[cfe] Fix inference for variant parameters leaking UnknownTypes.

Change-Id: I056535cbfc74554de408ce4f9d89c142c4c860cf
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/128771
Reviewed-by: Leaf Petersen <leafp@google.com>
Reviewed-by: Dmitry Stefantsov <dmitryas@google.com>
Commit-Queue: Kallen Tu <kallentu@google.com>

Author: kallentu

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

@@ -291,16 +291,17 @@ class TypeSchemaEnvironment extends HierarchyBasedTypeEnvironment
       }
 
       TypeConstraint constraint = constraints[typeParam];
-      if (downwardsInferPhase || !typeParam.isLegacyCovariant) {
+      if (downwardsInferPhase) {
         inferredTypes[i] = _inferTypeParameterFromContext(
-            constraint, extendsConstraint, clientLibrary,
-            isContravariant: typeParam.variance == Variance.contravariant);
+            constraint, extendsConstraint, clientLibrary);
       } else {
         inferredTypes[i] = _inferTypeParameterFromAll(
             typesFromDownwardsInference[i],
             constraint,
             extendsConstraint,
-            clientLibrary);
+            clientLibrary,
+            isContravariant: typeParam.variance == Variance.contravariant,
+            preferUpwardsInference: !typeParam.isLegacyCovariant);
       }
     }
 
@@ -451,10 +452,13 @@ class TypeSchemaEnvironment extends HierarchyBasedTypeEnvironment
       DartType typeFromContextInference,
       TypeConstraint constraint,
       DartType extendsConstraint,
-      Library clientLibrary) {
+      Library clientLibrary,
+      {bool isContravariant: false,
+      bool preferUpwardsInference: false}) {
     // See if we already fixed this type from downwards inference.
-    // If so, then we aren't allowed to change it based on argument types.
-    if (isKnown(typeFromContextInference)) {
+    // If so, then we aren't allowed to change it based on argument types unless
+    // [preferUpwardsInference] is true.
+    if (!preferUpwardsInference && isKnown(typeFromContextInference)) {
       return typeFromContextInference;
     }
 
@@ -463,14 +467,13 @@ class TypeSchemaEnvironment extends HierarchyBasedTypeEnvironment
       addUpperBound(constraint, extendsConstraint, clientLibrary);
     }
 
-    return solveTypeConstraint(constraint, grounded: true);
+    return solveTypeConstraint(constraint,
+        grounded: true, isContravariant: isContravariant);
   }
 
   DartType _inferTypeParameterFromContext(TypeConstraint constraint,
-      DartType extendsConstraint, Library clientLibrary,
-      {bool isContravariant: false}) {
-    DartType t =
-        solveTypeConstraint(constraint, isContravariant: isContravariant);
+      DartType extendsConstraint, Library clientLibrary) {
+    DartType t = solveTypeConstraint(constraint);
     if (!isKnown(t)) {
       return t;
     }

pkg/front_end/test/spell_checking_list_tests.txt

@@ -89,6 +89,7 @@ consts
 contract
 conversion
 conversions
+cov
 crashes
 cumulative
 dacoharkes

pkg/front_end/testcases/text_serialization.status

@@ -1225,3 +1225,4 @@ unified_collections/mixed_entries: TextSerializationFailure
 variance/class_type_parameter_modifier: TextSerializationFailure
 variance/generic_covariance_sound_variance: TextSerializationFailure
 variance/mixin_type_parameter_modifier: TextSerializationFailure
+variance/unconstrained_inference: TextSerializationFailure

pkg/front_end/testcases/variance/unconstrained_inference.dart

@@ -0,0 +1,22 @@
+// Copyright (c) 2019, 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.
+
+class Covariant<out T> {}
+class Contravariant<in T> {}
+class Invariant<inout T> {}
+
+void covariantListInfer<T>(Covariant<List<T>> x) {}
+void contravariantListInfer<T>(Contravariant<List<T>> x) {}
+void invariantListInfer<T>(Invariant<List<T>> x) {}
+
+main() {
+  var cov = new Covariant();
+  covariantListInfer(Covariant());
+
+  var contra = new Contravariant();
+  contravariantListInfer(Contravariant());
+
+  var inv = new Invariant();
+  invariantListInfer(Invariant());
+}

pkg/front_end/testcases/variance/unconstrained_inference.dart.outline.expect

@@ -0,0 +1,24 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+class Covariant<T extends core::Object* = dynamic> extends core::Object {
+  synthetic constructor •() → self::Covariant<self::Covariant::T*>*
+    ;
+}
+class Contravariant<contravariant T extends core::Object* = dynamic> extends core::Object {
+  synthetic constructor •() → self::Contravariant<self::Contravariant::T*>*
+    ;
+}
+class Invariant<invariant T extends core::Object* = dynamic> extends core::Object {
+  synthetic constructor •() → self::Invariant<self::Invariant::T*>*
+    ;
+}
+static method covariantListInfer<T extends core::Object* = dynamic>(self::Covariant<core::List<self::covariantListInfer::T*>*>* x) → void
+  ;
+static method contravariantListInfer<T extends core::Object* = dynamic>(self::Contravariant<core::List<self::contravariantListInfer::T*>*>* x) → void
+  ;
+static method invariantListInfer<T extends core::Object* = dynamic>(self::Invariant<core::List<self::invariantListInfer::T*>*>* x) → void
+  ;
+static method main() → dynamic
+  ;

