Demonstrating an ICE in Visual Studio 15.5 and 15.6 pre1.

ice_vs_15.5_15.6_if_constexpr.cpp

//this code causes an internal compiler error in Visual Studio 2017.
//in 15.5 and 15.6 the ICE occurs in 'f:\dd\vctools\compiler\cxxfe\sl\p1\c\parsetree.cpp' on line 1692.
//in the MSVC dailies (19.13.26014.0 used for this test),
//the ICE occurs in 'f:\dd\vctools\compiler\cxxfe\sl\p1\c\parsetree.cpp' on line 1711.
#include <string>
#include <sstream>
#include <iostream>
#include <utility>

template <class CHAR>
using basic_string_t = std::basic_string<CHAR, std::char_traits<CHAR>, std::allocator<CHAR>>;
template <class CHAR>
using basic_ostringstream_t = std::basic_ostringstream<CHAR, std::char_traits<CHAR>, std::allocator<CHAR>>;

namespace Detail
{
	template<typename CHAR, typename ARG, typename... ARGS>
	inline void Concatenate(basic_ostringstream_t<CHAR>& ss,
		ARG&& arg, ARGS&&... args) noexcept
	{
		ss << std::forward<ARG>(arg);
		if constexpr (sizeof...(ARGS) > 0)
			Concatenate<CHAR>(ss, std::forward<ARGS>(args)...);
	}
}

template<typename CHAR = char, typename... ARGS>
basic_string_t<CHAR> Concatenate(ARGS&&... args) noexcept
{
	if constexpr (sizeof...(ARGS) == 0)
		return {};
	else
	{
		basic_ostringstream_t<CHAR> ss;
		Detail::Concatenate<CHAR>(ss, std::forward<ARGS>(args)...);
		return ss.str();
	}
}

int main()
{
	std::cout << Concatenate("this does not compile in ", 15.5, " or ", 15.6, " pre1!");
	return 0;
}