Add is and as support for std::expected (v2)

include/cpp2util.h

@@ -1185,7 +1185,9 @@ inline constexpr auto is( auto const& x, auto&& value ) -> bool
     else if constexpr (requires{ bool{x == value}; }) {
         return x == value;
     }
-    return false;
+    else {
+        return false;
+    }
 }
 
 
@@ -1302,6 +1304,20 @@ auto as(X const& x CPP2_SOURCE_LOCATION_PARAM_WITH_DEFAULT) -> decltype(auto) {
     }
 }
 
+#ifdef __cpp_lib_expected
+// Ensure const-ref `as` for `std::expected` is visible before the template definition below since it won't be found by ADL.
+template<typename T, typename X>
+    requires std::is_same_v<X, std::expected<T, typename X::error_type>>
+constexpr auto as( X const &x ) -> decltype(auto);
+
+template<typename T, typename X>
+    requires (
+              std::is_same_v<T, std::unexpected<typename X::error_type>>
+              && std::is_same_v<X, std::expected<typename X::value_type, typename X::error_type>>
+             )
+constexpr auto as( X const &x ) -> decltype(auto);
+#endif
+
 template< typename C, typename X >
 auto as( X& x ) -> decltype(auto) {
     if constexpr (std::is_same_v<C, X>) {
@@ -1566,8 +1582,9 @@ inline constexpr auto is( std::any const& x, auto&& value ) -> bool
         auto pvalue = std::any_cast<CPP2_TYPEOF(value)>(&x);
         return pvalue && *pvalue == value;
     }
-    //  else
-    return false;
+    else {
+        return false;
+    }
 }
 
 
@@ -1613,7 +1630,9 @@ constexpr auto is( std::optional<T> const& x, auto&& value ) -> bool
     else if constexpr (requires{ bool{ x.value() == value }; }) {
         return x.has_value() && x.value() == value;
     }
-    return false;
+    else {
+        return false;
+    }
 }
 
 
@@ -1625,6 +1644,92 @@ constexpr auto as( X const& x ) -> decltype(auto)
     { return x.value(); }
 
 
+//-------------------------------------------------------------------------------------------------------------
+//  std::expected is and as
+//
+#ifdef __cpp_lib_expected
+//  is Type
+//
+template<typename T, typename X>
+    requires std::is_same_v<X, std::expected<T, typename X::error_type>>
+constexpr auto is( X const &x ) -> bool
+{
+    return x.has_value();
+}
+
+template<typename T, typename U, typename V>
+    requires std::is_same_v<T, empty>
+constexpr auto is( std::expected<U, V> const &x ) -> bool
+{
+    return !x.has_value();
+}
+
+//  is std::unexpected<T> Type
+//
+template<typename T, typename X>
+    requires (
+              std::is_same_v<T, std::unexpected<typename X::error_type>>
+              && std::is_same_v<X, std::expected<typename X::value_type, typename X::error_type>>
+             )
+constexpr auto is( X const &x ) -> bool
+{
+    return !x.has_value();
+}
+
+
+//  is Value
+//
+template<typename T, typename U>
+constexpr auto is( std::expected<T, U> const &x, auto &&value ) -> bool
+{
+    //  Predicate case
+    if constexpr (requires{ bool{ value(x) }; }) {
+        return value(x);
+    }
+    else if constexpr (std::is_function_v<decltype(value)> || requires{ &value.operator(); }) {
+        return false;
+    }
+
+    //  Value case
+    else if constexpr (requires{ bool{ x.value() == value }; }) {
+        return x.has_value() && x.value() == value;
+    }
+    else {
+        return false;
+    }
+}
+
+
+//  as
+//
+template<typename T, typename X>
+    requires std::is_same_v<X, std::expected<T, typename X::error_type>>
+constexpr auto as( X const &x ) -> decltype(auto)
+{
+    return x.value();
+}
+
+//  as std::unexpected<T>
+//
+template<typename T, typename X>
+    requires (
+              std::is_same_v<T, std::unexpected<typename X::error_type>>
+              && std::is_same_v<X, std::expected<typename X::value_type, typename X::error_type>>
+             )
+constexpr auto as( X const &x ) -> decltype(auto)
+{
+    // It's UB to call `error` if `has_value` is true.
+    if (x.has_value()) {
+        Throw(
+            std::runtime_error("Cannot cast 'expected' to 'unexpected' because it has a value"),
+            "Cannot cast 'expected' to 'unexpected' because it has a value");
+    }
+
+    return std::unexpected< typename X::error_type>(x.error());
+}
+#endif
+
+
 //-----------------------------------------------------------------------
 //
 //  A variation of GSL's final_action_success / finally

