infer constructor type arguments from values

[kasperpeulen]

Would it be possible that strong mode infers the type of otherMap below to be Map<String, String>

  var map = new Map<String, String>();   
  var otherMap = new Map.from(map);

Now otherMap has type Map<dynamic, dynamic>.

[jmesserly]

@kasperpeulen definitely! We can use upwards inference for constructors like we do for generic methods. Was chatting with @leafpetersen earlier this week about this.

[kasperpeulen]

awesome 👍

[jmesserly]

BTW, a related issue is that we don't (AFAIK) do downwards inference on generic methods. Current state of the world:

• constructor type parameters are only inferred from downwards inference
• generic function type parameters are only inferred from upwards inference

[jmesserly]

Also map/list literals: dart-archive/dev_compiler#108
and block bodied lambda return: #25487

[jmesserly]

note, we should fix LUB first: #25802, or we're going to get strange results in some cases

[jmesserly]

https://codereview.chromium.org/1717803003/ fixes literals. Still need constructors.

[jmesserly]

https://codereview.chromium.org/2115173002/ fixes constructors

[kasperpeulen]

@jmesserly Not sure if I understand correctly, but it doesn't seem to work as I expected:

[jmesserly]

@kasperpeulen the Map.from constructor (like List.from) is designed to be able to cast from one type to another, so it won't infer.
The signature is new Map.from(Map other)
if the signature was new Map.from(Map<K, V> other) then it would work, but you would not be able to use Map.from in a cast like this:

var m = <dynamic, dynamic>{1: 2}
var m2 = new Map<int, int>.from(m); // allowed, casts each key/value

that's an API design choice for the SDK. Inference however is supported for constructors. For example List.filled can infer from the value type.