Add @union initial test case

regression-tests/pure2-union.cpp2

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+
+name_or_number: @union type = {
+    name: std::string;
+    num : i32;
+}
+
+print_name: (non: name_or_number) = {
+    if non.is_name() {
+        std::cout << non.get_name() << "\n";
+    }
+    else {
+        std::cout << "(not a name)\n";
+    }
+}
+
+main: () = {
+    x: name_or_number = ();
+    std::cout << "sizeof(x) is (sizeof(x))$\n";
+
+    x.print_name();
+
+    s: std::string = "xyzzy";
+    x.set_name( s );
+
+    x.print_name();
+}

regression-tests/test-results/clang-12/pure2-union.cpp.execution

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+sizeof(x) is 33
+(not a name)
+xyzzy

regression-tests/test-results/gcc-10/pure2-union.cpp.output

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+pure2-union.cpp2:6:42: error: expected ‘;’ at end of member declaration
+In file included from pure2-union.cpp:7:
+../../../include/cpp2util.h:10005:47: error: static assertion failed: GCC 11 or higher is required to support variables and type-scope functions that have a 'requires' clause. This includes a type-scope 'forward' parameter of non-wildcard type, such as 'func: (this, forward s: std::string)', which relies on being able to add a 'requires' clause - in that case, use 'forward s: _' instead if you need the result to compile with GCC 10.
+pure2-union.cpp2:7:1: note: in expansion of macro ‘CPP2_REQUIRES_’
+pure2-union.cpp2:11:41: error: expected ‘;’ at end of member declaration
+In file included from pure2-union.cpp:7:
+../../../include/cpp2util.h:10005:47: error: static assertion failed: GCC 11 or higher is required to support variables and type-scope functions that have a 'requires' clause. This includes a type-scope 'forward' parameter of non-wildcard type, such as 'func: (this, forward s: std::string)', which relies on being able to add a 'requires' clause - in that case, use 'forward s: _' instead if you need the result to compile with GCC 10.
+pure2-union.cpp2:12:1: note: in expansion of macro ‘CPP2_REQUIRES_’
+pure2-union.cpp2:28:6: error: no declaration matches ‘void name_or_number::set_name(auto:82&&) & requires  is_same_v<typename std::remove_cv<typename std::remove_reference<decltype(name_or_number::set_name::value)>::type>::type, std::__cxx11::string>’
+pure2-union.cpp2:6:14: note: candidate is: ‘template<class auto:80> void name_or_number::set_name(auto:80&&) &’
+pure2-union.cpp2:2:7: note: ‘class name_or_number’ defined here
+pure2-union.cpp2:35:6: error: no declaration matches ‘void name_or_number::set_num(auto:83&&) & requires  is_same_v<typename std::remove_cv<typename std::remove_reference<decltype(name_or_number::set_num::value)>::type>::type, cpp2::i32>’
+pure2-union.cpp2:11:14: note: candidate is: ‘template<class auto:81> void name_or_number::set_num(auto:81&&) &’
+pure2-union.cpp2:2:7: note: ‘class name_or_number’ defined here

