Support FirDiagnostic.isValid properly

Diagnostic is considered valid in this commit if it's reported on
a syntactically non-erroneous element without erroneous last child

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/FirDiagnostic.kt

@@ -25,7 +25,7 @@ sealed class FirDiagnostic<out E : FirSourceElement> : Diagnostic {
         get() = factory.getTextRanges(this)
 
     override val isValid: Boolean
-        get() = true
+        get() = factory.isValid(this)
 }
 
 sealed class FirSimpleDiagnostic<out E : FirSourceElement> : FirDiagnostic<E>() {

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/FirDiagnosticFactory.kt

@@ -29,6 +29,11 @@ sealed class AbstractFirDiagnosticFactory<out E : FirSourceElement, D : FirDiagn
 
     fun getTextRanges(diagnostic: FirDiagnostic<*>): List<TextRange> =
         positioningStrategy.markDiagnostic(diagnostic)
+
+    fun isValid(diagnostic: FirDiagnostic<*>): Boolean {
+        val element = diagnostic.element
+        return positioningStrategy.isValid(element.lighterASTNode, element.treeStructure)
+    }
 }
 
 class FirDiagnosticFactory0<E : FirSourceElement, P : PsiElement>(

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/LightTreePositioningStrategy.kt

@@ -6,9 +6,13 @@
 package org.jetbrains.kotlin.fir.analysis.diagnostics
 
 import com.intellij.lang.LighterASTNode
+import com.intellij.openapi.util.Ref
 import com.intellij.openapi.util.TextRange
+import com.intellij.psi.TokenType
 import com.intellij.util.diff.FlyweightCapableTreeStructure
-import org.jetbrains.kotlin.KtNodeTypes
+import org.jetbrains.kotlin.lexer.KtSingleValueToken
+import org.jetbrains.kotlin.lexer.KtTokens
+import org.jetbrains.kotlin.lexer.KtTokens.WHITE_SPACE
 
 open class LightTreePositioningStrategy {
     open fun markDiagnostic(diagnostic: FirDiagnostic<*>): List<TextRange> {
@@ -20,6 +24,10 @@ open class LightTreePositioningStrategy {
         return markElement(node, tree)
     }
 
+    open fun isValid(node: LighterASTNode, tree: FlyweightCapableTreeStructure<LighterASTNode>): Boolean {
+        return !hasSyntaxErrors(node, tree)
+    }
+
     companion object {
         val DEFAULT = LightTreePositioningStrategies.DEFAULT
     }
@@ -32,3 +40,21 @@ fun markElement(node: LighterASTNode, tree: FlyweightCapableTreeStructure<Lighte
 fun markRange(from: LighterASTNode, to: LighterASTNode, tree: FlyweightCapableTreeStructure<LighterASTNode>): List<TextRange> {
     return listOf(TextRange(tree.getStartOffset(from), tree.getEndOffset(to)))
 }
+
+private val DOC_AND_COMMENT_TOKENS = setOf(
+    WHITE_SPACE, KtTokens.IDENTIFIER,
+    KtTokens.EOL_COMMENT, KtTokens.BLOCK_COMMENT, KtTokens.SHEBANG_COMMENT, KtTokens.DOC_COMMENT
+)
+
+private fun hasSyntaxErrors(node: LighterASTNode, tree: FlyweightCapableTreeStructure<LighterASTNode>): Boolean {
+    if (node.tokenType == TokenType.ERROR_ELEMENT) return true
+
+    val childrenRef = Ref<Array<LighterASTNode>>()
+    tree.getChildren(node, childrenRef)
+    val children = childrenRef.get()
+    return children.lastOrNull {
+        val tokenType = it.tokenType
+        tokenType !is KtSingleValueToken && tokenType !in DOC_AND_COMMENT_TOKENS
+    }?.let { hasSyntaxErrors(it, tree) } == true
+}
+