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

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

Parser (C++26 features):
- = delete("reason"): optional string message on deleted functions
  now consumed in both ctor and function declaration paths
- if consteval: C++23/26 immediate-function context check parsed;
  emitted as if constexpr(true) for runtime compatibility
- constexpr/constinit structured bindings: isStructuredBindingStart
  and parseStructuredBinding now recognize constexpr/constinit before auto

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

CppBuild preamble:
- C++23/26 using declarations added with version guards:
  std::expected, std::unexpected, std::print, std::println,
  std::inplace_vector, std::function_ref

Author: pepc84

mode/CppMode.jar

@@ -981,8 +981,8 @@ private CompilationUnit parseOrReportError(String code, RunnerListener listener)
     try {
       return Parser.parse(code);
     } catch (ParseException e) {
-      System.err.println(e.getMessage());
-      listener.statusError("Syntax error: " + e.getMessage());
+      System.err.println(ErrorFormatter.format(code, e));
+      listener.statusError(ErrorFormatter.statusLine(e));
       throw new AlreadyReportedException("Syntax error: " + e.getMessage());
     }
   }
@@ -1100,37 +1100,25 @@ private File writeSketchImpl(RunnerListener listener) throws IOException {
     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
-out.append("#if __cplusplus >= 202302L\n");
-out.append("using std::expected; using std::unexpected;\n");
-out.append("using std::print; using std::println;\n");
-out.append("#endif\n");
-// C++26 additions
-out.append("#if __cplusplus > 202302L\n");
-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
+    // C++23 standard library additions -- guarded by version check
+    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 standard library additions -- guarded by version check
+    out.append("#if __cplusplus > 202302L\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");
     out.append("namespace Processing {\n");
     out.append("using namespace std;\n");

src/java/CppBuild.java

@@ -103,6 +103,16 @@ private CppLexerToken advance() {
         return t;
     }
 
+    private CppLexerToken prev() {
+        // Walk backward from pos-1 skipping comments, return the last
+        // consumed real token. Used by expectPunct(";") to point the
+        // error caret at the end of the broken statement, not the start
+        // of the next token.
+        int p = pos - 1;
+        while (p > 0 && (tokens.get(p).type() == CppLexerTokenType.LINE_COMMENT
+                       || tokens.get(p).type() == CppLexerTokenType.BLOCK_COMMENT)) p--;
+        return p >= 0 ? tokens.get(p) : tokens.get(0);
+    }
     private boolean isAtEnd() {
         return peek().type() == CppLexerTokenType.EOF;
     }
@@ -149,6 +159,15 @@ private boolean matchOp(String op) {
 
     private CppLexerToken expectPunct(String p) {
         if (checkPunct(p)) return advance();
+        if (p.equals(";")) {
+            // Point the caret at the end of the previous token (where the
+            // semicolon should have been), not the start of the next token.
+            CppLexerToken last = prev();
+            throw new ParseException(
+                "expected ';' but found '" + peek().text() + "'",
+                last.line(), last.col() + last.text().length()
+            );
+        }
         throw error("expected '" + p + "' but found '" + peek().text() + "'");
     }
 
@@ -1321,6 +1340,8 @@ else if (checkPunct("{")) {
                     advance(); isDefault = true;
                 } else if (checkKeyword("delete")) {
                     advance(); isDelete = true;
+                    // C++26: = delete("reason")
+                    if (checkPunct("(")) { advance(); int _dd=1; while(!isAtEnd()&&_dd>0){if(checkPunct("("))_dd++;else if(checkPunct(")"))_dd--;advance();} }
                 } else {
                     pos = save;
                 }