fix #26120, sideways casts no longer supported

R=vsm@google.com

Review URL: https://codereview.chromium.org/2231273002 .

CHANGELOG.md

@@ -86,6 +86,9 @@
     }
     ```
 
+* Breaking change - sideways casts are no longer allowed
+    (SDK issue [26120](https://github.com/dart-lang/sdk/issues/26120)).
+
 ### Dart VM
 
 *   The dependency on BoringSSL has been rolled forward. Going forward, builds

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

@@ -354,8 +354,7 @@ class StrongTypeSystemImpl extends TypeSystem {
 
     // Don't allow implicit downcasts between function types
     // and call method objects, as these will almost always fail.
-    if ((fromType is FunctionType && getCallMethodType(toType) != null) ||
-        (toType is FunctionType && getCallMethodType(fromType) != null)) {
+    if (fromType is FunctionType && getCallMethodType(toType) != null) {
       return false;
     }
 
@@ -373,7 +372,7 @@ class StrongTypeSystemImpl extends TypeSystem {
 
     // If the subtype relation goes the other way, allow the implicit
     // downcast.
-    if (isSubtypeOf(toType, fromType) || toType.isAssignableTo(fromType)) {
+    if (isSubtypeOf(toType, fromType)) {
       // TODO(leafp,jmesserly): we emit warnings/hints for these in
       // src/task/strong/checker.dart, which is a bit inconsistent. That
       // code should be handled into places that use isAssignableTo, such as

pkg/analyzer/lib/src/task/strong/checker.dart

@@ -749,14 +749,10 @@ class CodeChecker extends RecursiveAstVisitor {
     // fromT <: toT, no coercion needed.
     if (rules.isSubtypeOf(from, to)) return;
 
-    // TODO(vsm): We can get rid of the second clause if we disallow
-    // all sideways casts - see TODO below.
-    // -------
     // Note: a function type is never assignable to a class per the Dart
     // spec - even if it has a compatible call method.  We disallow as
     // well for consistency.
-    if ((from is FunctionType && rules.getCallMethodType(to) != null) ||
-        (to is FunctionType && rules.getCallMethodType(from) != null)) {
+    if (from is FunctionType && rules.getCallMethodType(to) != null) {
       return;
     }
 
@@ -766,19 +762,9 @@ class CodeChecker extends RecursiveAstVisitor {
       return;
     }
 
-    // TODO(vsm): Once we have generic methods, we should delete this
-    // workaround.  These sideways casts are always ones we warn about
-    // - i.e., we think they are likely to fail at runtime.
-    // -------
-    // Downcast if toT <===> fromT
-    // The intention here is to allow casts that are sideways in the restricted
-    // type system, but allowed in the regular dart type system, since these
-    // are likely to succeed.  The canonical example is List<dynamic> and
-    // Iterable<T> for some concrete T (e.g. Object).  These are unrelated
-    // in the restricted system, but List<dynamic> <: Iterable<T> in dart.
-    if (from.isAssignableTo(to)) {
-      _recordImplicitCast(expr, from, to);
-    }
+    // Anything else is an illegal sideways cast.
+    // However, these will have been reported already in error_verifier, so we
+    // don't need to report them again.
   }
 
   void _checkFieldAccess(AstNode node, AstNode target, SimpleIdentifier field) {
@@ -1057,7 +1043,7 @@ class CodeChecker extends RecursiveAstVisitor {
     // toT <:_R fromT => to <: fromT
     // NB: classes with call methods are subtypes of function
     // types, but the function type is not assignable to the class
-    assert(toType.isSubtypeOf(fromType) || fromType.isAssignableTo(toType));
+    assert(toType.isSubtypeOf(fromType));
 
     // Inference "casts":
     if (expression is Literal || expression is FunctionExpression) {

pkg/analyzer/test/generated/type_system_test.dart

@@ -164,6 +164,7 @@ class LeastUpperBoundTest extends LeastUpperBoundTestBase {
     FunctionType expected = _functionType([objectType, numType, numType]);
     _checkLeastUpperBound(type1, type2, expected);
   }
+
   void test_nestedNestedFunctionsLubInnermostParamTypes() {
     FunctionType type1 = _functionType([
       _functionType([
@@ -620,7 +621,11 @@ class StrongAssignabilityTest {
       numType,
       bottomType
     ];
-    List<DartType> unrelated = <DartType>[intType, stringType, interfaceType,];
+    List<DartType> unrelated = <DartType>[
+      intType,
+      stringType,
+      interfaceType,
+    ];
 
     _checkGroups(doubleType,
         interassignable: interassignable, unrelated: unrelated);
@@ -750,7 +755,10 @@ class StrongAssignabilityTest {
       doubleType,
       bottomType
     ];
-    List<DartType> unrelated = <DartType>[stringType, interfaceType,];
+    List<DartType> unrelated = <DartType>[
+      stringType,
+      interfaceType,
+    ];
 
     _checkGroups(numType,
         interassignable: interassignable, unrelated: unrelated);
@@ -780,10 +788,12 @@ class StrongAssignabilityTest {
 
   void _checkCrossLattice(
       DartType top, DartType left, DartType right, DartType bottom) {
-    _checkGroups(top, interassignable: <DartType>[top, left, right, bottom]);
-    _checkGroups(left, interassignable: <DartType>[top, left, right, bottom]);
-    _checkGroups(right, interassignable: <DartType>[top, left, right, bottom]);
-    _checkGroups(bottom, interassignable: <DartType>[top, left, right, bottom]);
+    _checkGroups(top, interassignable: [top, left, right, bottom]);
+    _checkGroups(left,
+        interassignable: [top, left, bottom], unrelated: [right]);
+    _checkGroups(right,
+        interassignable: [top, right, bottom], unrelated: [left]);
+    _checkGroups(bottom, interassignable: [top, left, right, bottom]);
   }
 
   void _checkEquivalent(DartType type1, DartType type2) {