Incorrect expectations in co19/LanguageFeatures/Constructor-tear-offs/summary_A01_t01

[alexmarkov]

The test verifies that constructor tear-offs have the certain function types:

co19/LanguageFeatures/Constructor-tear-offs/summary_A01_t01.dart

Lines 26 to 29 in ef018b9

	typedef List<T> Func1<T>(int i, T t);
	typedef List<int> Func2(int i1, int i2);
	typedef List<List<T>> Func3<T>(int i, T t);
	typedef List<List<int>> Func4(int i1, int i2);

co19/LanguageFeatures/Constructor-tear-offs/summary_A01_t01.dart

Lines 32 to 39 in ef018b9

	const filledList = List.filled; // List<T> Function<T>(int, T)
	Expect.isTrue(filledList is Func1);
	const filledIntList = List<int>.filled; // List<int> Function(int, int)
	Expect.isTrue(filledIntList is Func2);
	const filledListList = ListList.filled; // List<List<T>> Function<T>(int, T)
	Expect.isTrue(filledListList is Func3);
	const filledIntListList = ListList<int>.filled; // List<List<int>> Function(int, int)
	Expect.isTrue(filledIntListList is Func4);

There are a couple of problems with these typedefs:

1. Func1 and Func3 declare generic type aliases to non-generic function types. For example, declaration of Func1 in the new syntax is equivalent to

typedef Func1<T> = List<T> Function(int i, T t);

As a result, is Func1 instantiates type parameter of generic Func1 with default dynamic, and the type check is actually testing is List<dynamic> Function(int i, dynamic t). Constructor tear-off is a generic function, and it doesn't pass type test against non-generic function type.

Instead, Func1 should be declared as a non-generic alias to a generic function type:

typedef Func1 = List<T> Function<T>(int i, T t);

2. Func3 and Func4 are used to test types of ListList.filled and ListList<int>.filled, where
   

   co19/LanguageFeatures/Constructor-tear-offs/summary_A01_t01.dart

   Line 24 in ef018b9

   typedef ListList<T> = List<List<T>>;

However, they do not properly substitute type parameter E of List to List<T> or List<int> for the 2nd argument (E fill) of List.filled. They use T or int which is not correct.

These type aliases can be corrected in the following way:

typedef Func3 = List<List<T>> Function<T>(int i, List<T> t);
typedef List<List<int>> Func4(int i1, List<int> i2);

[sgrekhov]

@alexmarkov thank you for the explanation!