Java: please emit covariant Map types

[rix0rrr]

🚀 Feature Request

Affected Languages

• Java
• .NET (C#, F#, ...) (maybe! I don't know)

Right now, a {[key: K]: V} (or Record<K, V> which I prefer writing because all those curlies make me dizzy :) ), renders to the following type in Java:

Map<K, V>

Example:

TS:  {[key: string]: any}

# renders to

Java: Map<String, Object> 

I'd like to use this in the IAM library in addCondition(), where the user needs to actually pass a Record<string, string | string[]>, but since we don't do unions we take the common supertype, which is any.

The problem is, most people are going to want to use an ImmutableMap<> to pass as an argument. That's fine, but the effective type of that map in 9 times out of 10 is going to be:

ImmutableMap<String, String>

And it so happens that you cannot assign a Map<String, String> to a Map<String, Object>, as Java doesn't know about the subtyping relationship you intend (and prefers not to guess).

Proof:

    Map<String, Object> x = new HashMap<String, String>();

Main.java:6: error: incompatible types: HashMap<String,String> cannot be converted to Map<String,Object>
    Map<String, Object> x = new HashMap<String, String>();

https://repl.it/repls/TalkativeUnrulyTheories

Proposed solution

The actual type we should be emitting in Java, to mirror the subtyping relationship in TypeScript, is the following:

Map<String, ? extends Object>

(In fact it should actually be Map<? extends String, ? extends Object>, but since we never have anything other than a String in the first position (because JavaScript), we can simplify a little...)

And the same for List.

You would need to investigate yourself whether we need to explicitly annotate C# as well, or not.

[RomainMuller]

I like this idea a lot!

I don't reckon C# needs something similar... But we should add a compliance test to confirm that.

[NetaNir]

Opening this cause we had to temporary revert the change