Add test for extracting a Java AST with an error expression

Also note that ErrorExpr can occur outside upgrade/downgrade scripts

Author: smowton

java/ql/lib/semmle/code/java/Expr.qll

@@ -276,7 +276,8 @@ class ExprParent extends @exprparent, Top { }
  * An error expression.
  *
  * These may be generated by upgrade or downgrade scripts when databases
- * cannot be fully converted.
+ * cannot be fully converted, or generated by the extractor when extracting
+ * source code containing errors.
  */
 class ErrorExpr extends Expr, @errorexpr {
   override string toString() { result = "<error expr>" }

java/ql/test/library-tests/errorexpr/PrintAst.expected

@@ -0,0 +1,30 @@
+Test.java:
+#    0| [CompilationUnit] Test
+#    1|   1: [Class] Test
+#    3|     2: [Method] yield
+#    3|       3: [TypeAccess] int
+#-----|       4: (Parameters)
+#    3|         0: [Parameter] x
+#    3|           0: [TypeAccess] int
+#    3|       5: [BlockStmt] { ... }
+#    3|         0: [ReturnStmt] return ...
+#    3|           0: [VarAccess] x
+#    5|     3: [Method] secondCall
+#    5|       3: [TypeAccess] void
+#    5|       5: [BlockStmt] { ... }
+#    7|     4: [Method] yieldWrapper
+#    7|       3: [TypeAccess] int
+#-----|       4: (Parameters)
+#    7|         0: [Parameter] x
+#    7|           0: [TypeAccess] int
+#    7|       5: [BlockStmt] { ... }
+#    8|         0: [LocalVariableDeclStmt] var ...;
+#    8|           0: [TypeAccess] int
+#    8|           1: [LocalVariableDeclExpr] ret
+#    8|             0: [ErrorExpr] <error expr>
+#    8|               0: [MethodAccess] yield(...)
+#    8|                 0: [VarAccess] x
+#    9|         1: [ExprStmt] <Expr>;
+#    9|           0: [MethodAccess] secondCall(...)
+#   10|         2: [ReturnStmt] return ...
+#   10|           0: [VarAccess] ret

java/ql/test/library-tests/errorexpr/PrintAst.qlref

@@ -0,0 +1 @@
+semmle/code/java/PrintAst.ql

java/ql/test/library-tests/errorexpr/Test.java

@@ -0,0 +1,13 @@
+public class Test {
+
+  public int yield(int x) { return x; }
+
+  public void secondCall() { }
+
+  public int yieldWrapper(int x) { 
+    int ret = yield(x); 
+    secondCall();
+    return ret;
+  }
+
+}

java/ql/test/library-tests/errorexpr/options

@@ -0,0 +1 @@
+//semmle-extractor-options: --no-strict-javac-errors --javac-args -source 17 -target 17