### Godot version

Beta 17, master

### System information

Windows 11

### Issue description

Godot gets a hard crash while attempting to code-complete for member `const` of a class.

### Steps to reproduce

```GDScript
enum E { V1 }

const e := E
```

And then type a `.` after the last `E`, which triggers code hints/auto-completion for subscripts based on `E` and causes a hard crash.

Does not happen for `var e := E.V1`.

### Minimal reproduction project

See above.
ERR_FAIL_COND_V_MSG(!p_target.is_set(), true, "Parser bug (please report): Trying to check compatibility of unset target type");
	ERR_FAIL_COND_V_MSG(!p_source.is_set(), true, "Parser bug (please report): Trying to check compatibility of unset value type");
	if (p_target.kind == GDScriptParser::DataType::VARIANT) {
		
		return true;
	}
	if (p_source.kind == GDScriptParser::DataType::VARIANT) {
		
		return true;
	}
	if (p_target.kind == GDScriptParser::DataType::BUILTIN) {
		bool valid = p_source.kind == GDScriptParser::DataType::BUILTIN && p_target.builtin_type == p_source.builtin_type;
		if (!valid && p_allow_implicit_conversion) {
			valid = Variant::can_convert_strict(p_source.builtin_type, p_target.builtin_type);
		}
		if (!valid && p_target.builtin_type == Variant::INT && p_source.kind == GDScriptParser::DataType::ENUM && !p_source.is_meta_type) {
			
			valid = true;
		}
		if (valid && p_target.builtin_type == Variant::ARRAY && p_source.builtin_type == Variant::ARRAY) {
			
			if (p_target.has_container_element_type() && p_source.has_container_element_type()) {
				valid = p_target.get_container_element_type() == p_source.get_container_element_type();
			}
		}
		return valid;
	}
	if (p_target.kind == GDScriptParser::DataType::ENUM) {