Report the correct column number for syntax errors

quickjs.c

@@ -22452,8 +22452,18 @@ int JS_PRINTF_FORMAT_ATTR(2, 3) js_parse_error(JSParseState *s, JS_PRINTF_FORMAT
     backtrace_flags = 0;
     if (s->cur_func && s->cur_func->backtrace_barrier)
         backtrace_flags = JS_BACKTRACE_FLAG_SINGLE_LEVEL;
+    /* s->col_num is not advanced during token scanning, so derive the column
+       as the 1-based offset of the token from the start of its line. */
+    int err_col_num = s->token.col_num;
+    if (s->token.ptr && s->token.ptr >= s->buf_start) {
+        const uint8_t *line_start = s->token.ptr;
+        while (line_start > s->buf_start &&
+               line_start[-1] != '\n' && line_start[-1] != '\r')
+            line_start--;
+        err_col_num = (int)(s->token.ptr - line_start) + 1;
+    }
     build_backtrace(ctx, ctx->rt->current_exception, JS_UNDEFINED, s->filename,
-                    s->line_num, s->col_num, backtrace_flags);
+                    s->token.line_num, err_col_num, backtrace_flags);
     return -1;
 }
 

tests/parse-error-column.js

@@ -0,0 +1,26 @@
+import { assert } from "./assert.js";
+
+/* A SyntaxError should report the column of the offending token, not always
+   column 1. The location is exposed through the error's stack trace. */
+function errorLocation(src) {
+    try {
+        eval(src);
+    } catch (e) {
+        const m = /<input>:(\d+):(\d+)/.exec(e.stack);
+        if (m)
+            return { line: +m[1], col: +m[2] };
+    }
+    return { line: -1, col: -1 };
+}
+
+assert(errorLocation("let x = ;").col, 9);
+assert(errorLocation("1 + + ;").col, 7);
+assert(errorLocation("hocuspocus(").col, 12);
+assert(errorLocation("a b").col, 3);
+
+/* line number stays correct and the column is relative to the line start */
+{
+    const loc = errorLocation("let ok = 1\nlet z = @");
+    assert(loc.line, 2);
+    assert(loc.col, 9);
+}