tests/language/inference_update_2/cascaded_field_promotion_test.dart

// Copyright (c) 2023, 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.

// Tests that field promotion works with cascades.

import 'package:expect/static_type_helper.dart';

class C {
  final Object? _field;
  C([this._field]);
  void f([_]) {}
}

void cascadedAccessReceivesTheBenefitOfPromotion(C c) {
  // If a field of an object is promoted prior to its use in a cascade, accesses
  // to the field within cascade sections retain the promotion.
  c._field as int;
  c._field.expectStaticType<Exactly<int>>();
  c
    .._field.expectStaticType<Exactly<int>>()
    .._field.expectStaticType<Exactly<int>>();
  // And the promotion remains on later accesses to the same variable.
  c._field.expectStaticType<Exactly<int>>();
}

void fieldAccessOnACascadeExpressionRetainsPromotion(C c) {
  // If a field of an object is promoted prior to its use in a cascade, accesses
  // to the field from outside the cascade retain the promotion.
  c._field as int;
  c._field.expectStaticType<Exactly<int>>();
  (c..f())._field.expectStaticType<Exactly<int>>();
  // And the promotion remains on later accesses to the same variable.
  c._field.expectStaticType<Exactly<int>>();
}

void fieldsPromotableWithinCascade(C c) {
  // Within a cascade, a field can be promoted using `!`.
  c
    .._field.expectStaticType<Exactly<Object?>>()
    .._field!.expectStaticType<Exactly<Object>>()
    .._field.expectStaticType<Exactly<Object>>();
  // And after the cascade, the promotion is retained.
  c._field.expectStaticType<Exactly<Object>>();
}

void cascadeExpressionIsNotPromotable(Object? o) {
  // However, null-checking, casting, or type checking the result of a cascade
  // expression does not promote the target of the cascade. (It could, in
  // principle, but it would be extra work to implement, and it seems unlikely
  // that it would be of much benefit).
  (o..toString())!;
  o.expectStaticType<Exactly<Object?>>();
  (o..toString()) as Object;
  o.expectStaticType<Exactly<Object?>>();
  if ((o..toString()) is Object) {
    o.expectStaticType<Exactly<Object?>>();
  }
}

void ephemeralValueFieldsArePromotable(C Function() getC) {
  // Fields of an ephemeral value (one that is not explicitly stored in a
  // variable) can still be promoted in one cascade section, and the results of
  // the promotion can be seen in later cascade sections.
  getC()
    .._field.expectStaticType<Exactly<Object?>>()
    .._field!.expectStaticType<Exactly<Object>>()
    .._field.expectStaticType<Exactly<Object>>();
  // But they won't be seen if a fresh value is created.
  getC()._field.expectStaticType<Exactly<Object?>>();
}

void writeCapturedValueFieldsArePromotable(C c) {
  // Fields of a write-captured variable can still be promoted in one cascade
  // section, and the results of the promotion can be seen in later cascade
  // sections. This is because the target of the cascade is stored in an
  // implicit temporary variable, separate from the write-captured variable.
  f() {
    c = C(null);
  }

  c
    .._field.expectStaticType<Exactly<Object?>>()
    .._field!.expectStaticType<Exactly<Object>>()
    .._field.expectStaticType<Exactly<Object>>();
  // But fields of the write-captured variable itself aren't promoted.
  c._field.expectStaticType<Exactly<Object?>>();
}

void writeDefeatsLaterAccessesButNotCascadeTarget(C c) {
  // If a write to a variable happens during a cascade, any promotions based on
  // that variable are invalidated, but the target of the cascade remains
  // promoted, since it's stored in an implicit temporary variable that's
  // unaffected by the write.
  c._field as C;
  c._field.expectStaticType<Exactly<C>>();
  c
    .._field.f([c = C(C()), c._field.expectStaticType<Exactly<Object?>>()])
    .._field.expectStaticType<Exactly<C>>();
  c._field.expectStaticType<Exactly<Object?>>();
}

void cascadedInvocationsPermitted(C c) {
  // A promoted field may be invoked inside a cascade.
  c._field as int Function();
  c._field.expectStaticType<Exactly<int Function()>>();
  c.._field().expectStaticType<Exactly<int>>();
}

main() {
  cascadedAccessReceivesTheBenefitOfPromotion(C(0));
  fieldAccessOnACascadeExpressionRetainsPromotion(C(0));
  fieldsPromotableWithinCascade(C(0));
  cascadeExpressionIsNotPromotable(0);
  ephemeralValueFieldsArePromotable(() => C(0));
  writeCapturedValueFieldsArePromotable(C(0));
  writeDefeatsLaterAccessesButNotCascadeTarget(C(C()));
  int f() => 0;
  cascadedInvocationsPermitted(C(f));
}