clang::Cursor::template_arg_value should behave defensively

[fitzgen]

We should check that the cursor's referent is a template specialization and that i is not out of bounds. If either of those things don't hold, we should return None and avoid the FFI call. There is no way to check the return value for validity if we mess this up: the behavior is undefined! Otherwise, we should make the FFI call and return Some.

This would be a nice removal of a foot gun.

http://clang.llvm.org/doxygen/group__CINDEX__TYPES.html#ga46e363545effaa0794a2ba4bcfae1fe3

It is undefined to call this function on a CXCursor that does not represent a FunctionDecl or whose I'th template argument is not an integral value.

I can mentor whoever would like to work on this bug.

[highfive]

Please make a comment here if you intend to work on this issue. Thank you!

[nikhilshagri]

I'll take a shot at this one!

[fitzgen]

Great! Let me know if you run into any road blocks :)