Don't take the address of an xvalue when printing an expr result

[teemperor]

Summary

If we have an xvalue here, we will always hit the err_typecheck_invalid_lvalue_addrof error
in 'Sema::CheckAddressOfOperand' when trying to take the address of the result. This patch
uses the fallback code path where we store the result in a local variable instead when we hit
this case.

Fixes rdar://problem/40613277

[teemperor]

• Needs a test

[teemperor]

• Added test case

[teemperor]

• Removed now unnecessary brackets.

[friss]

packages/Python/lldbsuite/test/expression_command/xvalue/TestXValuePrinting.py

33

Could you use a different value than 0? Getting a random zero from memory seems to easy. It would be better to test a real string.

[teemperor]

• Generalized test case a bit more and no longer checking only for 0.

[teemperor]

• Removed unnecessary include from the test.

[vsk]

Thanks, LGTM. This looks pretty cut and dry. The evaluator shouldn't try to take the address of an rvalue.

packages/Python/lldbsuite/test/expression_command/xvalue/Makefile

+LEVEL = ../../make
+
+CXX_SOURCES := main.cpp
+
+include $(LEVEL)/Makefile.rules

packages/Python/lldbsuite/test/expression_command/xvalue/TestXValuePrinting.py

+from __future__ import print_function
+
+
+import lldb
+from lldbsuite.test.decorators import *
+from lldbsuite.test.lldbtest import *
+from lldbsuite.test import lldbutil
+
+
+class ExprXValuePrintingTestCase(TestBase):
+
+    mydir = TestBase.compute_mydir(__file__)
+
+    def setUp(self):
+        # Call super's setUp().
+        TestBase.setUp(self)
+
+        self.main_source = "main.cpp"
+        self.main_source_spec = lldb.SBFileSpec(self.main_source)
+
+    def do_test(self, dictionary=None):
+        """Printing an xvalue should work."""
+        self.build(dictionary=dictionary)
+
+        (target, process, thread, bkpt) = lldbutil.run_to_source_breakpoint(self, 
+                                          '// Break here', self.main_source_spec)
+        frame = thread.GetFrameAtIndex(0)
+
+        value = frame.EvaluateExpression("foo().data")
+        self.assertTrue(value.IsValid())
+        self.assertTrue(value.GetError().Success())
+        self.assertEqual(value.GetValueAsUnsigned(), 0)
+
+    @expectedFailureAll(oslist=["windows"], bugnumber="llvm.org/pr21765")
+    def test(self):
+        self.do_test()
+

packages/Python/lldbsuite/test/expression_command/xvalue/main.cpp

+#include <stddef.h>
+
+struct StringRef
+{
+  const char *data = 0;
+};
+
+StringRef foo() { return StringRef(); }
+
+int main(int argc, char const *argv[])
+{
+  const char *something = foo().data;
+  return 0; // Break here
+}

source/Plugins/ExpressionParser/Clang/ASTResultSynthesizer.cpp

bool ASTResultSynthesizer::SynthesizeBodyResult(CompoundStmt *Body,
   //     are excised.
   //
   //   - During materialization, $0 (the result persistent variable) is
   //   populated with the location
   //     of a newly-allocated area of memory.
   //
   //   - During dematerialization, $0 is ignored.
 
-  bool is_lvalue = (last_expr->getValueKind() == VK_LValue ||
-                    last_expr->getValueKind() == VK_XValue) &&
+  bool is_lvalue = (last_expr->getValueKind() == VK_LValue) &&
                    (last_expr->getObjectKind() == OK_Ordinary);
 
   QualType expr_qual_type = last_expr->getType();
   const clang::Type *expr_type = expr_qual_type.getTypePtr();
 
   if (!expr_type)
     return false;