feat: 表达式作为值 — if/else 和 block 可产生值

src/codegen/codegen.c

@@ -451,6 +451,48 @@ static LLVMValueRef codegen_expr(CgCtx* ctx, AstNode* node) {
         return LLVMBuildLoad2(ctx->builder, elem_load_ty, elem_ptr, "arr_load");
     }
 
+    // 块表达式: { stmt*; expr } → 最后表达式的值
+    case AST_BLOCK: {
+        LLVMValueRef result = NULL;
+        for (size_t i = 0; i < node->as.block.stmt_count; i++) {
+            AstNode* stmt = node->as.block.stmts[i];
+            bool is_last = (i == node->as.block.stmt_count - 1);
+            if (is_last && stmt->kind == AST_EXPR_STMT && node->type.kind != TYPE_VOID) {
+                result = codegen_expr(ctx, stmt->as.expr_stmt.expr);
+            } else {
+                codegen_stmt(ctx, stmt);
+            }
+        }
+        return result;
+    }
+
+    // if 表达式: if cond { a } else { b }
+    case AST_IF_STMT: {
+        if (node->type.kind == TYPE_VOID) { codegen_stmt(ctx, node); return NULL; }
+        LLVMValueRef cond_val = codegen_expr(ctx, node->as.if_stmt.cond);
+        if (!cond_val) return NULL;
+        LLVMTypeRef res_ty = type_info_to_llvm(ctx, &node->type);
+        LLVMValueRef alloca = LLVMBuildAlloca(ctx->builder, res_ty, "if_res");
+        LLVMValueRef func = LLVMGetBasicBlockParent(LLVMGetInsertBlock(ctx->builder));
+        LLVMBasicBlockRef then_bb = LLVMAppendBasicBlockInContext(ctx->context, func, "then");
+        LLVMBasicBlockRef else_bb = LLVMAppendBasicBlockInContext(ctx->context, func, "else");
+        LLVMBasicBlockRef merge_bb = LLVMAppendBasicBlockInContext(ctx->context, func, "if_merge");
+        LLVMBuildCondBr(ctx->builder, cond_val, then_bb, else_bb);
+
+        LLVMPositionBuilderAtEnd(ctx->builder, then_bb);
+        LLVMValueRef then_val = codegen_expr(ctx, node->as.if_stmt.then_block);
+        if (then_val) LLVMBuildStore(ctx->builder, then_val, alloca);
+        LLVMBuildBr(ctx->builder, merge_bb);
+
+        LLVMPositionBuilderAtEnd(ctx->builder, else_bb);
+        LLVMValueRef else_val = codegen_expr(ctx, node->as.if_stmt.else_block);
+        if (else_val) LLVMBuildStore(ctx->builder, else_val, alloca);
+        LLVMBuildBr(ctx->builder, merge_bb);
+
+        LLVMPositionBuilderAtEnd(ctx->builder, merge_bb);
+        return LLVMBuildLoad2(ctx->builder, res_ty, alloca, "if_val");
+    }
+
     default:
         return NULL;
     }

src/parser/parser.c

@@ -258,7 +258,23 @@ static AstNode* parse_expr_prec(Parser* p, Precedence min_prec, ErrorInfo* error
     AstNode* left = NULL;
 
     // 前缀解析
-    if (tok->kind == TOK_MINUS || tok->kind == TOK_BANG) {
+    if (tok->kind == TOK_IF) {
+        // if-expr: if cond { then } else { else }  →  AST_IF_STMT (表达式位置)
+        const Token* if_tok = advance(p);
+        AstNode* cond = parse_expr(p, error);
+        if (!cond) return NULL;
+        AstNode* then_block = parse_block(p, error);
+        if (!then_block) return NULL;
+        AstNode* else_block = NULL;
+        if (match(p, TOK_ELSE)) {
+            if (peek(p)->kind == TOK_IF)
+                else_block = parse_expr_prec(p, min_prec, error);  // else if
+            else
+                else_block = parse_block(p, error);
+            if (!else_block) return NULL;
+        }
+        left = ast_make_if(p->arena, cond, then_block, else_block, tok_loc(if_tok));
+    } else if (tok->kind == TOK_MINUS || tok->kind == TOK_BANG) {
         left = parse_unary(p, error);
     } else if (tok->kind == TOK_LPAREN) {
         left = parse_group(p, error);

src/sema/sema.c

@@ -423,6 +423,8 @@ static void analyze_expr(AstNode* node, Scope* scope, ErrorList* errors, Arena*
     case AST_ENUM_VARIANT:   analyze_enum_variant(node, scope, errors, a);   break;
     case AST_INDEX_EXPR:     analyze_index_expr(node, scope, errors, a);     break;
     case AST_METHOD_CALL:    analyze_method_call(node, scope, errors, a);    break;
+    case AST_IF_STMT:        analyze_node(node, scope, errors, a);           break;
+    case AST_BLOCK:          analyze_node(node, scope, errors, a);           break;
     default: break;
     }
 }
@@ -602,6 +604,17 @@ static void analyze_node(AstNode* node, Scope* scope, ErrorList* errors, Arena*
         for (size_t i = 0; i < node->as.block.stmt_count; i++) {
             analyze_node(node->as.block.stmts[i], scope, errors, a);
         }
+        // 表达式作为值: 块类型 = 最后一条表达式语句的类型
+        if (node->as.block.stmt_count > 0) {
+            AstNode* last = node->as.block.stmts[node->as.block.stmt_count - 1];
+            if (last->kind == AST_EXPR_STMT) {
+                TypeKind ek = last->as.expr_stmt.expr->type.kind;
+                if (ek != TYPE_ERROR && ek != TYPE_VOID) {
+                    node->type.kind = ek;
+                    node->type.struct_name = last->as.expr_stmt.expr->type.struct_name;
+                }
+            }
+        }
         break;
 
     case AST_LET_STMT: {
@@ -773,6 +786,19 @@ static void analyze_node(AstNode* node, Scope* scope, ErrorList* errors, Arena*
         if (node->as.if_stmt.else_block) {
             analyze_node(node->as.if_stmt.else_block, scope, errors, a);
         }
+        // 表达式作为值: if 类型 = 两个分支的共同非 void 类型
+        {
+            AstNode* tb = node->as.if_stmt.then_block;
+            AstNode* eb = node->as.if_stmt.else_block;
+            if (tb && eb) {
+                TypeKind tt = tb->type.kind, et = eb->type.kind;
+                if (tt == et && tt != TYPE_VOID && tt != TYPE_ERROR) {
+                    node->type.kind = tt;
+                    if (tt == TYPE_STRUCT && tb->type.struct_name)
+                        node->type.struct_name = tb->type.struct_name;
+                }
+            }
+        }
         break;
 
     case AST_WHILE_STMT:

test/programs/30_if_expr.l

@@ -0,0 +1,14 @@
+fn main() -> i64 {
+    // if 作为表达式: let x = if cond { val1 } else { val2 }
+    let a = if true { 10; } else { 20; };
+    print_i64(a);  // 10
+
+    let b = if false { 10; } else { 20; };
+    print_i64(b);  // 20
+
+    // 嵌套 if 表达式
+    let c = if a > 5 { 100; } else { 0; };
+    print_i64(c);  // 100
+
+    return 0;
+}