Language/Variables/Evaluation_of_Implicit_Variable_Getters/definition_t06.dart

// Copyright (c) 2021, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

/**
 * @assertion Let d be the declaration of a variable v.  If d is a local or
 * instance variable, then the invocation of the implicit getter of v
 * evaluates to the value stored in v.
 * If d is a static or library variable then the implicit getter method of v
 * executes as follows:
 * - Non-constant variable declaration with initializer. If d is of one of the 
 *   forms var v = e; , T v = e; , final v = e; , final T v = e; , static 
 *   v = e; , static T v = e; , static final v = e;  or static final T v = e; 
 *   and no value has yet been stored into v then the initializer expression 
 *   e is evaluated. If, during the evaluation of e, the getter  for v is 
 *   invoked, a CyclicInitializationError is thrown. If the evaluation 
 *   succeeded yielding an object o, let r = o, otherwise let r = null. In 
 *   any case, r is stored into v. The result of executing the getter is r.
 * - Constant variable declaration. If d is of one of the forms const v = e; ,
 *   const T v = e; , static const v = e;  or static const T v = e; the result
 *   of the getter is the value of the compile time constant e. Note that a
 *   compile time constant cannot depend on itself, so no cyclic references 
 *   can occur. Otherwise
 * - Variable declaration without initializer. The result of executing the
 *   getter method is the value stored in v.
 *
 * @note The language specification says 'If, during the evaluation of [e]
 * (which is the initializing expression for [v]), the getter for [v] is invoked,
 * a [CyclicInitializationError] is thrown', but this is overridden by the
 * following text in the nnbd feature specification: 'If a variable or field is
 * read from during the process of evaluating its own initializer expression,
 * and no write to the variable has occurred, the read is treated as a first
 * read and the initializer expression is evaluated again.'
 * So CyclicInitializationErrors are gone here when null safety is enabled.
 * In the test, [f] returns [null] (typed as [dynamic]), and the evaluation of
 * the initializing expression for [sVar] as well as [sFinal] discards the
 * function literal (() => sVar respectively () => sFinal) without ever
 * executing it. So we do not have a situation where the getter for
 * [sVar]/[sFinal] is invoked during the evaluation of its initializing
 * expression, and both variables just get initialized to the value [null]. No
 * recursion and no errors.
 *
 * @description Checks that [CyclicInitializationError] is not thrown with null
 * safety turned on.
 *
 * @author sgrekhov@unipro.ru
 */
f(func) {}

class C {
  static var sVar = f(() => sVar);
  static final sFinal = f(() => sFinal);
}

main() {
  C? c;
}