test: add test case for C++26 pack indexing

regression-tests/pure2-variadics-indexing.cpp2

@@ -0,0 +1,11 @@
+f0: <T...> () == :T...[0] = 32;
+// f1: <T...> () == :T...[0]::value_type = 4; // Pending #533.
+// 'V...[0]' needs to be made an _id-expression_.
+// f2: <V...: _> () == V...[0];
+// f3: <V...: _> () == V...[0].front();
+main: () = {
+  static_assert(f0<i64>() == 32);
+  // static_assert(f1<std::integral_constant<i16, 8>>() == 4);
+  // static_assert(f2<2>() == 2);
+  // static_assert(f3<(:std::array = (1))>() == 1);
+}

regression-tests/test-results/pure2-variadics-indexing.cpp

@@ -0,0 +1,35 @@
+
+#define CPP2_IMPORT_STD          Yes
+
+//=== Cpp2 type declarations ====================================================
+
+
+#include "cpp2util.h"
+
+#line 1 "pure2-variadics-indexing.cpp2"
+
+
+//=== Cpp2 type definitions and function declarations ===========================
+
+#line 1 "pure2-variadics-indexing.cpp2"
+template<typename ...T> [[nodiscard]] constexpr auto f0() -> decltype(auto);
+// f1: <T...> () == :T...[0]::value_type = 4; // Pending #533.
+// 'V...[0]' needs to be made an _id-expression_.
+// f2: <V...: _> () == V...[0];
+// f3: <V...: _> () == V...[0].front();
+#line 6 "pure2-variadics-indexing.cpp2"
+auto main() -> int;
+
+//=== Cpp2 function definitions =================================================
+
+#line 1 "pure2-variadics-indexing.cpp2"
+template<typename ...T> [[nodiscard]] constexpr auto f0() -> decltype(auto) { return T...[0]{32};  }
+
+#line 6 "pure2-variadics-indexing.cpp2"
+auto main() -> int{
+  static_assert(f0<cpp2::i64>() == 32);
+  // static_assert(f1<std::integral_constant<i16, 8>>() == 4);
+  // static_assert(f2<2>() == 2);
+  // static_assert(f3<(:std::array = (1))>() == 1);
+}
+

regression-tests/test-results/pure2-variadics-indexing.cpp2.output

@@ -0,0 +1,2 @@
+pure2-variadics-indexing.cpp2... ok (all Cpp2, passes safety checks)
+