiagnosticsHandler.kt b/compiler/tests-common-new/tests/org/jetbrains/kotlin/test/frontend/fir/handlers/FirDiagnosticsHandler.kt
index 199f29e19c6..a3c647b0e33 100644
--- a/compiler/tests-common-new/tests/org/jetbrains/kotlin/test/frontend/fir/handlers/FirDiagnosticsHandler.kt
+++ b/compiler/tests-common-new/tests/org/jetbrains/kotlin/test/frontend/fir/handlers/FirDiagnosticsHandler.kt
@@ -5,11 +5,7 @@
 
 package org.jetbrains.kotlin.test.frontend.fir.handlers
 
-import com.intellij.lang.LighterASTNode
-import com.intellij.openapi.util.Ref
 import com.intellij.psi.PsiElement
-import com.intellij.psi.TokenType
-import com.intellij.util.diff.FlyweightCapableTreeStructure
 import org.jetbrains.kotlin.KtNodeTypes
 import org.jetbrains.kotlin.checkers.diagnostics.factories.DebugInfoDiagnosticFactory1
 import org.jetbrains.kotlin.checkers.utils.TypeOfCall
@@ -33,7 +29,6 @@ import org.jetbrains.kotlin.fir.types.ConeKotlinType
 import org.jetbrains.kotlin.fir.types.FirTypeRef
 import org.jetbrains.kotlin.fir.types.coneTypeSafe
 import org.jetbrains.kotlin.fir.visitors.FirDefaultVisitorVoid
-import org.jetbrains.kotlin.lexer.KtTokens
 import org.jetbrains.kotlin.name.FqNameUnsafe
 import org.jetbrains.kotlin.psi.KtDotQualifiedExpression
 import org.jetbrains.kotlin.resolve.AnalyzingUtils
@@ -118,11 +113,8 @@ class FirDiagnosticsHandler(testServices: TestServices) : FirAnalysisHandler(tes
                 FirErrors.SYNTAX.on(FirRealPsiSourceElement(it)).toMetaInfo(testFile, lightTreeEnabled, lightTreeComparingModeEnabled)
             }
         } else {
-            val source = firFile.source ?: return
-            val errorNodes = LightTreeErrorsCollector(source.treeStructure).collectErrorNodes(source.lighterASTNode)
-            errorNodes.map { node ->
-                FirErrors.SYNTAX.on(node.toFirLightSourceElement(source.treeStructure))
-                    .toMetaInfo(testFile, lightTreeEnabled, lightTreeComparingModeEnabled)
+            collectLightTreeSyntaxErrors(firFile).map { node ->
+                FirErrors.SYNTAX.on(node).toMetaInfo(testFile, lightTreeEnabled, lightTreeComparingModeEnabled)
             }
         }
 
@@ -301,32 +293,3 @@ class PsiLightTreeMetaInfoProcessor(testServices: TestServices) : AbstractTwoAtt
     }
 }
 
