[investigation] SequencesSpecialSupportDfaTest.testNilReference

(cherry picked from commit 18e19888c8ef616ea354e9cfec3b14c8b2e6e342)

Author: SrTobi

scala/scala-impl/src/org/jetbrains/plugins/scala/lang/dfa/analysis/invocations/specialSupport/SpecialSupportUtils.scala

@@ -42,7 +42,7 @@ object SpecialSupportUtils {
     }
   }
 
-  private def tryRetrieveCollectionSizeDirectly(dfType: DfType): Option[Int] = dfType match {
+  def tryRetrieveCollectionSizeDirectly(dfType: DfType): Option[Int] = dfType match {
     case referenceType: DfReferenceType => referenceType.getSpecialFieldType match {
       case intConstant: DfIntConstantType => Some(intConstant.getValue.intValue)
       case _ => None

scala/scala-impl/src/org/jetbrains/plugins/scala/lang/dfa/controlFlow/ScalaDfaVariableDescriptor.scala

@@ -23,7 +23,9 @@ case class ScalaDfaVariableDescriptor(variable: PsiElement,
     case _ => "<unknown>"
   }
 
-  override def getDfType(qualifier: DfaVariableValue): DfType = variable match {
+  override def getDfType(qualifier: DfaVariableValue): DfType = dfType
+
+  def dfType: DfType = variable match {
     case typeable: Typeable =>
       val scType = typeable.`type`().getOrAny
       val nullability = variable match {

scala/scala-impl/src/org/jetbrains/plugins/scala/lang/dfa/controlFlow/transform/ExpressionTransformation.scala

@@ -11,6 +11,7 @@ import com.intellij.psi.{CommonClassNames, PsiElement, PsiMethod}
 import com.intellij.util.containers.FList
 import org.jetbrains.plugins.scala.extensions._
 import org.jetbrains.plugins.scala.lang.dfa.analysis.framework.ScalaStatementAnchor
+import org.jetbrains.plugins.scala.lang.dfa.analysis.invocations.specialSupport.SpecialSupportUtils.tryRetrieveCollectionSizeDirectly
 import org.jetbrains.plugins.scala.lang.dfa.controlFlow.{ScalaDfaControlFlowBuilder, ScalaDfaVariableDescriptor, TransformationFailedException}
 import org.jetbrains.plugins.scala.lang.dfa.utils.ScalaDfaTypeUtils.{inferExpressionType, literalToDfType}
 import org.jetbrains.plugins.scala.lang.psi.api.base.{ScInterpolatedStringLiteral, ScLiteral}
@@ -194,7 +195,12 @@ trait ExpressionTransformation { this: ScalaDfaControlFlowBuilder =>
       ScalaDfaVariableDescriptor.fromReferenceExpression(expression) match {
         case Some(descriptor) =>
           rreq.result {
-            pushVariable(descriptor, expression)
+            val dfType = descriptor.dfType
+            // A reference like `Nil` already knows its collection size from its type, but the memory
+            // state only tracks sizes of values assigned into it, so pushing it as a variable would
+            // hide the size from collection access assertions. Push its df type directly instead.
+            if (tryRetrieveCollectionSizeDirectly(dfType).isDefined) push(dfType, ScalaStatementAnchor(expression))
+            else pushVariable(descriptor, expression)
             //buildImplicitConversion(Some(expression), Some(expectedType))
           }
         case _ =>

scala/scala-impl/test/org/jetbrains/plugins/scala/lang/dfa/analysis/tests/invocations/SequencesSpecialSupportDfaTest.scala

@@ -3,7 +3,7 @@ package org.jetbrains.plugins.scala.lang.dfa.analysis.tests.invocations
 import org.jetbrains.plugins.scala.lang.dfa.Messages._
 import org.jetbrains.plugins.scala.lang.dfa.analysis.ScalaDfaTestBase
 import org.jetbrains.plugins.scala.lang.dfa.analysis.framework.ScalaCollectionAccessProblem.{indexOutOfBoundsProblem, noSuchElementProblem}
-import org.junit.{Ignore, Test}
+import org.junit.Test
 
 class SequencesSpecialSupportDfaTest extends ScalaDfaTestBase {
 
@@ -80,7 +80,6 @@ class SequencesSpecialSupportDfaTest extends ScalaDfaTestBase {
     "list.head" -> noSuchElementProblem.alwaysMessage
   )
 
-  @Ignore("Failing test")
   @Test
   def testNilReference(): Unit = test(codeFromMethodBody(returnType = "Int") {
     """

scala/scala-impl/test/org/jetbrains/plugins/scala/lang/dfa/controlFlow/transform/tests/ReferenceExpressionsControlFlowTest.scala

@@ -118,12 +118,12 @@ class ReferenceExpressionsControlFlowTest extends ScalaDfaControlFlowBuilderTest
       |""".stripMargin
   }) {
     """
-      |0: PUSH Nil
+      |0: PUSH_VAL NOT_NULL UNMODIFIABLE size=0
       |1: ASSIGN_TO grades
       |2: POP
       |3: PUSH Student
       |4: PUSH_VAL 22
-      |5: PUSH grades
+      |5: PUSH_VAL NOT_NULL UNMODIFIABLE size=0
       |6: CALL Student#apply
       |7: ASSIGN_TO s1
       |8: POP