Add generics to new Map.fromIterable()

[nex3]

Currently, new Map.fromIterable() is very difficult to use in strong mode, because it doesn't support type propagation from the Iterable argument to the callbacks:

Map.fromIterable(Iterable elements, {K key(element), V value(element)});

We should fix this, either using #26391 to make the constructor generic, or by making it a static method instead of a constructor.

[jacehensley-wf]

I just ran into a case where this would be great, when adding some new lints.

[kevmoo]

@floitschG @lrhn – would be great to consider for v2

[lrhn]

Definitely. It's one of the clear use-cases for #26391.

[matanlurey]

Uh oh - this is now a warning in the latest dev released of the SDK:

class Foo {
  String bar;
}

void doThing(Iterable<Foo> foos) {
  new Map<Foo, String>.fromIterable(foos, value: (Foo foo) => foo.bar);
}

This is a hint, soon to be an error, because the definition dynamic -> String is expected:

value: (/* Dynamic as bottom */ foo) => (foo as Foo).bar

[natebosch]

With implicit-casts: false does this make it impossible to implement a fallback for these functions?

The current implementation has a default of the identify function for both key and value - I don't think it could do that if we had stricter generic types.

The tradeoff is probably worth it, but if no-implicit-casts becomes the default it's a bit harder to do.