Fix warnings on implicit casts

R=leafp@google.com

Review URL: https://codereview.chromium.org/1418653003 .

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

@@ -154,11 +154,28 @@ abstract class DownCast extends CoercionInfo {
       return new StaticTypeError(rules, expression, toT, reason: reason);
     }
 
+    // TODO(vsm): Change this to an assert when we have generic methods and
+    // fix TypeRules._coerceTo to disallow implicit sideways casts.
+    if (!rules.isSubTypeOf(toT, fromT)) {
+      assert(toT.isSubtypeOf(fromT) || fromT.isAssignableTo(toT));
+      return new DownCastComposite(rules, expression, cast);
+    }
+
     // Composite cast: these are more likely to fail.
     if (!rules.isGroundType(toT)) {
       // This cast is (probably) due to our different treatment of dynamic.
       // It may be more likely to fail at runtime.
-      return new DownCastComposite(rules, expression, cast);
+      if (fromT is InterfaceType) {
+        // For class types, we'd like to allow non-generic down casts, e.g.,
+        // Iterable<T> to List<T>.  The intuition here is that raw (generic)
+        // casts are problematic, and we should complain about those.
+        var typeArgs = fromT.typeArguments;
+        if (typeArgs.isEmpty || typeArgs.any((t) => t.isDynamic)) {
+          return new DownCastComposite(rules, expression, cast);
+        }
+      } else {
+        return new DownCastComposite(rules, expression, cast);
+      }
     }
 
     // Dynamic cast

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

@@ -376,8 +376,6 @@ class TypeRules {
 
   // Produce a coercion which coerces something of type fromT
   // to something of type toT.
-  // If wrap is true and both are function types, a closure
-  // wrapper coercion is produced using _wrapTo (see above)
   // Returns the error coercion if the types cannot be coerced
   // according to our current criteria.
   Coercion _coerceTo(DartType fromT, DartType toT) {
@@ -387,11 +385,12 @@ class TypeRules {
     // fromT <: toT, no coercion needed
     if (isSubTypeOf(fromT, toT)) return Coercion.identity(toT);
 
-    // For now, reject conversions between function types and
-    // call method objects.  We could choose to allow casts here.
-    // Wrapping a function type to assign it to a call method
-    // object will never succeed.  Wrapping the other way could
-    // be allowed.
+    // 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 ((fromT is FunctionType && getCallMethodType(toT) != null) ||
         (toT is FunctionType && getCallMethodType(fromT) != null)) {
       return Coercion.error();
@@ -400,6 +399,10 @@ class TypeRules {
     // Downcast if toT <: fromT
     if (isSubTypeOf(toT, fromT)) return Coercion.cast(fromT, toT);
 
+    // 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
@@ -409,6 +412,7 @@ class TypeRules {
     if (fromT.isAssignableTo(toT)) {
       return Coercion.cast(fromT, toT);
     }
+
     return Coercion.error();
   }
 

pkg/analyzer/test/src/task/strong/checker_test.dart

@@ -984,7 +984,7 @@ void main() {
           lOfAs = /*severe:StaticTypeError*/mOfOs;
           lOfAs = mOfAs;
           lOfAs = /*warning:DownCastComposite*/lOfDs;
-          lOfAs = /*warning:DownCastComposite*/lOfOs;
+          lOfAs = /*info:DownCastImplicit*/lOfOs;
           lOfAs = lOfAs;
         }
         {
@@ -993,7 +993,7 @@ void main() {
           mOfDs = mOfAs;
           mOfDs = /*info:DownCastImplicit*/lOfDs;
           mOfDs = /*info:DownCastImplicit*/lOfOs;
-          mOfDs = /*info:DownCastImplicit*/lOfAs;
+          mOfDs = /*warning:DownCastComposite*/lOfAs;
         }
         {
           mOfOs = mOfDs;
@@ -1005,11 +1005,11 @@ void main() {
         }
         {
           mOfAs = /*warning:DownCastComposite*/mOfDs;
-          mOfAs = /*warning:DownCastComposite*/mOfOs;
+          mOfAs = /*info:DownCastImplicit*/mOfOs;
           mOfAs = mOfAs;
           mOfAs = /*warning:DownCastComposite*/lOfDs;
-          mOfAs = /*warning:DownCastComposite*/lOfOs;
-          mOfAs = /*warning:DownCastComposite*/lOfAs;
+          mOfAs = /*info:DownCastImplicit*/lOfOs;
+          mOfAs = /*info:DownCastImplicit*/lOfAs;
         }
 
       }