regression-tests/pure2-expected-is-as.cpp2

@@ -0,0 +1,90 @@
+// `std::expected` requires C++23 so a dedicated test file is needed
+// since only MSVC supports it at time of writing, and there's no #ifdef
+// or `static if` support in Cpp2 (yet?).
+
+main: () -> int = {
+
+    ex1: std::expected<int, int> = (123);
+    ex2: std::expected<int, int> = std::unexpected(-1);
+    ex3: std::expected<std::string, size_t> = ("Expect the unexpected");
+
+    if ex1 is int {
+        std::cout << "ex1 is int\n";
+    }
+
+    if ex1 is bool {
+        std::cout << "BUG - ex1 is not a bool\n";
+        return -1;
+    }
+
+    if ex1 is void {
+        std::cout << "BUG - ex1 is not 'empty'\n";
+        return -1;
+    }
+
+    if ex1 is std::unexpected<int> {
+        std::cout << "BUG - ex1 is not unexpected\n";
+        return -1;
+    }
+
+    if ex1 is 123 {
+        std::cout << "ex1 is 123\n";
+    }
+
+    if ex1 is 100 {
+        std::cout << "BUG - ex1's value is not 100\n";
+        return -1;
+    }
+
+    val1:= ex1 as int;
+    std::cout << "ex1 as int = " << val1 << "\n";
+
+    if ex2 is int {
+        std::cout << "BUG - ex2 is not an int\n";
+        return -1;
+    }
+
+    if ex2 is bool {
+        std::cout << "BUG - ex2 is not a bool\n";
+        return -1;
+    }
+
+    if ex2 is 123 {
+        std::cout << "BUG - ex2 does not have a value\n";
+        return -1;
+    }
+
+    if ex2 is std::unexpected<int> {
+        std::cout << "ex2 is unexpected<int> and error is: " << ex2.error() << "\n";
+    }
+
+    if ex2 is void {
+        std::cout << "ex2 is 'empty' aka unexpected<int> and error is: " << ex2.error() << "\n";
+    }
+
+    ex2_err:= ex2 as std::unexpected<int>;
+    std::cout << "ex2 as std::unexpected<int> and error = " << ex2_err.error() << "\n";
+
+    test_inspect(ex1, "expected<int, int> with value");
+    test_inspect(ex2, "expected<int, int> with unexpected");
+    test_inspect(ex3, "expected<string, size_t> with value");
+
+    return 0;
+}
+
+test_inspect: ( x: _, msg: _ ) = {
+
+    unwrap:= :(unexp: std::unexpected<int>) -> _ = {
+        return unexp.error();
+    };
+
+    std::cout
+        << "\n" << msg << "\n    ..."
+        << inspect x -> std::string {
+            is int = "integer " + std::to_string(x as int);
+            is std::unexpected<int> = "unexpected<int> " + std::to_string(unwrap(x as std::unexpected<int>));
+            is std::string = "string " + x as std::string;
+            is _   = " no match";
+        }
+        << "\n";
+}

