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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.framework.util.typeinference.constraint;

import java.util.Set;
import org.checkerframework.framework.type.AnnotatedTypeFactory;
import org.checkerframework.framework.type.AnnotatedTypeMirror;

/**
 * A2FReducer takes an A2F constraint that is not irreducible (@see AFConstraint.isIrreducible) and
 * reduces it by one step. The resulting constraint may still be reducible.
 *
 * Generally reductions should map to corresponding rules in
 * https://docs.oracle.com/javase/specs/jls/se17/html/jls-15.html#jls-15.12.2.7
 */
public class A2FReducer implements AFReducer {

  protected final A2FReducingVisitor visitor;

  public A2FReducer(AnnotatedTypeFactory typeFactory) {
    this.visitor = new A2FReducingVisitor(typeFactory);
  }

  @Override
  public boolean reduce(AFConstraint constraint, Set newConstraints) {
    if (constraint instanceof A2F) {
      A2F a2f = (A2F) constraint;
      visitor.visit(a2f.argument, a2f.formalParameter, newConstraints);
      return true;

    } else {
      return false;
    }
  }

  /**
   * Given an A2F constraint of the form: A2F( typeFromMethodArgument, typeFromFormalParameter )
   *
   * 
A2FReducingVisitor visits the constraint as follows: visit ( typeFromMethodArgument,
   * typeFromFormalParameter, newConstraints )
   *
   * 
The visit method will determine if the given constraint should either:
   *
   * 

   *   be discarded -- in this case, the visitor just returns
   *   
reduced to a simpler constraint or set of constraints -- in this case, the new constraint
   *       or set of constraints is added to newConstraints
   * 
   */
  private static class A2FReducingVisitor extends AFReducingVisitor {

    public A2FReducingVisitor(AnnotatedTypeFactory typeFactory) {
      super(A2F.class, typeFactory);
    }

    @Override
    public AFConstraint makeConstraint(AnnotatedTypeMirror subtype, AnnotatedTypeMirror supertype) {
      return new A2F(subtype, supertype);
    }

    @Override
    public AFConstraint makeInverseConstraint(
        AnnotatedTypeMirror subtype, AnnotatedTypeMirror supertype) {
      return new F2A(subtype, supertype);
    }

    @Override
    public AFConstraint makeEqualityConstraint(
        AnnotatedTypeMirror subtype, AnnotatedTypeMirror supertype) {
      return new FIsA(supertype, subtype);
    }
  }
}