Program: 18833
Project: 18833
Buggy source:
/** 
 * Visits a   {@link Token#FUNCTION} node.
 * @param t The node traversal object that supplies context, such as thescope chain to use in name lookups as well as error reporting.
 * @param n The node being visited.
 */
private void visitFunction(NodeTraversal t,Node n){
  JSDocInfo info=n.getJSDocInfo();
  FunctionType functionType=(FunctionType)n.getJSType();
  String functionPrivateName=n.getFirstChild().getString();
  if (functionType.isInterface() || functionType.isConstructor()) {
    FunctionType baseConstructor=functionType.getPrototype().getImplicitPrototype().getConstructor();
    if (baseConstructor != null && baseConstructor != getNativeType(OBJECT_FUNCTION_TYPE) && (baseConstructor.isConstructor() && functionType.isInterface() || baseConstructor.isInterface() && functionType.isConstructor())) {
      compiler.report(t.makeError(n,CONFLICTING_EXTENDED_TYPE,functionPrivateName));
    }
    for (    JSType baseInterface : functionType.getImplementedInterfaces()) {
      boolean badImplementedType=false;
      ObjectType baseInterfaceObj=ObjectType.cast(baseInterface);
      if (baseInterfaceObj != null) {
        FunctionType interfaceConstructor=baseInterfaceObj.getConstructor();
        if (interfaceConstructor != null && !interfaceConstructor.isInterface()) {
          badImplementedType=true;
        }
      }
 else {
        badImplementedType=true;
      }
      if (badImplementedType) {
        report(t,n,BAD_IMPLEMENTED_TYPE,functionPrivateName);
      }
    }
    if (functionType.isConstructor()) {
      validator.expectAllInterfacePropertiesImplemented(functionType);
    }
  }
}

Fixed version (ground truth):
/** 
 * Visits a   {@link Token#FUNCTION} node.
 * @param t The node traversal object that supplies context, such as thescope chain to use in name lookups as well as error reporting.
 * @param n The node being visited.
 */
private void visitFunction(NodeTraversal t,Node n){
  JSDocInfo info=n.getJSDocInfo();
  FunctionType functionType=(FunctionType)n.getJSType();
  String functionPrivateName=n.getFirstChild().getString();
  if (functionType.isInterface() || functionType.isConstructor()) {
    FunctionType baseConstructor=functionType.getPrototype().getImplicitPrototype().getConstructor();
    if (baseConstructor != null && baseConstructor != getNativeType(OBJECT_FUNCTION_TYPE) && (baseConstructor.isConstructor() && functionType.isInterface() || baseConstructor.isInterface() && functionType.isConstructor())) {
      compiler.report(t.makeError(n,CONFLICTING_EXTENDED_TYPE,functionPrivateName));
    }
    for (    JSType baseInterface : functionType.getImplementedInterfaces()) {
      boolean badImplementedType=false;
      ObjectType baseInterfaceObj=ObjectType.cast(baseInterface);
      if (baseInterfaceObj != null) {
        FunctionType interfaceConstructor=baseInterfaceObj.getConstructor();
        if (interfaceConstructor != null && !interfaceConstructor.isInterface()) {
          badImplementedType=true;
        }
      }
 else {
        badImplementedType=true;
      }
      if (badImplementedType) {
        report(t,n,BAD_IMPLEMENTED_TYPE,functionPrivateName);
      }
    }
    if (functionType.isConstructor()) {
      validator.expectAllInterfaceProperties(t,n,functionType);
    }
  }
}