This comment was originally written by @olostan

"assert(x is B);" should be "assert(x is A);", of course

Removed Type-Defect label.
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This is non-trivial issue. We've discussed it before, and will again.

Set owner to @gbracha.
Added this to the Later milestone.
Added Accepted label.

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Issue #22424 has been merged into this issue.

I guess https://github.com/gbracha/metaclasses/blob/master/proposal.md is supposed to lead to a solution (as reference for people finding this issue first).

Still new to Dart and I also ran into this same issue. I see its closed (as obsolete). I also see a link to a proposed solution, but find no action on that proposal.

So, apologies for asking again, but how exactly am I supposed to instantiate a generic type?

Still new to Dart and I also ran into this same issue. I see its closed (as obsolete). I also see a link to a proposed solution, but find no action on that proposal.

So, apologies for asking again, but how exactly am I supposed to instantiate a generic type?

There is the reflectable, json_serializer packages etc

I use source generation

@jodinathan Thank you. I appreciate the pointer. Unfortunately, its a bit overkill for my requirements. I've already implemented a quite ugly workaround, but I'm still hoping for a language-supported fix.

@techfan101 wrote:

how exactly am I supposed to instantiate a generic type?

It is basically a questionable idea. However, a solution that will work in general is to pair up the type argument with a corresponding factory function:

class B<T> {
  final T Function() factoryOfT;
  B(this.factoryOfT);
  T getBySmth() => factoryOfT();
}

That factory function may need to embed a lot of decisions. For instance, if T is int then you'd have to return some existing integer rather than a "new" one; if T is a class that only has constructors that take many arguments and require some serious building effort in order to obtain an object that will actually work, then the factory function will have to perform that building work. If T is the empty type (Never) then the factory may be forced to throw an exception or loop forever: There is no way to satisfy the request "give me an element from the empty set".

C# allows for a constraint on type parameters that they must support new(), which is basically syntactic sugar for (1) requiring that the type argument denotes a class that has a constructor that accepts the empty argument list, and (2) passing a factory function around (or using a heterogeneous compilation strategy such that we get a fresh copy of the whole class for each distinct value of the type argument). This kind of syntactic sugar might be convenient in some situations, but it isn't very general. The solution where we declare the factory function explicitly is more general (e.g., it could take whatever parameters we'd want, and we could use different factory functions for different instances of the enclosing class).

@techfan101 wrote:

how exactly am I supposed to instantiate a generic type?

It is basically a questionable idea. However, a solution that will work in general is to pair up the type argument with a corresponding factory function:

class B<T> {
  final T Function() factoryOfT;
  B(this.factoryOfT);
  T getBySmth() => factoryOfT();
}

That factory function may need to embed a lot of decisions. For instance, if T is int then you'd have to return some existing integer rather than a "new" one; if T is a class that only has constructors that take many arguments and require some serious building effort in order to obtain an object that will actually work, then the factory function will have to perform that building work. If T is the empty type (Never) then the factory may be forced to throw an exception or loop forever: There is no way to satisfy the request "give me an element from the empty set".

C# allows for a constraint on type parameters that they must support new(), which is basically syntactic sugar for (1) requiring that the type argument denotes a class that has a constructor that accepts the empty argument list, and (2) passing a factory function around (or using a heterogeneous compilation strategy such that we get a fresh copy of the whole class for each distinct value of the type argument). This kind of syntactic sugar might be convenient in some situations, but it isn't very general. The solution where we declare the factory function explicitly is more general (e.g., it could take whatever parameters we'd want, and we could use different factory functions for different instances of the enclosing class).

The syntactic sugar of C# is convenient in a lot of cases, if not most of them. And it does not stops you from using a factory function explicitly if needed.
I am guessing that you usually needs T() with business models and they are also usually have empty constructors. Our projects all use source generated generic factories with no argument, so a simple T() would have been nice.

@eernstg Thank you, but unfortunately your suggestion is a non-starter for my requirements.

@jodinathan Exactly what I need. I am completely comfortable requiring a default/parameterless constructor. While I have the occasional gripe, I think the new() constraint from C# (or some similar syntactic clue) strikes the right balance between convenience and insanity.

Short of some support within the language, I am reluctant to believe that other solutions will truly suffice. Honestly open to suggestions, but I need something broadly applicable that requires little additional plumbing.

I've been at this (engineering not Dart :)) for over 35 years. I can always find ugly workarounds, which is (sadly) what I'm currently using. That said, given that no "good" solution seems apparent, I'm a bit surprised this issue was closed.