Exclude ?? operator from true/false literal check in createFlowCondition

Author: ahejlsberg

src/compiler/binder.ts

@@ -951,11 +951,10 @@ namespace ts {
             if (!expression) {
                 return flags & FlowFlags.TrueCondition ? antecedent : unreachableFlow;
             }
-            if (expression.kind === SyntaxKind.TrueKeyword && flags & FlowFlags.FalseCondition ||
-                expression.kind === SyntaxKind.FalseKeyword && flags & FlowFlags.TrueCondition) {
-                if (!isExpressionOfOptionalChainRoot(expression)) {
-                    return unreachableFlow;
-                }
+            if ((expression.kind === SyntaxKind.TrueKeyword && flags & FlowFlags.FalseCondition ||
+                expression.kind === SyntaxKind.FalseKeyword && flags & FlowFlags.TrueCondition) &&
+                !isExpressionOfOptionalChainRoot(expression) && !isQuestionQuestionExpression(expression.parent)) {
+                return unreachableFlow;
             }
             if (!isNarrowingExpression(expression)) {
                 return antecedent;

src/compiler/utilities.ts

@@ -5953,6 +5953,10 @@ namespace ts {
         return isOptionalChainRoot(node.parent) && node.parent.expression === node;
     }
 
+    export function isQuestionQuestionExpression(node: Node) {
+        return node.kind === SyntaxKind.BinaryExpression && (<BinaryExpression>node).operatorToken.kind === SyntaxKind.QuestionQuestionToken;
+    }
+
     export function isNewExpression(node: Node): node is NewExpression {
         return node.kind === SyntaxKind.NewExpression;
     }