Fixed https://bugs.eclipse.org/bugs/show_bug.cgi?id=67834. Thanks to Dave for the JUnit test.

core/org.eclipse.cdt.core.tests/parser/org/eclipse/cdt/core/parser/tests/ScannerTestCase.java

@@ -674,6 +674,29 @@ public class ScannerTestCase extends BaseScannerTest
 		}
 	}
 
+	public void testBug67834() throws Exception {
+		initializeScanner(
+				"#if ! BAR\n" + //$NON-NLS-1$
+				"foo\n" + //$NON-NLS-1$
+				"#else\n" + //$NON-NLS-1$
+				"bar\n" + //$NON-NLS-1$
+				"#endif\n"  //$NON-NLS-1$
+				); //$NON-NLS-1$
+		validateIdentifier("foo"); //$NON-NLS-1$
+		validateEOF();
+		validateBalance();
+		
+		initializeScanner(
+				"#if ! (BAR)\n" + //$NON-NLS-1$
+				"foo\n" + //$NON-NLS-1$
+				"#else\n" + //$NON-NLS-1$
+				"bar\n" + //$NON-NLS-1$
+				"#endif\n"  //$NON-NLS-1$
+				); //$NON-NLS-1$
+		validateIdentifier("foo"); //$NON-NLS-1$
+		validateEOF();
+		validateBalance();
+	}
 	public void testPreprocessorMacros()
 	{
 		try

core/org.eclipse.cdt.core/parser/org/eclipse/cdt/internal/core/parser/ExpressionParser.java

@@ -1408,12 +1408,20 @@ public class ExpressionParser implements IExpressionParser, IParserData {
 						typeId.freeReferences(astFactory.getReferenceManager());
 					throw bte;
 				}
-				mark = null; // clean up mark so that we can garbage collect
+				
 				if (templateIdScopes != null) {
 					templateIdScopes.pop();
 					popped = true;
 				}
 				IASTExpression castExpression = castExpression(scope, kind, key);
+				if( castExpression != null && castExpression.getExpressionKind() == IASTExpression.Kind.PRIMARY_EMPTY )
+				{
+					backup( mark );
+					if (typeId != null)
+						typeId.freeReferences(astFactory.getReferenceManager());
+					return unaryExpression(scope, kind, key);
+				}
+				mark = null; // clean up mark so that we can garbage collect
 				try {
 					return astFactory.createExpression(scope,
 							IASTExpression.Kind.CASTEXPRESSION, castExpression,