Wrong type parameter constraint checking error

[sledorze]

typedef A = {
  var y : Int;
}

typedef B = {
  var x : Int;
}

class Test {


  static function foo<V, T : (V, A), U : (V, B)>(x : T) : Void {
  }

  static function main() {
    foo("toto");
  }
}

ouputs this error:

Build failure
Test.hx:16: characters 4-7 : Constraint check failure for foo.T
Test.hx:16: characters 4-7 : String should be B
Test.hx:16: characters 4-7 : String should be { x : Int }
Test.hx:16: characters 4-7 : String has no field x
Test.hx:16: characters 4-7 : Constraint check failure for foo.T
Test.hx:16: characters 4-7 : String should be A
Test.hx:16: characters 4-7 : String should be { y : Int }
Test.hx:16: characters 4-7 : String has no field y

Whereas 'String should be A' is correct, 'String should be B' is incorrect.

[Simn]

That looks like fun! Let's see if we can understand what happens:

Unification	Assign	To	Result
Call	String	T	T/String
Constraint check for T	T/String	V	V/String
Constraint check for T	T/String	A	"String should be A"
Constraint check for U	U	V/String	U/String
Constraint check for U	U/String	B	"String should be B"

This might be working as intended.

[sledorze]

Mmmm, thanks for the explanation.
I was thinking that the compiler would infer U as a subtype of both V (string) and B (typedef); a kind of type union.
From your explanation, I understand that no ad'hoc subtype representation exist in the compiler.

Note: that maybe using 'String' for the example is misleading, the non shrinked version was involving only typedefs / anonymous