pkg/front_end/testcases/variance/unconstrained_inference.dart.strong.expect

@@ -0,0 +1,30 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+class Covariant<T extends core::Object* = dynamic> extends core::Object {
+  synthetic constructor •() → self::Covariant<self::Covariant::T*>*
+    : super core::Object::•()
+    ;
+}
+class Contravariant<contravariant T extends core::Object* = dynamic> extends core::Object {
+  synthetic constructor •() → self::Contravariant<self::Contravariant::T*>*
+    : super core::Object::•()
+    ;
+}
+class Invariant<invariant T extends core::Object* = dynamic> extends core::Object {
+  synthetic constructor •() → self::Invariant<self::Invariant::T*>*
+    : super core::Object::•()
+    ;
+}
+static method covariantListInfer<T extends core::Object* = dynamic>(self::Covariant<core::List<self::covariantListInfer::T*>*>* x) → void {}
+static method contravariantListInfer<T extends core::Object* = dynamic>(self::Contravariant<core::List<self::contravariantListInfer::T*>*>* x) → void {}
+static method invariantListInfer<T extends core::Object* = dynamic>(self::Invariant<core::List<self::invariantListInfer::T*>*>* x) → void {}
+static method main() → dynamic {
+  self::Covariant<dynamic>* cov = new self::Covariant::•<dynamic>();
+  self::covariantListInfer<dynamic>(new self::Covariant::•<core::List<dynamic>*>());
+  self::Contravariant<core::Null?>* contra = new self::Contravariant::•<core::Null?>();
+  self::contravariantListInfer<core::Null?>(new self::Contravariant::•<core::List<core::Null?>*>());
+  self::Invariant<dynamic>* inv = new self::Invariant::•<dynamic>();
+  self::invariantListInfer<core::Null?>(new self::Invariant::•<core::List<core::Null?>*>());
+}

pkg/front_end/testcases/variance/unconstrained_inference.dart.strong.transformed.expect

@@ -0,0 +1,30 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+class Covariant<T extends core::Object* = dynamic> extends core::Object {
+  synthetic constructor •() → self::Covariant<self::Covariant::T*>*
+    : super core::Object::•()
+    ;
+}
+class Contravariant<contravariant T extends core::Object* = dynamic> extends core::Object {
+  synthetic constructor •() → self::Contravariant<self::Contravariant::T*>*
+    : super core::Object::•()
+    ;
+}
+class Invariant<invariant T extends core::Object* = dynamic> extends core::Object {
+  synthetic constructor •() → self::Invariant<self::Invariant::T*>*
+    : super core::Object::•()
+    ;
+}
+static method covariantListInfer<T extends core::Object* = dynamic>(self::Covariant<core::List<self::covariantListInfer::T*>*>* x) → void {}
+static method contravariantListInfer<T extends core::Object* = dynamic>(self::Contravariant<core::List<self::contravariantListInfer::T*>*>* x) → void {}
+static method invariantListInfer<T extends core::Object* = dynamic>(self::Invariant<core::List<self::invariantListInfer::T*>*>* x) → void {}
+static method main() → dynamic {
+  self::Covariant<dynamic>* cov = new self::Covariant::•<dynamic>();
+  self::covariantListInfer<dynamic>(new self::Covariant::•<core::List<dynamic>*>());
+  self::Contravariant<core::Null?>* contra = new self::Contravariant::•<core::Null?>();
+  self::contravariantListInfer<core::Null?>(new self::Contravariant::•<core::List<core::Null?>*>());
+  self::Invariant<dynamic>* inv = new self::Invariant::•<dynamic>();
+  self::invariantListInfer<core::Null?>(new self::Invariant::•<core::List<core::Null?>*>());
+}

tests/language_2/variance/variance_unconstrained_inference_test.dart

@@ -0,0 +1,26 @@
+// Copyright (c) 2019, 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.
+
+// Tests unconstrained inferencing with sound variance.
+
+// SharedOptions=--enable-experiment=variance
+
+class Covariant<out T> {}
+class Contravariant<in T> {}
+class Invariant<inout T> {}
+
+void covariantListInfer<T>(Covariant<List<T>> x) {}
+void contravariantListInfer<T>(Contravariant<List<T>> x) {}
+void invariantListInfer<T>(Invariant<List<T>> x) {}
+
+main() {
+  var cov = new Covariant();
+  covariantListInfer(Covariant());
+
+  var contra = new Contravariant();
+  contravariantListInfer(Contravariant());
+
+  var inv = new Invariant();
+  invariantListInfer(Invariant());
+}