The return type 'num' isn't a 'T', as required by the closure's contex-CodePudding

I've defined an extension:

extension MyIterable<T extends num> on Iterable<T> {
  T sum() => reduce((v, e) => v   e); // Error
}

Why is v e not considered T, why is there an error?

The return type 'num' isn't a 'T', as required by the closure's context.

I'm using it in the main:

void main() {
  int a = MyIterable<int>([1, 2]).sum();
  double b = MyIterable<double>([1.0, 2]).sum();
  num c = MyIterable<num>([1.0, 2]).sum();
}

CodePudding user response：

Although v and e are statically known to be T, which is some subtype of num, the exact subtype isn't known, so v e is only statically known to conform to num.operator . Consequently, the result is num (the base type), not necessarily a T (the more specific subtype).

In this case, I think you'll probably need to add an as T explicit cast.