Am I misunderstanding precedence?

I've encountered something which confuses me, but I'm not sure if it's a bug or just something I don't understand.

I'm looking at this file,
https://github.com/mysql/mysql-server/blob/5c8c085ba96d30d697d0baa54d67b102c232116b/sql/sql_yacc.yy#L1169

https://github.com/mysql/mysql-server/blob/5c8c085ba96d30d697d0baa54d67b102c232116b/sql/sql_yacc.yy#L9300

https://github.com/mysql/mysql-server/blob/5c8c085ba96d30d697d0baa54d67b102c232116b/sql/sql_yacc.yy#L9582

Of note are,

```
%left   EQ EQUAL_SYM GE GT_SYM LE LT NE IS LIKE REGEXP IN_SYM
...
%left  INTERVAL_SYM

...

bool_pri:
          bool_pri IS NULL_SYM %prec IS
...

simple_expr:
        | INTERVAL_SYM expr interval '+' expr %prec INTERVAL_SYM
```

From the above snippet, it looks like the intention is for the `INTERVAL_SYM` rule to have a higher precedence than the `IS NULL` rule.

Given the input string `INTERVAL 0 DAY + NULL IS NULL`, there are two valid parse trees, ignoring precedence,

+ (INTERVAL 0 DAY + NULL) IS NULL
+ INTERVAL 0 DAY + (NULL IS NULL)

If we include precedence, it seems we would want,

+ (INTERVAL 0 DAY + NULL) IS NULL

The MySQL documentation seems to agree,
https://dev.mysql.com/doc/refman/5.7/en/operator-precedence.html

It shows that INTERVAL should have a higher precedence than IS.

However, when I actually run this input against MySQL,
```
SELECT
    INTERVAL 0 DAY + NULL IS NULL,
    (INTERVAL 0 DAY + NULL) IS NULL,
    INTERVAL 0 DAY + (NULL IS NULL)
```
https://www.db-fiddle.com/f/cMPJfx5kZKoMeX4PKDrTWC/0


I find that IS NULL has higher precedence. Which is unexpected.

Is this a bug with bison? Or am I misunderstanding something?

I'm not sure what specific version of bison is being used but I found it should be 2.1+
https://dev.mysql.com/doc/refman/5.7/en/source-installation-prerequisites.html