LUCENE-8142: Fix AssertingImpactsEnum and add missing javadoc.

lucene/core/src/java/org/apache/lucene/index/Impacts.java

@@ -23,6 +23,9 @@ import java.util.List;
  */
 public abstract class Impacts {
 
+  /** Sole constructor. Typically invoked by sub classes. */
+  protected Impacts() {}
+
   /**
    * Return the number of levels on which we have impacts.
    * The returned value is always greater than 0 and may not always be the

lucene/core/src/java/org/apache/lucene/search/DisjunctionMaxScorer.java

@@ -53,6 +53,7 @@ final class DisjunctionMaxScorer extends DisjunctionScorer {
     float scoreMax = 0;
     double otherScoreSum = 0;
     for (Scorer scorer : subScorers) {
+      scorer.advanceShallow(0);
       float subScore = scorer.getMaxScore(DocIdSetIterator.NO_MORE_DOCS);
       if (subScore >= scoreMax) {
         otherScoreSum += scoreMax;

lucene/core/src/java/org/apache/lucene/search/DisjunctionSumScorer.java

@@ -36,6 +36,7 @@ final class DisjunctionSumScorer extends DisjunctionScorer {
     super(weight, subScorers, needsScores);
     double maxScore = 0;
     for (Scorer scorer : subScorers) {
+      scorer.advanceShallow(0);
       maxScore += scorer.getMaxScore(DocIdSetIterator.NO_MORE_DOCS);
     }
     // The error of sums depends on the order in which values are summed up. In

lucene/core/src/java/org/apache/lucene/search/ReqExclScorer.java

@@ -76,6 +76,11 @@ class ReqExclScorer extends Scorer {
     return reqScorer.score(); // reqScorer may be null when next() or skipTo() already return false
   }
 
+  @Override
+  public int advanceShallow(int target) throws IOException {
+    return reqScorer.advanceShallow(target);
+  }
+
   @Override
   public float getMaxScore(int upTo) throws IOException {
     return reqScorer.getMaxScore(upTo);

lucene/core/src/java/org/apache/lucene/search/ReqOptSumScorer.java

@@ -52,6 +52,7 @@ class ReqOptSumScorer extends Scorer {
     this.reqScorer = reqScorer;
     this.optScorer = optScorer;
 
+    reqScorer.advanceShallow(0);
     this.reqMaxScore = reqScorer.getMaxScore(DocIdSetIterator.NO_MORE_DOCS);
     this.maxScorePropagator = new MaxScoreSumPropagator(Arrays.asList(reqScorer, optScorer));
 
@@ -209,6 +210,12 @@ class ReqOptSumScorer extends Scorer {
     return score;
   }
 
+  @Override
+  public int advanceShallow(int target) throws IOException {
+    optScorer.advanceShallow(target);
+    return reqScorer.advanceShallow(target);
+  }
+
   @Override
   public float getMaxScore(int upTo) throws IOException {
     float maxScore = reqScorer.getMaxScore(upTo);

lucene/test-framework/src/java/org/apache/lucene/search/AssertingScorer.java

@@ -40,7 +40,7 @@ public class AssertingScorer extends Scorer {
   IteratorState state = IteratorState.ITERATING;
   int doc;
   float minCompetitiveScore = 0;
-  int lastShallowTarget;
+  int lastShallowTarget = -1;
 
   private AssertingScorer(Random random, Scorer in, ScoreMode scoreMode) {
     super(in.weight);