feat: ?? 空值合并运算符 — a ?? b → if a!=0 {a} else {b}

Token(76): +TOK_QMARK_QMARK, 优先级 PREC_COALESCE=15 (||之上, |>之下)
parser 去糖为 if-expr, 零 sema/codegen 改动

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/lexer/lexer.c

@@ -155,6 +155,7 @@ Token* lex(Arena* a, const char* source, const char* filename,
         else if (c == '&' && peek_next(&l) == '&') { tokens[idx++] = make_token(&l, TOK_AND_AND, l.pos, 2); advance(&l); advance(&l); }
         else if (c == '|' && peek_next(&l) == '>') { tokens[idx++] = make_token(&l, TOK_PIPE, l.pos, 2); advance(&l); advance(&l); }
         else if (c == '|' && peek_next(&l) == '|') { tokens[idx++] = make_token(&l, TOK_PIPE_PIPE, l.pos, 2); advance(&l); advance(&l); }
+        else if (c == '?' && peek_next(&l) == '?') { tokens[idx++] = make_token(&l, TOK_QMARK_QMARK, l.pos, 2); advance(&l); advance(&l); }
         else if (c == '.') { tokens[idx++] = make_token(&l, TOK_DOT, l.pos, 1); advance(&l); }
         else if (c == '[') { tokens[idx++] = make_token(&l, TOK_LBRACKET, l.pos, 1); advance(&l); }
         else if (c == ']') { tokens[idx++] = make_token(&l, TOK_RBRACKET, l.pos, 1); advance(&l); }

src/lexer/token.c

@@ -30,6 +30,7 @@ static const char* NAMES[] = {
     [TOK_COMMA] = ",", [TOK_COLON] = ":", [TOK_SEMICOLON] = ";",
     [TOK_ASSIGN] = "=",
     [TOK_DOT] = ".", [TOK_COLON_COLON] = "::", [TOK_HASH] = "#",
+    [TOK_QMARK_QMARK] = "??",
     [TOK_EOF] = "EOF", [TOK_ERROR] = "错误",
 };
 

src/lexer/token.h

@@ -26,7 +26,7 @@ typedef enum {
     TOK_LBRACKET, TOK_RBRACKET,
     TOK_COMMA, TOK_COLON, TOK_SEMICOLON, TOK_ASSIGN,
     // 特殊
-    TOK_DOT, TOK_COLON_COLON, TOK_HASH,
+    TOK_DOT, TOK_COLON_COLON, TOK_HASH, TOK_QMARK_QMARK,
     TOK_EOF, TOK_ERROR,
 } TokenKind;
 

src/parser/expr.c

@@ -9,6 +9,7 @@ int parse_depth = 0;
 // === 运算符优先级 → Precedence 映射 ===
 Precedence tok_to_prec(TokenKind kind) {
     switch (kind) {
+        case TOK_QMARK_QMARK: return PREC_COALESCE;
         case TOK_PIPE_PIPE: return PREC_OR;
         case TOK_AND_AND:   return PREC_AND;
         case TOK_EQ_EQ: case TOK_BANG_EQ:
@@ -419,6 +420,24 @@ AstNode* parse_expr_prec(Parser* p, Precedence min_prec, ErrorInfo* error) {
             continue;
         }
 
+        // ?? 空值合并: a ?? b → if a != 0 { a; } else { b; }
+        if (kind == TOK_QMARK_QMARK) {
+            if (PREC_COALESCE <= min_prec) break;
+            advance(p);
+            AstNode* right = parse_expr_prec(p, PREC_COALESCE, error);
+            if (!right) return NULL;
+            AstNode* cond = ast_make_binary(p->arena, OP_NE, left,
+                ast_make_literal_i64(p->arena, 0, left->loc), left->loc);
+            AstNode** t_arr = (AstNode**)arena_alloc_impl(p->arena, sizeof(AstNode*));
+            t_arr[0] = ast_make_expr_stmt(p->arena, left, left->loc);
+            AstNode* then_block = ast_make_block(p->arena, t_arr, 1, left->loc);
+            AstNode** e_arr = (AstNode**)arena_alloc_impl(p->arena, sizeof(AstNode*));
+            e_arr[0] = ast_make_expr_stmt(p->arena, right, right->loc);
+            AstNode* else_block = ast_make_block(p->arena, e_arr, 1, right->loc);
+            left = ast_make_if(p->arena, cond, then_block, else_block, left->loc);
+            continue;
+        }
+
         // 中缀运算符
         Precedence prec = tok_to_prec(kind);
         if (prec <= min_prec) break;

src/parser/parse_internal.h

@@ -38,6 +38,7 @@ static inline const Token* expect(Parser* p, TokenKind k, ErrorInfo* e, const ch
 typedef enum {
     PREC_NONE = 0,
     PREC_PIPE = 10,
+    PREC_COALESCE = 15,
     PREC_OR = 20,
     PREC_AND = 30,
     PREC_COMPARE = 40,