Implement least upper bound.

tests/language/least_upper_bound_expansive_test.dart

+// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// Test least upper bound through type checking of conditionals.
+
+class N<T> {
+  T get n => null;
+}
+
+class C1<T> extends N<N<C1<T>>> {
+  T get c1 => null;
+}
+
+class C2<T> extends N<N<C2<N<C2<T>>>>> {
+  T get c2 => null;
+}
+
+/**
+ * Test that we don't try to find the least upper bound by applying the
+ * algorithm for finding the most specific common declaration recursively on
+ * type arguments.
+ *
+ * For C1<int> and N<C1<String>> this would result in this infinite chain of
+ * computations:
+ *
+ * lub(C1<int>,N<C1<String>>)=lub(N<N<C1<int>>>,N<C1<String>>)

[karlklose, 2013/11/21 13:29:10]

Please add space between arguments and around '='.

[Johnni Winther, 2013/11/25 13:58:27]

Done.

+ * =>
+ * lub(N<C1<int>>,C1<String>)=lub(N<C1<int>>,N<N<C1<String>>>)
+ * =>
+ * lub(C1<int>,N<C1<String>>)=lub(N<N<C1<int>>>,N<C1<String>>>)
+ * => ...
+ */
+void testC1(bool z, C1<int> a, N<C1<String>> b) {
+  if (z) {

[karlklose, 2013/11/21 13:29:10]

This guard against dead-code elimination is brittle.  You could use `new
DateTime.now().millisecondsSinceEpoch == 42` instead.

[Johnni Winther, 2013/11/25 13:58:27]

It doesn't really matter. I don't want to run it, but any analyzer should
provide the warnings.

+    // The least upper bound of C1<int> and N<C1<String>> is Object since the
+    // supertypes are
+    //     {C1<int>, N<N<C1<int>>>, Object} for C1<int> and
+    //     {N<C1<String>>, Object} for N<C1<String>> and
+    // Object is the most specific type in the intersection of the supertypes.
+
+    // Is least upper bound dynamic?
+    (z ? a : b).z; /// 01: static type warning
+    // Is least upper bound N<...> ?
+    (z ? a : b).n; /// 02: static type warning
+    // Is least upper bound C1<...> ?
+    (z ? a : b).c1; /// 03: static type warning
+    // Is least upper bound N<dynamic> ?
+    (z ? a : b).n.z; /// 04: static type warning
+    // Is least upper bound N<N<...>> ?
+    (z ? a : b).n.n; /// 05: static type warning
+    // Is least upper bound N<C1<...>> ?
+    (z ? a : b).n.c1; /// 06: static type warning
+  }
+}
+
+/**
+ * Test that we don't try to find the least upper bound by applying the
+ * algorithm for finding the most specific common declaration recursively on
+ * type arguments.
+ *
+ * For C1<int> and N<C1<String>> this would result in this infinite and
+ * expanding chain of computations:
+ *
+ * lub(C2<int>,N<C2<String>>)=lub(N<N<C2<N<C2<int>>>>>,N<C2<String>>)
+ * =>
+ * lub(N<C2<N<C2<int>>>>,C2<String>)=
+*                                 lub(N<C2<N<C2<int>>>>,N<N<C2<N<C2<String>>>>>)
+ * =>
+ * lub(C2<N<C2<int>>>,N<C2<N<C2<String>>>>)=
+                                lub(N<N<C2<N<C2<int>>>>>,N<C2<N<C2<String>>>>>)
+ * => ...
+ */
+
+void testC2(bool z, C2<int> a, N<C2<String>> b) {
+  if (z) {
+    // The least upper bound of C2<int> and N<C2<String>> is Object since the
+    // supertypes are
+    //     {C2<int>, N<N<C2<N<C2<int>>>>>, Object} for C1<int> and
+    //     {N<C2<String>>, Object} for N<C1<String>> and
+    // Object is the most specific type in the intersection of the supertypes.
+
+    // Is least upper bound dynamic?
+    (z ? a : b).z; /// 07: static type warning
+    // Is least upper bound N<...> ?
+    (z ? a : b).n; /// 08: static type warning
+    // Is least upper bound C2<...> ?
+    (z ? a : b).c2; /// 09: static type warning
+    // Is least upper bound N<dynamic> ?
+    (z ? a : b).n.z; /// 10: static type warning
+    // Is least upper bound N<N<...>> ?
+    (z ? a : b).n.n; /// 11: static type warning
+    // Is least upper bound N<C2<...>> ?
+    (z ? a : b).n.c2; /// 12: static type warning
+  }
+}
+
+void main() {
+  testC1(false, null, null);
+  testC2(false, null, null);
+}