Type inference is dependent on the order that type parameters are defined

[leonsenft]

Versions:

• DartPad on SDK 2.7.0
• VM/dart2js/DDC on 2.8.292505767-edge+google3-v2 (google3)

Here's a working reproduction https://dartpad.dev/a8a92c56149499dd479488f1cca5bff9.

class Value {}

class Renderer<T> {}

// When V is defined before R, type inference works in all cases.
class WorkingOrder<V extends Value, R extends Renderer<V>> {}

// When V is defined after R, type inference fails in some cases.
class BrokenOrder<R extends Renderer<V>, V extends Value> {}

void main() {
  // Correctly infers type.
  var working1 = WorkingOrder();
  print(working1.runtimeType); // WorkingOrder<Value, Renderer<Value>>.

  // Correctly infers type arguments to bounds.
  WorkingOrder working2 = WorkingOrder();
  print(working2.runtimeType); // WorkingOrder<Value, Renderer<Value>>.

  // ERROR: Incorrectly infers BrokenOrder<Renderer<dynamic>, Value>.
  var broken = BrokenOrder();

  // Correctly infers type arguments to bounds.
  BrokenOrder working3 = BrokenOrder();
  print(working3.runtimeType); // BrokenOrder<Renderer<Value>, Value>.
}

• Inference behaves differently when inferring the type of a variable declared as var versus the raw type - this is unexpected.
• The inference algorithm used for inferring the type of a variable declared as var appears to be dependent on the order in which type parameters are declared on the resulting type, when one of those type parameters depends on the other.

[leonsenft]

@leafpetersen @vsmenon @Markzipan @nshahan this is the inference issue I was showing you all yesterday.

[leonsenft]

@a-siva Sorry if I wasn't clear enough, this isn't restricted to the web. If you run my reproduction on the VM, the same error occurs.

$ dart main.dart

main.dart:19:17: Error: Inferred type argument 'Renderer<dynamic>' doesn't conform to the bound 'Renderer<V>' of the type variable 'R' on 'BrokenOrder'.
 - 'Renderer' is from 'main.dart'.
Try specifying type arguments explicitly so that they conform to the bounds.
  var broken1 = BrokenOrder();
                ^
main.dart:9:19: Context: This is the type variable whose bound isn't conformed to.
class BrokenOrder<R extends Renderer<V>, V extends Value> {}

[leonsenft]

The error also appears in the analyzer, which I didn't think was using the front end, but that would imply both the CFE and analyzer have the exact same bug.

[johnniwinther]

cc @stereotype441 @stefantsov

[srawlins]

@eernstg Is this the issue that was fixed recently with dart-lang/language#3009 ? I haven't looked at enabling that experiment personally...

[leonsenft]

As of Dart SDK 3.7.0-183.0.dev, this is still broken from a quick test in DartPad.

[srawlins]

Yeah I don't think dartpad has that experiment enabled. Even on the main branch.