Add support for inspecting values (not just types), closes hsutter#79

Also cleaned up _alternative_ to use _type-id_ rather than _id-expression_.

Author: hsutter

include/cpp2util.h

@@ -616,6 +616,23 @@ auto is( X const& x ) -> bool {
 }
 
 
+//-------------------------------------------------------------------------------------------------------------
+//  Built-in is (values)
+//
+
+inline constexpr auto is( auto const& x, auto const& value ) -> bool
+    requires requires{ x == value; }
+{
+    return x == value;
+}
+
+inline constexpr auto is( auto const& x, auto const& value ) -> bool
+    requires (!requires{ x == value; })
+{
+    return false;
+}
+
+
 //-------------------------------------------------------------------------------------------------------------
 //  Built-in as (partial)
 //

regression-tests/pure2-inspect-values.cpp2

@@ -0,0 +1,18 @@
+main: ()->int = {
+    test(42);
+    test(3.14);
+    test(0);
+    test(-42);
+    test("xyzzy" as std::string);
+}
+
+test: (x:_) = {
+    forty_two := 42;
+    std::cout << inspect x -> std::string {
+        is 0           = "zero";
+        is (forty_two) = "the answer";
+        is int         = "integer " + cpp2::to_string(x);
+        is std::string = x;
+        is _           = "(no match)";
+    } << "\n";
+}

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

@@ -0,0 +1,5 @@
+the answer
+(no match)
+zero
+integer -42
+xyzzy

regression-tests/test-results/clang-12/pure2-inspect-values.cpp.output

@@ -0,0 +1,5 @@
+the answer
+(no match)
+zero
+integer -42
+xyzzy

regression-tests/test-results/gcc-10/pure2-inspect-values.cpp.execution

@@ -0,0 +1,5 @@
+the answer
+(no match)
+zero
+integer -42
+xyzzy

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

@@ -0,0 +1 @@
+pure2-inspect-values.cpp

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

@@ -0,0 +1,32 @@
+// ----- Cpp2 support -----
+#define CPP2_USE_MODULES         Yes
+#include "cpp2util.h"
+
+
+#line 1 "pure2-inspect-values.cpp2"
+[[nodiscard]] auto main() -> int;
+#line 9 "pure2-inspect-values.cpp2"
+auto test(auto const& x) -> void;
+
+//=== Cpp2 definitions ==========================================================
+
+#line 1 "pure2-inspect-values.cpp2"
+[[nodiscard]] auto main() -> int{
+    test(42);
+    test(3.14);
+    test(0);
+    test(-42);
+    test(cpp2::as<  std::string>("xyzzy"));
+}
+
+auto test(auto const& x) -> void{
+    auto forty_two { 42 }; 
+    std::cout << [&] () -> std::string { auto&& __expr = x;
+        if (cpp2::is(__expr, 0)) { if constexpr( requires{"zero";} ) if constexpr( std::is_convertible_v<CPP2_TYPEOF(("zero")),std::string> ) return "zero"; else return std::string{}; else return std::string{}; }
+        else if (cpp2::is(__expr, (std::move(forty_two)))) { if constexpr( requires{"the answer";} ) if constexpr( std::is_convertible_v<CPP2_TYPEOF(("the answer")),std::string> ) return "the answer"; else return std::string{}; else return std::string{}; }
+        else if (cpp2::is<int>(__expr)) { if constexpr( requires{"integer " + cpp2::to_string(x);} ) if constexpr( std::is_convertible_v<CPP2_TYPEOF(("integer " + cpp2::to_string(x))),std::string> ) return "integer " + cpp2::to_string(x); else return std::string{}; else return std::string{}; }
+        else if (cpp2::is<std::string>(__expr)) { if constexpr( requires{x;} ) if constexpr( std::is_convertible_v<CPP2_TYPEOF((x)),std::string> ) return x; else return std::string{}; else return std::string{}; }
+        else return "(no match)"; }
+    ()
+ << "\n";}
+

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

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