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The Checker Framework enhances Java's type system to make it more powerful and useful. This lets software developers detect and prevent errors in their Java programs. The Checker Framework includes compiler plug-ins ("checkers") that find bugs or verify their absence. It also permits you to write your own compiler plug-ins.

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package org.checkerframework.checker.index.samelen;

import com.sun.source.tree.ExpressionTree;
import com.sun.source.tree.Tree;
import java.util.Collection;
import java.util.Collections;
import java.util.TreeSet;
import javax.lang.model.element.AnnotationMirror;
import org.checkerframework.checker.compilermsgs.qual.CompilerMessageKey;
import org.checkerframework.checker.index.IndexUtil;
import org.checkerframework.checker.index.qual.PolySameLen;
import org.checkerframework.checker.index.qual.SameLen;
import org.checkerframework.common.basetype.BaseTypeChecker;
import org.checkerframework.common.basetype.BaseTypeVisitor;
import org.checkerframework.dataflow.expression.JavaExpression;
import org.checkerframework.framework.type.AnnotatedTypeMirror;
import org.checkerframework.javacutil.AnnotationUtils;
import org.checkerframework.javacutil.TreeUtils;

public class SameLenVisitor extends BaseTypeVisitor {
  public SameLenVisitor(BaseTypeChecker checker) {
    super(checker);
  }

  /**
   * Merges SameLen annotations, then calls super.
   *
   * {@inheritDoc}
   */
  @Override
  protected void commonAssignmentCheck(
      AnnotatedTypeMirror varType,
      AnnotatedTypeMirror valueType,
      Tree valueTree,
      @CompilerMessageKey String errorKey,
      Object... extraArgs) {
    if (IndexUtil.isSequenceType(valueType.getUnderlyingType())
        && TreeUtils.isExpressionTree(valueTree)
        // if both annotations are @PolySameLen, there is nothing to do
        && !(valueType.hasAnnotation(PolySameLen.class)
            && varType.hasAnnotation(PolySameLen.class))) {

      JavaExpression rhs = JavaExpression.fromTree((ExpressionTree) valueTree);
      if (rhs != null && SameLenAnnotatedTypeFactory.mayAppearInSameLen(rhs)) {
        String rhsExpr = rhs.toString();
        AnnotationMirror sameLenAnno = valueType.getAnnotation(SameLen.class);
        Collection exprs;
        if (sameLenAnno == null) {
          exprs = Collections.singletonList(rhsExpr);
        } else {
          exprs =
              new TreeSet<>(
                  AnnotationUtils.getElementValueArray(
                      sameLenAnno, atypeFactory.sameLenValueElement, String.class));
          exprs.add(rhsExpr);
        }
        AnnotationMirror newSameLen = atypeFactory.createSameLen(exprs);
        valueType.replaceAnnotation(newSameLen);
      }
    }
    super.commonAssignmentCheck(varType, valueType, valueTree, errorKey, extraArgs);
  }
}