Incorrect type inference when swapping type parameters in a constructor call

[stereotype441]

The following code (inspired by HashBiMap.inverse from https://github.com/google/quiver-dart/blob/master/lib/src/collection/bimap.dart) produces strong mode errors:

    class Pair<T, U> {
      T t;
      U u;
      Pair(this.t, this.u);
      Pair<U, T> get reversed => new Pair(u, t);
    }

The errors are:

• At new Pair(u, t): The return type 'Pair<U, U>' is not a 'Pair<U, T>', as defined by the method 'reversed'.
• At the t in new Pair(u, t): The argument type 'T' cannot be assigned to the parameter type 'U'.

It seems like incorrect type parameters are being inferred for the call to the Pair constructor.

This behavior started happening in 8d2c6d0.

[bwilkerson]

@jmesserly

[jmesserly]

good catch!

it looks like an incorrect capture bug.

I wonder if we have a similar bug with generic functions. We certainly tried very hard to avoid this class of bug in them :)

If not, there's a few possibilities on what could be going on. Constructors have some notable differences:

• their downward inference uses matchType instead of the type variable constraint solver used for inference on generic functions/methods. (This is currently a TODO in the code)
• they go through some contortions to get back to the "uninstantiated" constructor function type, which generic functions/methods don't need to worry about (this one might be a TODO as well :) )
• we never fully refactored interface type generics to avoid capture like we did with function types

Anyway I'll dig in, but that's kind of a scary bug. Hopefully it's something little and not "omg this implementation is fundamentally broken" :)

[jmesserly]

Couldn't reproduce this with generic functions, so it seems specific to constructors (which makes sense)

[jmesserly]

got it. Our substitution for the "extends" clause w/ recursive bounds was incorrectly affecting inference for this. Fix on the way

[jmesserly]

https://codereview.chromium.org/2205993002/

[scheglov]

Possibly related: https://codereview.chromium.org/2376213003