[cfe] Fix value of the top type in least closure of a type schema

Previously, the algorithm for the least closure was abstracted over
the top type. However, all call sites only pass `Object?` as the top
type, and this CL fixes that value, which is expected in null-safe
Dart.

Change-Id: I67cea1ccf910e96d4441dd0fbee9579f5cf22155
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/400080
Reviewed-by: Johnni Winther <[email protected]>
Commit-Queue: Chloe Stefantsova <[email protected]>

Author: chloestefantsova

pkg/front_end/lib/src/type_inference/standard_bounds.dart

@@ -21,7 +21,7 @@ mixin TypeSchemaStandardBounds on StandardBounds {
   DartType leastClosureForUpperBound(DartType typeSchema) {
     //  - We replace all uses of `T1 <: T2` in the `UP` algorithm by `S1 <: S2`
     //  where `Si` is the least closure of `Ti` with respect to `_`.
-    return leastClosure(typeSchema, topType: coreTypes.objectNullableRawType);
+    return leastClosure(typeSchema, coreTypes: coreTypes);
   }
 
   @override

pkg/front_end/lib/src/type_inference/type_schema_elimination.dart

@@ -3,6 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import 'package:kernel/ast.dart';
+import 'package:kernel/core_types.dart';
 import 'package:kernel/src/replacement_visitor.dart';
 
 import 'type_schema.dart' show UnknownType;
@@ -38,9 +39,9 @@ DartType greatestClosure(DartType schema, {required DartType topType}) {
 ///
 /// Note that the least closure of a type schema is always a subtype of any type
 /// which matches the schema.
-DartType leastClosure(DartType schema, {required DartType topType}) {
+DartType leastClosure(DartType schema, {required CoreTypes coreTypes}) {
   return _TypeSchemaEliminationVisitor.run(schema,
-      computeLeastClosure: true, topType: topType);
+      computeLeastClosure: true, topType: coreTypes.objectNullableRawType);
 }
 
 /// Visitor that computes least and greatest closures of a type schema.

pkg/front_end/lib/src/type_inference/type_schema_environment.dart

@@ -409,7 +409,7 @@ class TypeSchemaEnvironment extends HierarchyBasedTypeEnvironment
       if (constraint.lower is! SharedUnknownTypeSchemaView<DartType>) {
         return grounded
             ? leastClosure(constraint.lower.unwrapTypeSchemaView(),
-                topType: topType)
+                coreTypes: coreTypes)
             : constraint.lower.unwrapTypeSchemaView();
       } else if (constraint.upper is! UnknownType) {
         return grounded
@@ -440,7 +440,7 @@ class TypeSchemaEnvironment extends HierarchyBasedTypeEnvironment
       } else if (constraint.lower is! UnknownType) {
         return grounded
             ? leastClosure(constraint.lower.unwrapTypeSchemaView(),
-                topType: topType)
+                coreTypes: coreTypes)
             :
             // Coverage-ignore(suite): Not run.
             constraint.lower.unwrapTypeSchemaView();

pkg/front_end/test/type_inference/type_schema_elimination_nnbd_test.dart

@@ -29,8 +29,7 @@ class TypeSchemaEliminationTest {
   }
 
   DartType leastClosure(DartType schema) {
-    return typeSchemaElimination.leastClosure(schema,
-        topType: env.coreTypes.objectNullableRawType);
+    return typeSchemaElimination.leastClosure(schema, coreTypes: env.coreTypes);
   }
 
   void testGreatest(String type, String expectedClosure) {

pkg/front_end/test/type_inference/type_schema_elimination_test.dart

@@ -29,8 +29,7 @@ class TypeSchemaEliminationTest {
   }
 
   DartType leastClosure(DartType schema) {
-    return typeSchemaElimination.leastClosure(schema,
-        topType: new DynamicType());
+    return typeSchemaElimination.leastClosure(schema, coreTypes: env.coreTypes);
   }
 
   void testGreatest(String type, String expectedClosure) {
@@ -69,13 +68,13 @@ class TypeSchemaEliminationTest {
   }
 
   void test_leastClosure_contravariant() {
-    testLeast("(UNKNOWN) ->* dynamic", "(dynamic) ->* dynamic");
-    testLeast("({UNKNOWN foo}) ->* dynamic", "({dynamic foo}) ->* dynamic");
+    testLeast("(UNKNOWN) ->* dynamic", "(Object?) ->* dynamic");
+    testLeast("({UNKNOWN foo}) ->* dynamic", "({Object? foo}) ->* dynamic");
   }
 
   void test_leastClosure_contravariant_contravariant() {
     testLeast("((UNKNOWN) ->* UNKNOWN) ->* dynamic",
-        "((Never) ->* dynamic) ->* dynamic");
+        "((Never) ->* Object?) ->* dynamic");
   }
 
   void test_leastClosure_covariant() {
@@ -85,7 +84,7 @@ class TypeSchemaEliminationTest {
 
   void test_leastClosure_function_multipleUnknown() {
     testLeast("(UNKNOWN, UNKNOWN, {UNKNOWN a, UNKNOWN b}) ->* UNKNOWN",
-        "(dynamic, dynamic, {dynamic a, dynamic b}) ->* Never");
+        "(Object?, Object?, {Object? a, Object? b}) ->* Never");
   }
 
   void test_leastClosure_simple() {