A column declared with `NOT NULL DEFAULT ''` apply-loops because the
catalog reads it back as a bareword empty string and the default
normaliser doesn't wrap it the way it wraps non-empty barewords for
ENUM / SET / CHAR / temporal types. Parser stores `''` (quoted), so
they never compare equal and every post-apply plan re-emits
`MODIFY COLUMN … DEFAULT ''`.

Surfaced while writing the evolution scenario in PR #31; worked
around there by dropping the explicit DEFAULT. Filed under
"High — correctness bugs" because it produces a permanent drift loop
that doesn't self-resolve.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Two Copilot nits on the entry I just added in PR #32:

  - normalizeColumnDefault lives in catalog/tables.go, not
    catalog/catalog.go. (Copilot #1.)
  - The function isn't ENUM/SET/CHAR/temporal-specific — it ignores
    typeName entirely and uses vitess to classify any value as
    bareword-vs-non-bareword. The bug is the `def == ""` short-circuit
    at the top of the function, which returns "" before the bareword
    check runs. The parser side stores `''` (quoted), so they never
    compare equal. (Copilot #2.)

Note also expands the suggested fix: either wrap the empty-string
case for column types where MySQL preserves it, or simply drop the
short-circuit and let vitess classify "".

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Copilot correctly noted the second fix path I'd suggested ("drop the
short-circuit and let vitess classify") doesn't work — vitess can't
parse `SELECT ` and returns a syntax error. The empty-string case
needs an explicit mapping, and since the right behaviour depends on
the column type (varchar/char/text/enum/set preserve `''`; numeric
types don't take a literal empty string at all), the fix has to
start consuming `typeName`, which the current implementation ignores.

Verified with a one-liner go run that `SELECT ''` parses fine but
`SELECT ` (trailing-space empty expression) errors out.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Copilot review: TEXT (and BLOB) cannot take a literal DEFAULT in
MySQL, so listing `text` in the suggested-fix type set documents an
impossible case. Narrow to CHAR / VARCHAR / BINARY / VARBINARY /
ENUM / SET — the types that actually accept `DEFAULT ''` as the
bareword empty string we'd need to round-trip — and explicitly call
out that TEXT / BLOB are excluded because MySQL forbids a literal
default on those.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Copilot review #4 style nit: the long line with the parenthetical
aside broke the rest of TODO.md's wrapping convention, and inline
`SELECT ` (with trailing space) is brittle in renderers that
collapse trailing whitespace. Rewrapped at the standard width and
turned the parenthetical into its own sentence; the inline code is
now `SELECT` followed by the prose "with an empty expression".

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>