FIR Preserve nullability of ConeErrorType when possible

compiler/fir/cones/src/org/jetbrains/kotlin/fir/diagnostics/ConeDiagnostic.kt

@@ -9,6 +9,10 @@ interface ConeDiagnostic {
     val reason: String
 }
 
+interface ConeDiagnosticWithNullability : ConeDiagnostic {
+    val isNullable: Boolean
+}
+
 class ConeStubDiagnostic(val original: ConeDiagnostic) : ConeDiagnostic {
     override val reason: String get() = original.reason
 }

compiler/fir/cones/src/org/jetbrains/kotlin/fir/types/ConeTypes.kt

@@ -6,6 +6,7 @@
 package org.jetbrains.kotlin.fir.types
 
 import org.jetbrains.kotlin.fir.diagnostics.ConeDiagnostic
+import org.jetbrains.kotlin.fir.diagnostics.ConeDiagnosticWithNullability
 import org.jetbrains.kotlin.fir.symbols.ConeClassLikeLookupTag
 import org.jetbrains.kotlin.fir.symbols.ConeClassifierLookupTag
 import org.jetbrains.kotlin.fir.types.impl.ConeClassLikeTypeImpl
@@ -50,7 +51,9 @@ class ConeErrorType(
         get() = ConeClassLikeErrorLookupTag(ClassId.fromString("<error>"))
 
     override val nullability: ConeNullability
-        get() = ConeNullability.UNKNOWN
+        get() = if (diagnostic is ConeDiagnosticWithNullability) {
+            if (diagnostic.isNullable) ConeNullability.NULLABLE else ConeNullability.NOT_NULL
+        } else ConeNullability.UNKNOWN
 
     override fun equals(other: Any?) = this === other
     override fun hashCode(): Int = System.identityHashCode(this)

compiler/fir/semantics/src/org/jetbrains/kotlin/fir/resolve/diagnostics/ConeDiagnostics.kt

@@ -11,6 +11,7 @@ import org.jetbrains.kotlin.fir.FirElement
 import org.jetbrains.kotlin.fir.contracts.description.ConeContractDescriptionElement
 import org.jetbrains.kotlin.fir.declarations.FirVariable
 import org.jetbrains.kotlin.fir.diagnostics.ConeDiagnostic
+import org.jetbrains.kotlin.fir.diagnostics.ConeDiagnosticWithNullability
 import org.jetbrains.kotlin.fir.diagnostics.ConeDiagnosticWithSource
 import org.jetbrains.kotlin.fir.expressions.FirConstExpression
 import org.jetbrains.kotlin.fir.expressions.FirOperation
@@ -58,7 +59,8 @@ class ConeUnresolvedSymbolError(val classId: ClassId) : ConeUnresolvedError {
     override val reason: String get() = "Symbol not found for $classId"
 }
 
-class ConeUnresolvedTypeQualifierError(val qualifiers: List<FirQualifierPart>, val isNullable: Boolean) : ConeUnresolvedError {
+class ConeUnresolvedTypeQualifierError(val qualifiers: List<FirQualifierPart>, override val isNullable: Boolean)
+    : ConeUnresolvedError, ConeDiagnosticWithNullability {
     override val qualifier: String get() = qualifiers.joinToString(separator = ".") { it.name.asString() }
     override val reason: String get() = "Symbol not found for $qualifier${if (isNullable) "?" else ""}"
 }