Implementation of null-aware operators.

pkg/compiler/lib/src/resolution/members.dart

 // Copyright (c) 2012, 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.
 
 part of resolution;
 
 abstract class TreeElements {
   AnalyzableElement get analyzedElement;
   Iterable<Node> get superUses;
 
@@ skipping 2597 matching lines @@
         registry.registerAssert(node);
         registry.registerSendStructure(node, sendStructure);
         return const AssertResult();
       }
 
       return node.selector.accept(this);
     }
 
     var oldCategory = allowedCategory;
     allowedCategory |= ElementCategory.PREFIX | ElementCategory.SUPER;
+
+    bool oldSendIsMemberAccess = sendIsMemberAccess;
+    int oldAllowedCategory = allowedCategory;
+
+    // Conditional sends like `e?.foo` treat the receiver as an expression.  So
+    // `C?.foo` needs to be treated like `(C).foo`, not like C.foo. Prefixes and
+    // super are not allowed on their own in that context.
+    if (node.isConditional) {
+      sendIsMemberAccess = false;
+      allowedCategory =
+          ElementCategory.VARIABLE |
+          ElementCategory.FUNCTION |
+          ElementCategory.IMPLIES_TYPE;
+    }
     ResolutionResult resolvedReceiver = visit(node.receiver);
+    if (node.isConditional) {
+      sendIsMemberAccess = oldSendIsMemberAccess;
+      allowedCategory = oldAllowedCategory;
+    }
+
     allowedCategory = oldCategory;
 
     Element target;
     String name = node.selector.asIdentifier().source;
     if (identical(name, 'this')) {
       // TODO(ahe): Why is this using GENERIC?
       error(node.selector, MessageKind.GENERIC,
             {'text': "expected an identifier"});
       return null;
     } else if (node.isSuperCall) {
@@ skipping 104 matching lines @@
     if (node.isIndex) {
       return isSet ? new Selector.indexSet() : new Selector.index();
     }
 
     if (node.isOperator) {
       String source = node.selector.asOperator().source;
       String string = source;
       if (identical(string, '!') ||
           identical(string, '&&') || identical(string, '||') ||
           identical(string, 'is') || identical(string, 'as') ||
-          identical(string, '?') ||
+          identical(string, '?') || identical(string, '??') ||
           identical(string, '>>>')) {
         return null;
       }
       String op = source;
       if (!isUserDefinableOperator(source)) {
         op = Elements.mapToUserOperatorOrNull(source);
       }
       if (op == null) {
         // Unsupported operator. An error has been reported during parsing.
         return new Selector.call(
@@ skipping 184 matching lines @@
           registry.registerAsCheck(type);
         }
         resolvedArguments = true;
       } else if (identical(operatorString, '&&')) {
         doInPromotionScope(node.arguments.head,
             () => resolveArguments(node.argumentsNode));
         sendStructure = const LogicalAndStructure();
         resolvedArguments = true;
       } else if (operatorString == '||') {
         sendStructure = const LogicalOrStructure();
+      } else if (operatorString == '??') {
+        sendStructure = const IfNullStructure();
       }
       if (sendStructure != null) {
         registry.registerSendStructure(node, sendStructure);
       }
     }
 
     if (!resolvedArguments) {
       resolveArguments(node.argumentsNode);
     }
 
@@ skipping 94 matching lines @@
                 MessageKind.CANNOT_RESOLVE_SETTER, const {});
             registry.registerThrowNoSuchMethod();
           }
         }
         if (isComplex && getter == null && !inInstanceContext) {
           getter = reportAndCreateErroneousElement(node.selector, field.name,
               MessageKind.CANNOT_RESOLVE_GETTER, const {});
           registry.registerThrowNoSuchMethod();
         }
       } else if (target.impliesType) {
-        setter = reportAndCreateErroneousElement(node.selector, target.name,
-            MessageKind.ASSIGNING_TYPE, const {});
-        registry.registerThrowNoSuchMethod();
+        if (node.isIfNullAssignment) {
+          setter = reportAndCreateErroneousElement(node.selector, target.name,
+              MessageKind.IF_NULL_ASSIGNING_TYPE, const {});
+          // In this case, no assignment happens, the rest of the compiler can
+          // treat the expression `C ??= e` as if it's just reading `C`.
+        } else {
+          setter = reportAndCreateErroneousElement(node.selector, target.name,
+              MessageKind.ASSIGNING_TYPE, const {});
+          registry.registerThrowNoSuchMethod();
+        }
         registerTypeLiteralAccess(node, target);
       } else if (target.isFinal || target.isConst) {
         if (Elements.isStaticOrTopLevelField(target) || target.isLocal) {
           setter = reportAndCreateErroneousElement(
               node.selector, target.name, MessageKind.CANNOT_RESOLVE_SETTER,
               const {});
         } else if (node.isSuperCall) {
           setter = reportAndCreateErroneousElement(
               node.selector, target.name, MessageKind.SETTER_NOT_FOUND_IN_SUPER,
               {'name': target.name, 'className': currentClass.name});
@@ skipping 2119 matching lines @@
 }
 
 /// The result for the resolution of the `assert` method.
 class AssertResult implements ResolutionResult {
   const AssertResult();
 
   Element get element => null;
 
   String toString() => 'AssertResult()';
 }