-private class LightTreeErrorsCollector(private val tree: FlyweightCapableTreeStructure<LighterASTNode>) {
-
-    private fun LighterASTNode.getChildrenAsArray(): Array<out LighterASTNode?> {
-        val kidsRef = Ref<Array<LighterASTNode?>>()
-        return if (tree.getChildren(this, kidsRef) > 0) kidsRef.get() else emptyArray()
-    }
-
-    private inline fun LighterASTNode.forEachChildren(f: (LighterASTNode) -> Unit) {
-        val kidsArray = this.getChildrenAsArray()
-        for (kid in kidsArray) {
-            if (kid == null) break
-            val tokenType = kid.tokenType
-            if (KtTokens.COMMENTS.contains(tokenType) || tokenType == KtTokens.WHITE_SPACE || tokenType == KtTokens.SEMICOLON) continue
-            f(kid)
-        }
-    }
-
-    fun collectErrorNodes(node: LighterASTNode, acc: MutableList<LighterASTNode> = mutableListOf()): List<LighterASTNode> {
-        if (node.tokenType == TokenType.ERROR_ELEMENT) {
-            acc.add(node)
-        } else {
-            node.forEachChildren { child ->
-                collectErrorNodes(child, acc)
-            }
-        }
-        return acc
-    }
-
-}
diff --git a/compiler/tests-common-new/tests/org/jetbrains/kotlin/test/frontend/fir/handlers/lightTreeDiagnostics.kt b/compiler/tests-common-new/tests/org/jetbrains/kotlin/test/frontend/fir/handlers/lightTreeDiagnostics.kt
new file mode 100644
index 00000000000..49553727119
--- /dev/null
+++ b/compiler/tests-common-new/tests/org/jetbrains/kotlin/test/frontend/fir/handlers/lightTreeDiagnostics.kt
@@ -0,0 +1,77 @@
+/*
+ * Copyright 2010-2021 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.test.frontend.fir.handlers
+
+import com.intellij.lang.LighterASTNode
+import com.intellij.openapi.util.Ref
+import com.intellij.psi.TokenType
+import com.intellij.util.diff.FlyweightCapableTreeStructure
+import org.jetbrains.kotlin.fir.FirElement
+import org.jetbrains.kotlin.fir.FirLightSourceElement
+import org.jetbrains.kotlin.fir.declarations.FirFile
+import org.jetbrains.kotlin.fir.toFirLightSourceElement
+import org.jetbrains.kotlin.fir.visitors.FirVisitor
+import org.jetbrains.kotlin.lexer.KtTokens
+
+private typealias Tree = FlyweightCapableTreeStructure<LighterASTNode>
+
+private class LightTreeErrorsCollector(private val tree: Tree) {
+
+    private fun LighterASTNode.getChildrenAsArray(): Array<out LighterASTNode?> {
+        val kidsRef = Ref<Array<LighterASTNode?>>()
+        return if (tree.getChildren(this, kidsRef) > 0) kidsRef.get() else emptyArray()
+    }
+
+    private inline fun LighterASTNode.forEachChildren(f: (LighterASTNode) -> Unit) {
+        val kidsArray = this.getChildrenAsArray()
+        for (kid in kidsArray) {
+            if (kid == null) break
+            val tokenType = kid.tokenType
+            if (KtTokens.COMMENTS.contains(tokenType) || tokenType == KtTokens.WHITE_SPACE || tokenType == KtTokens.SEMICOLON) continue
+            f(kid)
+        }
+    }
+
+    fun collectErrorNodes(node: LighterASTNode, acc: MutableList<LighterASTNode> = mutableListOf()): List<LighterASTNode> {
+        if (node.tokenType == TokenType.ERROR_ELEMENT) {
+            acc.add(node)
+        } else {
+            node.forEachChildren { child ->
+                collectErrorNodes(child, acc)
+            }
+        }
+        return acc
+    }
+
+}
+
+private data class OffsetTree(val tree: Tree, val offset: Int)
+
+private object FirTreesExtractVisitor : FirVisitor<Unit, Pair<Tree?, MutableList<OffsetTree>>>() {
+    override fun visitElement(element: FirElement, data: Pair<Tree?, MutableList<OffsetTree>>) {
+        val source = element.source ?: return
+        val currentTree = source.treeStructure
+        val (lastTree, trees) = data
+        val newData = if (lastTree !== currentTree) {
+            val newTreeWithOffset = OffsetTree(currentTree, element.source?.startOffset ?: 0)
+            trees.add(newTreeWithOffset)
+            currentTree to trees
+        } else {
+            data
+        }
+        element.acceptChildren(this, newData)
+    }
+}
+
+internal fun collectLightTreeSyntaxErrors(file: FirFile): List<FirLightSourceElement> {
+    val trees = mutableListOf<OffsetTree>()
+    file.accept(FirTreesExtractVisitor, null to trees)
+    return trees.flatMap { (tree, offset) ->
+        LightTreeErrorsCollector(tree).collectErrorNodes(tree.root).map {
+            it.toFirLightSourceElement(tree, startOffset = it.startOffset + offset, endOffset = it.endOffset + offset)
+        }
+    }
+}