[clang] [Clang] [C++26] P1306R5: Iterating expansion statements using string injection (PR #208877)

[via cfe-commits]

ordinary-github-user wrote:

am i using template for correctly?
```c++
#include <cstdint>
#include <tuple>
struct RangeIterator
{
	std::uint8_t value;
	constexpr std::uint8_t operator*()const noexcept { return value; }
	constexpr RangeIterator& operator++()noexcept { ++value; return *this; }
	constexpr bool operator!=(RangeIterator r)const noexcept { return value != r.value; }
	friend constexpr std::uint8_t operator-(RangeIterator l, RangeIterator r) noexcept { return std::uint8_t(l.value - r.value); }
	friend constexpr RangeIterator operator-(RangeIterator l, u8 r)noexcept { return { std::uint8_t(l.value - r) }; }
	friend constexpr RangeIterator operator+(RangeIterator l, u8 r)noexcept { return { std::uint8_t(l.value + r) }; }
};

template<std::uint8_t End>
struct Range
{
	static consteval RangeIterator begin()noexcept { return { 0 }; }
	static consteval RangeIterator end()noexcept { return { End }; }
	static consteval std::uint8_t size()noexcept { return End; }
	template<std::size_t Index>requires(Index < size())consteval friend std::uint8_t get(Range)noexcept { return Index; }
};
namespace std
{
	template<std::uint8_t End>
	struct tuple_size<Range<End>>
	{
		static constexpr std::uint8_t value{ End };
	};
	template<std::size_t Index, std::uint8_t End>
	struct tuple_element<Index, Range<End>>
	{
		using type = std::uint8_t;
	};
}

static_assert([]static
{
	template for (constexpr auto _ : Range<2>{})
	{
		static constexpr auto [...c0] { Range<2>{} };
		(..., c0);
	}
	return true;
}());
```
```
error: static assertion expression is not an integral constant expression
 1411 | static_assert([]static
      |               ^~~~~~~~
 1412 | {
      | ~
 1413 |         template for (constexpr auto _ : Range<2>{})
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 1414 |         {
      |         ~
 1415 |                 static constexpr auto [...c0] { Range<2>{} };
      |                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 1416 |                 (..., c0);
      |                 ~~~~~~~~~~
 1417 |         }
      |         ~
 1418 |         return true;
      |         ~~~~~~~~~~~~
 1419 | }());
      | ~~~
1 error generated.
```

https://github.com/llvm/llvm-project/pull/208877