regression-tests/test-results/apple-clang-14/pure2-expected-is-as.cpp.output

@@ -0,0 +1,72 @@
+pure2-expected-is-as.cpp2:7:10: error: no member named 'expected' in namespace 'std'
+    std::expected<int,int> ex1 {123}; 
+    ~~~~~^
+pure2-expected-is-as.cpp2:7:22: error: expected '(' for function-style cast or type construction
+    std::expected<int,int> ex1 {123}; 
+                  ~~~^
+pure2-expected-is-as.cpp2:8:10: error: no member named 'expected' in namespace 'std'
+    std::expected<int,int> ex2 {std::unexpected(-1)}; 
+    ~~~~~^
+pure2-expected-is-as.cpp2:8:22: error: expected '(' for function-style cast or type construction
+    std::expected<int,int> ex2 {std::unexpected(-1)}; 
+                  ~~~^
+pure2-expected-is-as.cpp2:9:10: error: no member named 'expected' in namespace 'std'
+    std::expected<std::string,size_t> ex3 {"Expect the unexpected"}; 
+    ~~~~~^
+pure2-expected-is-as.cpp2:9:30: error: expected '(' for function-style cast or type construction
+    std::expected<std::string,size_t> ex3 {"Expect the unexpected"}; 
+                  ~~~~~~~~~~~^
+In file included from pure2-expected-is-as.cpp:7:
+In file included from ../../../include/cpp2util.h:46:
+In file included from /Applications/Xcode_14.3.1.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/c++/v1/algorithm:1712:
+In file included from /Applications/Xcode_14.3.1.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/c++/v1/memory:877:
+In file included from /Applications/Xcode_14.3.1.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/c++/v1/iterator:684:
+In file included from /Applications/Xcode_14.3.1.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/c++/v1/__iterator/common_iterator.h:22:
+/Applications/Xcode_14.3.1.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/c++/v1/variant:1282:3: error: static_assert failed due to requirement '0 < sizeof...(_Types)' "variant must consist of at least one alternative."
+  static_assert(0 < sizeof...(_Types),
+  ^             ~~~~~~~~~~~~~~~~~~~~~
+pure2-expected-is-as.cpp2:11:23: note: in instantiation of template class 'std::variant<>' requested here
+    if (cpp2::is<int>(ex1)) {
+                      ^
+pure2-expected-is-as.cpp2:11:23: error: use of undeclared identifier 'ex1'
+    if (cpp2::is<int>(ex1)) {
+                      ^
+pure2-expected-is-as.cpp2:15:24: error: use of undeclared identifier 'ex1'
+    if (cpp2::is<bool>(ex1)) {
+                       ^
+pure2-expected-is-as.cpp2:20:24: error: use of undeclared identifier 'ex1'
+    if (cpp2::is<void>(ex1)) {
+                       ^
+pure2-expected-is-as.cpp2:25:23: error: no member named 'unexpected' in namespace 'std'
+    if (cpp2::is<std::unexpected<int>>(ex1)) {
+                 ~~~~~^
+pure2-expected-is-as.cpp2:25:37: error: expected '(' for function-style cast or type construction
+    if (cpp2::is<std::unexpected<int>>(ex1)) {
+                                 ~~~^
+pure2-expected-is-as.cpp2:25:40: error: use of undeclared identifier 'ex1'; did you mean 'exp'?
+    if (cpp2::is<std::unexpected<int>>(ex1)) {
+                                       ^~~
+                                       exp
+/Applications/Xcode_14.3.1.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/c++/v1/math.h:895:1: note: 'exp' declared here
+exp(_A1 __lcpp_x) _NOEXCEPT {return ::exp((double)__lcpp_x);}
+^
+pure2-expected-is-as.cpp2:30:18: error: use of undeclared identifier 'ex1'
+    if (cpp2::is(ex1, 123)) {
+                 ^
+pure2-expected-is-as.cpp2:34:18: error: use of undeclared identifier 'ex1'
+    if (cpp2::is(ex1, 100)) {
+                 ^
+pure2-expected-is-as.cpp2:39:31: error: use of undeclared identifier 'ex1'
+    auto val1 {cpp2::as_<int>(ex1)}; 
+                              ^
+pure2-expected-is-as.cpp2:42:23: error: use of undeclared identifier 'ex2'
+    if (cpp2::is<int>(ex2)) {
+                      ^
+pure2-expected-is-as.cpp2:47:24: error: use of undeclared identifier 'ex2'
+    if (cpp2::is<bool>(ex2)) {
+                       ^
+pure2-expected-is-as.cpp2:52:18: error: use of undeclared identifier 'ex2'
+    if (cpp2::is(ex2, 123)) {
+                 ^
+fatal error: too many errors emitted, stopping now [-ferror-limit=]
+20 errors generated.