Make LUB algorithm aware of non-null types

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

 // Copyright (c) 2015, 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.
 
 // TODO(jmesserly): this was ported from package:dev_compiler, and needs to be
 // refactored to fit into analyzer.
 library analyzer.src.task.strong.checker;
 
 import 'package:analyzer/analyzer.dart';
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 144 matching lines @@
       DartType expectedType = _elementType(element);
       if (expectedType == null) expectedType = DynamicTypeImpl.instance;
       checkArgument(arg, expectedType);
     }
   }
 
   void checkAssignment(Expression expr, DartType type) {
     if (expr is ParenthesizedExpression) {
       checkAssignment(expr.expression, type);
     } else {
-      _checkDowncast(expr, type);
-      _checkNonNullAssignment(expr, type);
+      if (_checkNonNullAssignment(expr, type)) {
+        _checkDowncast(expr, type);
+      }
     }
   }
 
   /// Analyzer checks boolean conversions, but we need to check too, because
   /// it uses the default assignability rules that allow `dynamic` and `Object`
   /// to be assigned to bool with no message.
   void checkBoolean(Expression expr) =>
       checkAssignment(expr, typeProvider.boolType);
 
   void checkFunctionApplication(
@@ skipping 655 matching lines @@
       return;
     }
     if (declElement.enclosingElement is ExecutableElement) {
       // Variable declaration is inside a function or method, so it's safe.
       return;
     }
     _recordMessage(node, StrongModeCode.UNSAFE_BLOCK_CLOSURE_INFERENCE,
         [declElement.name]);
   }
 
-  void _checkNonNullAssignment(Expression expression, DartType type) {
+  /// Checks if the assignment is valid with respect to non-nullable types.
+  /// Returns `false` if a nullable expression is assigned to a variable of
+  /// non-nullable type and `true` otherwise.
+  bool _checkNonNullAssignment(Expression expression, DartType type) {
     var exprType = expression.staticType;
     if (rules.isNonNullableType(type) && !rules.isNonNullableType(exprType)) {

[Leaf, 2016/08/03 23:45:07]

I'd suggest a helper "isNullableType", to avoid double negation bugs.

[stanm, 2016/08/04 00:31:15]

Good idea, thanks!

       _recordMessage(expression, StaticTypeWarningCode.INVALID_ASSIGNMENT,
           [exprType, type]);
+      return false;
     }
+    return true;
   }
 
   void _checkReturnOrYield(Expression expression, AstNode node,
       {bool yieldStar: false}) {
     FunctionBody body = node.getAncestor((n) => n is FunctionBody);
     var type = _getExpectedReturnType(body, yieldStar: yieldStar);
     if (type == null) {
       // We have a type mismatch: the async/async*/sync* modifier does
       // not match the return or yield type.  We should have already gotten an
       // analyzer error in this case.
@@ skipping 631 matching lines @@
     var visited = new Set<InterfaceType>();
     do {
       visited.add(current);
       current.mixins.reversed.forEach(
           (m) => _checkIndividualOverridesFromClass(node, m, seen, true));
       _checkIndividualOverridesFromClass(node, current.superclass, seen, true);
       current = current.superclass;
     } while (!current.isObject && !visited.contains(current));
   }
 }