regression-tests/test-results/gcc-13/pure2-union.cpp.execution

@@ -0,0 +1,3 @@
+sizeof(x) is 33
+(not a name)
+xyzzy

regression-tests/test-results/msvc-2022/pure2-union.cpp.execution

@@ -0,0 +1,3 @@
+sizeof(x) is 25
+(not a name)
+xyzzy

regression-tests/test-results/msvc-2022/pure2-union.cpp.output

@@ -0,0 +1 @@
+pure2-union.cpp

regression-tests/test-results/pure2-union.cpp

@@ -0,0 +1,91 @@
+
+#define CPP2_USE_MODULES         Yes
+
+//=== Cpp2 type declarations ====================================================
+
+
+#include "cpp2util.h"
+
+
+#line 2 "pure2-union.cpp2"
+class name_or_number;
+
+
+//=== Cpp2 type definitions and function declarations ===========================
+
+
+#line 2 "pure2-union.cpp2"
+class name_or_number {
+private: std::array<std::byte,cpp2::max(sizeof(std::string), sizeof(cpp2::i32))> storage__ {}; private: cpp2::i8 discriminator__ {-1}; public: [[nodiscard]] auto is_name() const& -> bool;
+public: [[nodiscard]] auto get_name() const& -> auto&&;
+public: [[nodiscard]] auto get_name() & -> auto&&;
+public: auto set_name(auto&& value) & -> void
+CPP2_REQUIRES_ (std::is_same_v<CPP2_TYPEOF(value), std::string>);
+public: [[nodiscard]] auto is_num() const& -> bool;
+public: [[nodiscard]] auto get_num() const& -> auto&&;
+public: [[nodiscard]] auto get_num() & -> auto&&;
+public: auto set_num(auto&& value) & -> void
+CPP2_REQUIRES_ (std::is_same_v<CPP2_TYPEOF(value), cpp2::i32>);
+private: auto destroy() & -> void;
+public: ~name_or_number() noexcept;
+
+    public: name_or_number() = default;
+    public: name_or_number(name_or_number const&) = delete; /* No 'that' constructor, suppress copy */
+    public: auto operator=(name_or_number const&) -> void = delete;
+
+#line 5 "pure2-union.cpp2"
+};
+
+auto print_name(cpp2::in<name_or_number> non) -> void;
+
+
+#line 16 "pure2-union.cpp2"
+auto main() -> int;
+
+
+//=== Cpp2 function definitions =================================================
+
+
+
+    [[nodiscard]] auto name_or_number::is_name() const& -> bool { return discriminator__ == 0; }
+[[nodiscard]] auto name_or_number::get_name() const& -> auto&& { 
+                                                              cpp2::Default.expects(is_name(), "");return *cpp2::assert_not_null(reinterpret_cast<std::string const*>(&storage__)); }
+[[nodiscard]] auto name_or_number::get_name() & -> auto&& { 
+                                                                    cpp2::Default.expects(is_name(), "");return *cpp2::assert_not_null(reinterpret_cast<std::string*>(&storage__)); }
+auto name_or_number::set_name(auto&& value) & -> void
+requires (std::is_same_v<CPP2_TYPEOF(value), std::string>){if (!(is_name())) {destroy();std::construct_at(reinterpret_cast<std::string*>(&storage__), value);}else {*cpp2::assert_not_null(reinterpret_cast<std::string*>(&storage__)) = value;}discriminator__ = 0;}
+[[nodiscard]] auto name_or_number::is_num() const& -> bool { return discriminator__ == 1; }
+[[nodiscard]] auto name_or_number::get_num() const& -> auto&& { 
+                                                            cpp2::Default.expects(is_num(), "");return *cpp2::assert_not_null(reinterpret_cast<cpp2::i32 const*>(&storage__)); }
+[[nodiscard]] auto name_or_number::get_num() & -> auto&& { 
+                                                                  cpp2::Default.expects(is_num(), "");return *cpp2::assert_not_null(reinterpret_cast<cpp2::i32*>(&storage__)); }
+auto name_or_number::set_num(auto&& value) & -> void
+requires (std::is_same_v<CPP2_TYPEOF(value), cpp2::i32>){if (!(is_num())) {destroy();std::construct_at(reinterpret_cast<cpp2::i32*>(&storage__), value);}else {*cpp2::assert_not_null(reinterpret_cast<cpp2::i32*>(&storage__)) = value;}discriminator__ = 1;}
+auto name_or_number::destroy() & -> void{
+    if (discriminator__ == 0) {std::destroy_at(reinterpret_cast<std::string*>(&storage__));}
+    if (discriminator__ == 1) {std::destroy_at(reinterpret_cast<cpp2::i32*>(&storage__));}
+    }
+
+    name_or_number::~name_or_number() noexcept{destroy();}
+#line 7 "pure2-union.cpp2"
+auto print_name(cpp2::in<name_or_number> non) -> void{
+    if (CPP2_UFCS_0(is_name, non)) {
+        std::cout << CPP2_UFCS_0(get_name, non) << "\n";
+    }
+    else {
+        std::cout << "(not a name)\n";
+    }
+}
+
+auto main() -> int{
+    name_or_number x {}; 
+    std::cout << "sizeof(x) is " + cpp2::to_string(sizeof(x)) + "\n";
+
+    CPP2_UFCS_0(print_name, x);
+
+    std::string s {"xyzzy"}; 
+    CPP2_UFCS(set_name, x, std::move(s));
+
+    CPP2_UFCS_0(print_name, std::move(x));
+}
+

regression-tests/test-results/pure2-union.cpp2.output

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+pure2-union.cpp2... ok (all Cpp2, passes safety checks)
+