[lldb] Support "dereferencing" std::optional in frame var

lldb/source/Plugins/Language/CPlusPlus/GenericOptional.cpp

@@ -37,6 +37,8 @@ class GenericOptionalFrontend : public SyntheticChildrenFrontEnd {
   GenericOptionalFrontend(ValueObject &valobj, StdLib stdlib);
 
   size_t GetIndexOfChildWithName(ConstString name) override {
+    if (name == "$$dereference$$")
+      return 0;
     return formatters::ExtractIndexFromString(name.GetCString());
   }
 

lldb/test/API/functionalities/data-formatter/data-formatter-stl/generic/optional/TestDataFormatterGenericOptional.py

@@ -79,6 +79,18 @@ def cleanup():
             ],
         )
 
+        self.expect_var_path("*number_engaged", value="42")
+        self.expect_var_path("*x", children=[ValueCheck(name="x", value="42")])
+        self.expect_var_path("x->x", value="42")
+
+        # The error message could use some improvement, but at least we can
+        # check we don't crash.
+        self.expect(
+            "frame variable *number_not_engaged",
+            error=True,
+            substrs=["not a pointer or reference type"],
+        )
+
     @add_test_categories(["libc++"])
     ## Clang 7.0 is the oldest Clang that can reliably parse newer libc++ versions
     ## with -std=c++17.

lldb/test/API/functionalities/data-formatter/data-formatter-stl/generic/optional/main.cpp

@@ -12,6 +12,10 @@
 #define HAVE_OPTIONAL 0
 #endif
 
+struct X {
+  int x;
+};
+
 int main() {
   bool has_optional = HAVE_OPTIONAL;
 
@@ -25,6 +29,7 @@ int main() {
 
   optional_int number_not_engaged;
   optional_int number_engaged = 42;
+  std::optional<X> x = X{42};
 
   printf("%d\n", *number_engaged);