fix #27110, mark implicit downcasts for op assign

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

Index: pkg/analyzer/lib/src/task/strong/checker.dart
diff --git a/pkg/analyzer/lib/src/task/strong/checker.dart b/pkg/analyzer/lib/src/task/strong/checker.dart
index d0b55fed93a80bdca1f0ba935f98e22dae82274d..245443683ee7574c5b5325a891233bd9eccb1c96 100644
--- a/pkg/analyzer/lib/src/task/strong/checker.dart
+++ b/pkg/analyzer/lib/src/task/strong/checker.dart
@@ -186,9 +186,7 @@ class CodeChecker extends RecursiveAstVisitor {
         // so no need to insert an error for this here.
         continue;
       }
-      DartType expectedType = _elementType(element);
-      if (expectedType == null) expectedType = DynamicTypeImpl.instance;
-      checkArgument(arg, expectedType);
+      checkArgument(arg, _elementType(element));
     }
   }
 
@@ -701,34 +699,28 @@ class CodeChecker extends RecursiveAstVisitor {
       assert(functionType.namedParameterTypes.isEmpty);
       assert(functionType.optionalParameterTypes.isEmpty);
 
-      // Check the LHS type.
+      // Refine the return type.
       var rhsType = _getDefiniteType(expr.rightHandSide);
       var lhsType = _getDefiniteType(expr.leftHandSide);
       var returnType = rules.refineBinaryExpressionType(
           typeProvider, lhsType, op, rhsType, functionType.returnType);
 
-      if (!rules.isSubtypeOf(returnType, lhsType)) {
-        final numType = typeProvider.numType;
-        // TODO(jmesserly): this seems to duplicate logic in StaticTypeAnalyzer.
-        // Try to fix up the numerical case if possible.
-        if (rules.isSubtypeOf(lhsType, numType) &&
-            rules.isSubtypeOf(lhsType, rhsType)) {
-          // This is also slightly different from spec, but allows us to keep
-          // compound operators in the int += num and num += dynamic cases.
-          _recordImplicitCast(expr.rightHandSide, rhsType, lhsType);
-        } else {
-          // TODO(jmesserly): this results in a duplicate error, because
-          // ErrorVerifier also reports it.
-          _recordMessage(expr, StrongModeCode.STATIC_TYPE_ERROR,
-              [expr, returnType, lhsType]);
-        }
-      } else {
-        // Check the RHS type.
-        //
-        // This is only needed if we didn't already need a cast, and avoids
-        // emitting two messages for the same expression.
-        _checkDowncast(expr.rightHandSide, paramTypes.first);
-      }
+      // Check the argument for an implicit cast.
+      _checkDowncast(expr.rightHandSide, paramTypes[0], from: rhsType);

[Jennifer Messerly, 2016/08/25 22:27:15]

this is my attempt to fix by simplification :)

[Leaf, 2016/08/25 22:35:34]

:)

+
+      // Check the return type for an implicit cast.
+      //
+      // If needed, mark the left size to indicate a down cast when we assign

[Leaf, 2016/08/25 22:35:34]

size -> side?

+      // back to it. so these two implicit casts are equivalent:
+      //
+      //     y = /*implicit cast*/(y + 42);
+      //     y/*implicit cast*/ += 42;
+      //
+      // TODO(jmesserly): this is an unambiguous way to represent it, but it's

[Jennifer Messerly, 2016/08/25 22:27:15]

https://github.com/dart-lang/dev_compiler/issues/632 for the DDC side

+      // a bit sneaky. We can't use the rightHandSide because that could be a
+      // downcast on its own, and we can't use the entire expression because its
+      // result value could used and then implicitly downcast.
+      _checkDowncast(expr.leftHandSide, lhsType, from: returnType);
     }
   }