Update LUB for Null vs star.

As specified in dart-lang/language#700

Change-Id: I324841799a485600bf1ebfd54cd852465efac715
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/126981
Reviewed-by: Mike Fairhurst <mfairhurst@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

Author: scheglov

pkg/analyzer/lib/src/generated/type_system.dart

@@ -428,12 +428,22 @@ class Dart2TypeSystem extends TypeSystem {
       }
     }
 
+    var T1_impl = T1 as TypeImpl;
+    var T2_impl = T2 as TypeImpl;
+
+    var T1_nullability = T1_impl.nullabilitySuffix;
+    var T2_nullability = T2_impl.nullabilitySuffix;
+
     // UP(T1, T2) where NULL(T1)
     if (T1_isNull) {
       // * T2 if T2 is nullable
+      // * T2* if Null <: T2 or T1 <: Object (that is, T1 or T2 is legacy)
       // * T2? otherwise
       if (isNullable(T2)) {
         return T2;
+      } else if (T1_nullability == NullabilitySuffix.star ||
+          T2_nullability == NullabilitySuffix.star) {
+        return T2_impl.withNullability(NullabilitySuffix.star);
       } else {
         return makeNullable(T2);
       }
@@ -442,9 +452,13 @@ class Dart2TypeSystem extends TypeSystem {
     // UP(T1, T2) where NULL(T2)
     if (T2_isNull) {
       // * T1 if T1 is nullable
+      // * T1* if Null <: T1 or T2 <: Object (that is, T1 or T2 is legacy)
       // * T1? otherwise
       if (isNullable(T1)) {
         return T1;
+      } else if (T1_nullability == NullabilitySuffix.star ||
+          T2_nullability == NullabilitySuffix.star) {
+        return T1_impl.withNullability(NullabilitySuffix.star);
       } else {
         return makeNullable(T1);
       }
@@ -486,12 +500,6 @@ class Dart2TypeSystem extends TypeSystem {
       }
     }
 
-    var T1_impl = T1 as TypeImpl;
-    var T2_impl = T2 as TypeImpl;
-
-    var T1_nullability = T1_impl.nullabilitySuffix;
-    var T2_nullability = T2_impl.nullabilitySuffix;
-
     // UP(T1*, T2*) = S* where S is UP(T1, T2)
     // UP(T1*, T2?) = S? where S is UP(T1, T2)
     // UP(T1?, T2*) = S? where S is UP(T1, T2)

pkg/analyzer/test/generated/type_system_test.dart

@@ -2348,20 +2348,17 @@ class LeastUpperBoundFunctionsTest extends BoundTestBase {
 
 @reflectiveTest
 class LeastUpperBoundTest extends BoundTestBase {
-  @FailingTest(reason: 'With new rules UP(Never*, T)=T?')
   void test_bottom_function() {
     _checkLeastUpperBound(neverStar, functionTypeStar(returnType: voidNone),
         functionTypeStar(returnType: voidNone));
   }
 
-  @FailingTest(reason: 'With new rules UP(Never*, T)=T?')
   void test_bottom_interface() {
     var A = class_(name: 'A');
     var typeA = interfaceTypeStar(A);
     _checkLeastUpperBound(neverStar, typeA, typeA);
   }
 
-  @FailingTest(reason: 'With new rules UP(Never*, T)=T?')
   void test_bottom_typeParam() {
     var T = typeParameter('T');
     var typeT = typeParameterTypeStar(T);

pkg/analyzer/test/src/dart/element/upper_lower_bound_test.dart

@@ -1774,23 +1774,23 @@ class UpperBoundTest extends _BoundsTestBase {
 
     check(nullNone, intNone, intQuestion);
     check(nullNone, intQuestion, intQuestion);
-    check(nullNone, intStar, intQuestion);
+    check(nullNone, intStar, intStar);
 
     check(nullQuestion, intNone, intQuestion);
     check(nullQuestion, intQuestion, intQuestion);
-    check(nullQuestion, intStar, intQuestion);
+    check(nullQuestion, intStar, intStar);
 
-    check(nullStar, intNone, intQuestion);
+    check(nullStar, intNone, intStar);
     check(nullStar, intQuestion, intQuestion);
-    check(nullStar, intStar, intQuestion);
+    check(nullStar, intStar, intStar);
 
     check(nullNone, listNone(intNone), listQuestion(intNone));
     check(nullNone, listQuestion(intNone), listQuestion(intNone));
-    check(nullNone, listStar(intNone), listQuestion(intNone));
+    check(nullNone, listStar(intNone), listStar(intNone));
 
     check(nullNone, futureOrNone(intNone), futureOrQuestion(intNone));
     check(nullNone, futureOrQuestion(intNone), futureOrQuestion(intNone));
-    check(nullNone, futureOrStar(intNone), futureOrQuestion(intNone));
+    check(nullNone, futureOrStar(intNone), futureOrStar(intNone));
 
     check(nullNone, futureOrNone(intQuestion), futureOrNone(intQuestion));
     check(nullNone, futureOrStar(intQuestion), futureOrStar(intQuestion));