Working Around a Visual Studio Internal Compiler Error With Nested Templated Variables

Question

I'm trying to write code that will let me index into the parameter types of a function:

template <typename R, typename... ARGS>
R function_return(R(*)(ARGS...));

template <typename R, typename... ARGS>
std::tuple<ARGS...> function_parameters(R(*)(ARGS...));

template <int I, typename T>
using get_type = typename std::conditional_t<(I < 0), std::tuple_element<static_cast<int>(std::tuple_size_v<T>) + I, T>, std::tuple_element<I, T>>::type;

template <int I, typename T>
using parameter_type = get_type<I, decltype(function_parameters(std::declval<T>()))>;

Live Example (ICE under VS) Live Example (working on GCC)

But when I try to use this on visual-studio-2017 I get an internal compiler error:

fatal error C1001: An internal error has occurred in the compiler.

Is there another way that I can do this which might work around the internal compiler error?

Filed a bug ticket already? That said, there's one thing in your code that I'd avoid: Use of `ALL_UPPERCASE` for regular identifiers. If any win32 header somehow leaks into that code, you get a bunch of macros with that and some of them are plain evil. — Ulrich Eckhardt, May 30 '19 at 13:50
Definitely. Prefer `ReturnType` and `ArgType` or something like that — Lightness Races in Orbit, May 30 '19 at 13:55
@UlrichEckhardt The live example gives output and obvs doesn't ICE. The bug is in VS not in GCC 6.3 — Lightness Races in Orbit, May 30 '19 at 13:58
@UlrichEckhardt That is the correct behavior. I'm just demonstrating that I'm writing good code as far as gcc is concerned. The ICE is only on VS2017. — Jonathan Mee, May 30 '19 at 13:59
Gotta say, though, [it works for me](https://godbolt.org/z/0lLJ75). — Lightness Races in Orbit, May 30 '19 at 14:00
You can show the example on godbolt.org or upload to some other online compiler site that actually uses MSVC. — n. m. could be an AI, May 30 '19 at 14:00
@LightnessRacesinOrbit Perhaps it is a subversion issue. I'm stuck on 15.6.7. — Jonathan Mee, May 30 '19 at 14:08
It absolutely is so, as shown. (You really mean 19.13, [the version number of the compiler itself](https://en.wikipedia.org/wiki/Microsoft_Visual_C%2B%2B#Internal_version_numbering) rather than of the IDE) — Lightness Races in Orbit, May 30 '19 at 14:10

Angew is no longer proud of SO · Answer 1 · 2019-05-30T14:04:02.417

It probably depends on the exact (sub)version of VS2017, as mine does not produce an ICE on the code. However, the code is still problematic, as it can potentially instantiate std::tuple_element<2147483647, T> or something similar. You need to make sure only the correct branch is ever evaluated. Replace your definition of get_type with this:

template <int I, typename T, bool negative = (I < 0)>
struct get_type_impl;

template <int I, typename T>
struct get_type_impl<I, T, true>
{
  using type = typename std::tuple_element<static_cast<int>(std::tuple_size<T>::value) + I, T>::type;
};

template <int I, typename T>
struct get_type_impl<I, T, false>
{
  using type = typename std::tuple_element<I, T>::type;
};

template <int I, typename T>
using get_type = typename get_type_impl<I, T>::type;

This works for me on my VS 2017 (cl version 19.12)

Can confirm [this fixes the problem on the affected version](https://godbolt.org/z/QBP2JL). Good job! — Lightness Races in Orbit, May 30 '19 at 14:03

score 1 · Accepted Answer · answered May 30 '19 at 14:03

As mentioned here it seems like visual-studio-2017 struggles with templated using statements being passed to each other. (I'm seeing the internal compiler error on 15.6.7; as mentioned here this may have been fixed by patches.)

I've been able to work around it by capturing all the functionality in a single using statement:

template <typename R, typename... ARGS>
R function_return(R(*)(ARGS...));

template <typename R, typename... ARGS>
std::tuple<ARGS...> function_parameters(R(*)(ARGS...));

template <int I, typename T, typename X = decltype(function_parameters(std::declval<T>()))>
using parameter_type = typename std::conditional_t<(I < 0), std::tuple_element<static_cast<int>(std::tuple_size_v<X>) + I, X>, std::tuple_element<I, X>>::type;

Working Around a Visual Studio Internal Compiler Error With Nested Templated Variables

2 Answers2