feat: evaluate program-defined metafunctions (based on #797)

A metafunction is normal Cpp2 code compiled as part of a library.
When parsing a declaration that `@`-uses the metafunction,
the library is loaded and the metafunction invoked on the declaration.

The reflection API is available by default to Cpp2 code (via `cpp2util.h`).
The implementation of the API is provided by the `cppfront` executable.
For this to work, compiling `cppfront` should export its symbols
(for an explanation, see <https://cmake.org/cmake/help/latest/prop_tgt/ENABLE_EXPORTS.html>).

For `cppfront` to emit program-defined metafunctions,
the environment variable `CPPFRONT_METAFUNCTION_LIBRARY`
should be set to the library's path.

For `cppfront` to load program-defined metafunctions,
the environment variable `CPPFRONT_METAFUNCTION_LIBRARIES`
should be set to the `:`-separated library paths of the used metafunctions.

Here is an example of program-defined metafunctions.
The commands were cleaned up from the CMake buildsystem in #797.

`metafunctions.cpp2`:
```Cpp2
greeter: (inout t: cpp2::meta::type_declaration) = {
  t.add_member($R"(say_hi: () = std::cout << "Hello, world!\nFrom (t.name())$\n";)");
}
```

`main.cpp2`:
```Cpp2
my_class: @greeter type = { }
main: ()                = my_class().say_hi();
```

Build `cppfront`:
```bash
g++ -std=c++20 -o cppfront.cpp.o -c cppfront.cpp
g++ -Wl,--export-dynamic -rdynamic cppfront.cpp.o -o cppfront
```

Build `metafunctions`:
```bash
CPPFRONT_METAFUNCTION_LIBRARY=libmetafunctions.so ./cppfront metafunctions.cpp2
g++ -std=c++20 -fPIC -o metafunctions.cpp.o -c metafunctions.cpp
g++ -fPIC -shared -Wl,-soname,libmetafunctions.so -o libmetafunctions.so metafunctions.cpp.o
```

Build and run `main`:
```bash
CPPFRONT_METAFUNCTION_LIBRARIES=libmetafunctions.so ./cppfront main.cpp2
g++ -std=c++20 -o main.cpp.o -c main.cpp
g++ main.cpp.o -o main
./main
```

Output:
```output
metafunctions.cpp2... ok (all Cpp2, passes safety checks)

main.cpp2... ok (all Cpp2, passes safety checks)

Hello, world!
From my_class
```

@edo9300 Please, share your GitHub-provided `no-reply` email (see <https://docs.github.com/en/pull-requests/committing-changes-to-your-project/creating-and-editing-commits/creating-a-commit-with-multiple-authors>).
Co-authored-by: Edoardo Lolletti <>

Co-authored-by: Dylam De La Torre <[email protected]>

Author: JohelEGP

include/cpp2reflect.h

@@ -375,6 +375,16 @@ constexpr auto gcc_clang_msvc_min_versions(
     #define CPP2_CONSTEXPR constexpr
 #endif
 
+
+#if defined(_WIN32)
+#define CPPFRONTAPI __declspec(dllexport)
+#else
+#define CPPFRONTAPI  __attribute__ ((visibility ("default")))
+#endif
+
+#define CPP2_C_API extern "C" CPPFRONTAPI
+
+
 namespace cpp2 {
 
 
@@ -1842,7 +1852,9 @@ constexpr auto is( auto const& x, auto&& value ) -> bool
     else if constexpr (requires{ bool{x == value}; }) {
         return x == value;
     }
-    return false;
+    else {
+        return false;
+    }
 }
 
 //-----------------------------------------------------------------------
@@ -2883,4 +2895,15 @@ using cpp2::cpp2_new;
     #pragma clang diagnostic pop
 #endif
 
+#ifdef _MSC_VER
+#pragma warning(push)
+#pragma warning(disable: 4251)
+#endif
+
+#include "cpp2reflect.h"
+
+#ifdef _MSC_VER
+#pragma warning(pop)
+#endif
+
 #endif // CPP2_CPP2UTIL_H

include/cpp2util.h

@@ -14,7 +14,7 @@
 //  We want cppfront to build cleanly at very high warning levels, with warnings
 //  as errors -- so disable a handful that fire incorrectly due to compiler bugs
 #ifdef _MSC_VER
-    #pragma warning(disable: 4456 4706)
+    #pragma warning(disable: 4456 4706 4996)
 #endif
 #if defined(__GNUC__) && __GNUC__ >= 13 && !defined(__clang_major__)
     #pragma GCC diagnostic ignored "-Wdangling-reference"
@@ -103,28 +103,11 @@ struct source_line
 };
 
 
-using lineno_t = int32_t;
-using colno_t  = int32_t;   // not int16_t... encountered >80,000 char line during testing
-using index_t  = int32_t;
+using cpp2::meta::lineno_t;
+using cpp2::meta::colno_t;
+using cpp2::meta::index_t;
 
-struct source_position
-{
-    lineno_t    lineno;     // one-based offset into program source
-    colno_t     colno;      // one-based offset into line
-
-    source_position(lineno_t l = 1, colno_t  c = 1 )
-        : lineno{ l }, colno{ c }
-    {
-    }
-
-    auto operator<=>(source_position const&) const = default;
-
-    auto to_string() const
-        -> std::string
-    {
-        return "(" + std::to_string(lineno) + "," + std::to_string(colno) + ")";
-    }
-};
+using cpp2::meta::source_position;
 
 struct comment
 {
@@ -521,6 +504,36 @@ auto to_upper_and_underbar(std::string_view s)
 }
 
 
+auto to_lower_and_collapsed_underbar(
+    std::string_view s,
+    bool prev_was_underbar = false,
+    bool drop_back_underbar = false
+    )
+    -> std::string
+{
+    auto ret = std::string{};
+    for (char c : s) {
+        if (std::isalnum(c)) {
+            ret.push_back(string_util::safe_tolower(c));
+            prev_was_underbar = false;
+        }
+        else if (!prev_was_underbar) {
+            ret.push_back('_');
+            prev_was_underbar = true;
+        }
+    }
+    if (
+        drop_back_underbar
+        && !ret.empty()
+        && ret.back() == '_'
+        )
+    {
+        ret.pop_back();
+    }
+    return ret;
+}
+
+
 auto is_empty_or_a_decimal_number(std::string_view s)
     -> bool
 {