Implement subtype for type variables.

sdk/lib/_internal/compiler/implementation/dart_types.dart

Index: sdk/lib/_internal/compiler/implementation/dart_types.dart
diff --git a/sdk/lib/_internal/compiler/implementation/dart_types.dart b/sdk/lib/_internal/compiler/implementation/dart_types.dart
index 8f209240b8ec5101e4b2b1dce44466973471d422..6705302c045ab3e5330f6db4fd94e2f1e2d429b8 100644
--- a/sdk/lib/_internal/compiler/implementation/dart_types.dart
+++ b/sdk/lib/_internal/compiler/implementation/dart_types.dart
@@ -122,6 +122,8 @@ class TypeVariableType extends DartType {
 
   TypeVariableType(this.element);
 
+  DartType get upperBound => element.upperBound;
+
   TypeKind get kind => TypeKind.TYPE_VARIABLE;
 
   SourceString get name => element.name;
@@ -788,8 +790,9 @@ class Types {
       }
       return true;
     } else if (t is TypeVariableType) {
-      if (s is !TypeVariableType) return false;
-      return (identical(t.element, s.element));
+      if (t == s) return true;
+      TypeVariableType typeVariable = t;
+      return isSubtype(typeVariable.upperBound, s);

[ahe, 2013/02/05 15:14:40]

How do you deal with:

class Foo<T extends S, S extends T> {
  haveFun(T t, S s) {
    t = s;
    s = t;
  }
}

[Johnni Winther, 2013/02/05 18:45:57]

I don't. I think this should be part of the (still unimplemented) check on
bounds.

[ahe, 2013/02/18 08:22:18]

I don't like having known unbounded recursion in a type checker. I think this
would be really simple to implement using a variant of asInstanceOf. This method
can keep looking for an upper bound as long as it isn't a type variable or
itself.

     } else {
       throw 'internal error: unknown type kind';
     }