Don't allow SBValue::Cast to cast from a smaller type to a larger,

as we don't in general know where the extra data should come from.

Differential Revision: https://reviews.llvm.org/D153657

lldb/test/API/python_api/value/TestValueAPI.py

@@ -146,6 +146,19 @@ class ValueAPITestCase(TestBase):
         self.assertTrue(val_s.GetChildMemberWithName("a").AddressOf(), VALID_VARIABLE)
         self.assertTrue(val_a.Cast(val_i.GetType()).AddressOf(), VALID_VARIABLE)
 
+        # Test some other cases of the Cast API.  We allow casts from one struct type
+        # to another, which is a little weird, but we don't support casting from a
+        # smaller type to a larger as we often wouldn't know how to get the extra data:
+        val_f = target.EvaluateExpression("f")
+        bad_cast = val_s.Cast(val_f.GetType())
+        self.assertFailure(bad_cast.GetError(),
+                           "Can only cast to a type that is equal to or smaller than the orignal type.")
+        weird_cast = val_f.Cast(val_s.GetType())
+        self.assertSuccess(weird_cast.GetError(),
+                        "Can cast from a larger to a smaller")
+        self.assertEqual(weird_cast.GetChildMemberWithName("a").GetValueAsSigned(0), 33,
+                         "Got the right value")
+
         # Check that lldb.value implements truth testing.
         self.assertFalse(lldb.value(frame0.FindVariable("bogus")))
         self.assertTrue(lldb.value(frame0.FindVariable("uinthex")))

lldb/test/API/python_api/value/main.c

@@ -29,6 +29,13 @@ struct MyStruct
   int b;
 };
 
+struct MyBiggerStruct
+{
+  int a;
+  int b;
+  int c;
+};
+
 int main (int argc, char const *argv[])
 {
     uint32_t uinthex = 0xE0A35F10;
@@ -37,6 +44,7 @@ int main (int argc, char const *argv[])
     int i;
     MyInt a = 12345;
     struct MyStruct s = { 11, 22 };
+    struct MyBiggerStruct f = { 33, 44, 55 }; 
     int *my_int_ptr = &g_my_int;
     printf("my_int_ptr points to location %p\n", my_int_ptr);
     const char **str_ptr = days_of_week;