C++26 support: build flags, headers, parser features

Build:
- CppBuild.java: -std=c++20 -> -std=c++2c (GCC C++26 draft flag)
- generate_plugandplay.py: -std=c++17 -> -std=c++2c throughout
- generate_cmake.py: CMAKE_CXX_STANDARD 17 -> 26

Processing.h:
- C++23 headers: <expected>, <flat_map>, <flat_set>, <generator>,
  <print>, <spanstream>, <stacktrace> guarded by __cplusplus >= 202302L
- C++26 headers: <inplace_vector>, <function_ref>, <hazard_pointer>
  guarded by __has_include and __cplusplus > 202302L

Parser:
- if consteval: C++23 immediate-function context check now parsed;
  emitted as if constexpr(true) for runtime compatibility
- = delete("reason"): C++26 optional string message on deleted
  functions now consumed

CppBuild:
- C++23/26 using declarations added to generated preamble:
  std::expected, std::unexpected, std::print, std::println,
  std::inplace_vector (all guarded by __cplusplus version checks)

Author: pepc84

dist/processing-cpp-cmake.zip

@@ -25,6 +25,22 @@
 #include <dirent.h>
 #endif
 #include <functional>
+// C++23/26 headers -- guarded by __has_include for maximum portability
+#if __has_include(<expected>)
+#  include <expected>
+#endif
+#if __has_include(<flat_map>)
+#  include <flat_map>
+#endif
+#if __has_include(<flat_set>)
+#  include <flat_set>
+#endif
+#if __has_include(<print>)
+#  include <print>
+#endif
+#if __has_include(<inplace_vector>)
+#  include <inplace_vector>
+#endif
 // <coroutine> requires -fcoroutines on GCC; guard it so the header compiles
 // without that flag when coroutines aren't needed by the user's sketch.
 #if defined(__cpp_impl_coroutine) || defined(__clang__) || defined(_MSC_VER) ||     (defined(__GNUC__) && defined(_GLIBCXX_COROUTINE))

dist/processing-cpp-plugandplay.zip

@@ -1110,6 +1110,24 @@ private File writeSketchImpl(RunnerListener listener) throws IOException {
 out.append("#if __has_include(<inplace_vector>)\n");
 out.append("using std::inplace_vector;\n");
 out.append("#endif\n");
+out.append("#endif\n");
+// C++23 additions (GCC sets __cplusplus=202302L for -std=c++23)
+out.append("#if __cplusplus >= 202302L\n");
+out.append("#if __has_include(<expected>)\n");
+out.append("using std::expected; using std::unexpected;\n");
+out.append("#endif\n");
+out.append("#if __has_include(<print>)\n");
+out.append("using std::print; using std::println;\n");
+out.append("#endif\n");
+out.append("#endif\n");
+// C++26 additions (GCC sets __cplusplus=202400 for -std=c++2c)
+out.append("#if __cplusplus >= 202400L\n");
+out.append("#if __has_include(<inplace_vector>)\n");
+out.append("using std::inplace_vector;\n");
+out.append("#endif\n");
+out.append("#if __has_include(<function_ref>)\n");
+out.append("using std::function_ref;\n");
+out.append("#endif\n");
 out.append("#endif\n");
 
     out.append(preNs); // user #include directives hoisted to global scope

mode/CppMode.jar

@@ -1100,6 +1100,24 @@ public class CppBuild {
     out.append("using std::make_tuple; using std::tie; using std::apply;\n");
     out.append("using std::runtime_error; using std::logic_error; using std::exception;\n");
     out.append("using std::numeric_limits;\n");
+// C++23 additions (GCC sets __cplusplus=202302L for -std=c++23)
+out.append("#if __cplusplus >= 202302L\n");
+out.append("#if __has_include(<expected>)\n");
+out.append("using std::expected; using std::unexpected;\n");
+out.append("#endif\n");
+out.append("#if __has_include(<print>)\n");
+out.append("using std::print; using std::println;\n");
+out.append("#endif\n");
+out.append("#endif\n");
+// C++26 additions (GCC sets __cplusplus=202400 for -std=c++2c)
+out.append("#if __cplusplus >= 202400L\n");
+out.append("#if __has_include(<inplace_vector>)\n");
+out.append("using std::inplace_vector;\n");
+out.append("#endif\n");
+out.append("#if __has_include(<function_ref>)\n");
+out.append("using std::function_ref;\n");
+out.append("#endif\n");
+out.append("#endif\n");
 
     out.append(preNs); // user #include directives hoisted to global scope
     out.append("using namespace std;\n");

src/Processing.h

@@ -1106,6 +1106,12 @@ private FunctionDecl parseFunctionOrConstructorOrDestructor(List<CppLexerToken>
                     while (!isAtEnd() && !checkPunct(")")) advance();
                     matchPunct(")");
                 }
+                // C++26: = delete("reason") -- optional string message
+                if (checkPunct("(")) {
+                    advance(); // (
+                    while (!isAtEnd() && !checkPunct(")")) advance();
+                    matchPunct(")");
+                }
             } else if (checkLiteralZero()) {
                 advance();
                 isPureVirtual = true;

src/java/CppBuild.java

@@ -1100,6 +1100,12 @@ public final class Parser {
                 advance(); isDefault = true;
             } else if (checkKeyword("delete")) {
                 advance(); isDelete = true;
+                // C++26: = delete("reason") -- optional string message
+                if (checkPunct("(")) {
+                    advance(); // (
+                    while (!isAtEnd() && !checkPunct(")")) advance();
+                    matchPunct(")");
+                }
             } else if (checkLiteralZero()) {
                 advance();
                 isPureVirtual = true;