Meta-issue for discussion of the proposal to allow generic constructors

[leafpetersen]

This issue is to provide a forum for community discussion and feedback related to the proposal to allow constructors for classes to take additional generic arguments independent of the generic arguments of their class.

Currently strong mode allows generic functions and methods with a syntax like int foo<T>(List <T> l) ..., but constructors are not generic - they can only get the type parameters of the class.
An example case could be List.mapped<S>(Iterable<S> source, E map(S value)) ... where the type of the source doesn't depend on the type of the created list List<E>.

This entry will be updated when a proposal is available.

cc @munificent @eernstg @floitschG @lrhn

[brianegan]

I wrote in an issue for this, and I'm fully in favor of supporting generics in constructors! #30041

The status quo creates a weird dichotomy between factory constructors and static constructors, because I sometimes need constructors with generics. It's confusing for library consumers (e.g. is this a factory or static constructor? Autocompletion is affected within your IDE). It also adds complexity for folks learning the language: Why isn't there one true way for class construction?

Thanks for opening up the discussion and hearing our thoughts :)

[eernstg]

This feature was also requested in #31360.

[natebosch]

Closing in favor of https://github.com/dart-lang/sdk/issues/26391 since it's older.