checking and inference of null coalescing operator, fixes #24564 and fixes #25387

[email protected]

Review URL: https://codereview.chromium.org/1575173005 .

Author: John Messerly

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

@@ -8435,7 +8435,11 @@ class ResolverVisitor extends ScopedVisitor {
       }
     } else {
       // TODO(leafp): Do downwards inference using the declared type
-      // of the binary operator.
+      // of the binary operator for other cases.
+      if (operatorType == TokenType.QUESTION_QUESTION) {
+        InferenceContext.setTypeFromNode(leftOperand, node);
+        InferenceContext.setTypeFromNode(rightOperand, node);
+      }
       safelyVisit(leftOperand);
       safelyVisit(rightOperand);
     }

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

@@ -330,7 +330,7 @@ class StaticTypeAnalyzer extends SimpleAstVisitor<Object> {
   @override
   Object visitBinaryExpression(BinaryExpression node) {
     if (node.operator.type == TokenType.QUESTION_QUESTION) {
-      // Evaluation of an if-null expresion e of the form e1 ?? e2 is
+      // Evaluation of an if-null expression e of the form e1 ?? e2 is
       // equivalent to the evaluation of the expression
       // ((x) => x == null ? e2 : x)(e1).  The static type of e is the least
       // upper bound of the static type of e1 and the static type of e2.

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

@@ -189,11 +189,12 @@ class CodeChecker extends RecursiveAstVisitor {
   @override
   void visitAssignmentExpression(AssignmentExpression node) {
     var token = node.operator;
-    if (token.type != TokenType.EQ) {
-      _checkCompoundAssignment(node);
-    } else {
+    if (token.type == TokenType.EQ ||
+        token.type == TokenType.QUESTION_QUESTION_EQ) {
       DartType staticType = _getStaticType(node.leftHandSide);
       checkAssignment(node.rightHandSide, staticType);
+    } else {
+      _checkCompoundAssignment(node);
     }
     node.visitChildren(this);
   }

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

@@ -1493,6 +1493,23 @@ void main() {
        '''
   });
 
+  testChecker('null coalescing operator', {
+    '/main.dart': '''
+          class A {}
+          class C<T> {}
+          main() {
+            A a, b;
+            a ??= new A();
+            b = b ?? new A();
+
+            // downwards inference
+            C<int> c, d;
+            c ??= /*info:INFERRED_TYPE_ALLOCATION*/new C();
+            d = d ?? /*info:INFERRED_TYPE_ALLOCATION*/new C();
+          }
+       '''
+  });
+
   testChecker('compound assignments', {
     '/main.dart': '''
           class A {