feat: advanced ALTER COLUMN SET/DROP NOT NULL & DEFAULT, SET DATA TYPE (ADR-0035 Am2)

The standard-first ALTER COLUMN constraint gap-fill advanced mode lacked:

- ALTER COLUMN <c> SET DATA TYPE <ty> — ISO canonical synonym for the
  PostgreSQL TYPE shorthand (same AlterColumnType action + executor).
- SET NOT NULL / DROP NOT NULL — reuse the ADR-0029 do_add_constraint /
  do_drop_constraint executors (dry-run + internal-table guards free).
- SET DEFAULT <expr> / DROP DEFAULT — SET DEFAULT uses a dedicated
  raw-SQL executor (do_set_column_default); sql_expr yields no typed
  Value, so it can't go through do_add_constraint. DROP DEFAULT reuses
  do_drop_constraint.

Grammar: AT_ALTER_COLUMN gains a tail Choice (type / set / drop), reusing
SQL_TYPE and the CREATE TABLE DEFAULT_NODES; builder dispatch routes the
new column-attribute forms; runtime decomposes to the executors.

ADR-0035 Am2 corrected in-place: SET DEFAULT decomposes to
do_set_column_default, not do_add_constraint (Value-based) — found during
build.

Tests (test-first): 6 parse + 7 Tier-3 execution via run_replay. Suite
1962/0/1; clippy clean.

src/app.rs

@@ -1614,6 +1614,21 @@ impl App {
                     Some(table.as_str()),
                     Some(column.as_str()),
                 ),
+                // ADR-0035 Amendment 2: the column-attribute set/drop forms
+                // decompose to add/drop constraint, and name a column (so
+                // the friendly error can pinpoint it).
+                AlterTableAction::SetColumnNotNull { column }
+                | AlterTableAction::SetColumnDefault { column, .. } => (
+                    Operation::AddConstraint,
+                    Some(table.as_str()),
+                    Some(column.as_str()),
+                ),
+                AlterTableAction::DropColumnNotNull { column }
+                | AlterTableAction::DropColumnDefault { column } => (
+                    Operation::DropConstraint,
+                    Some(table.as_str()),
+                    Some(column.as_str()),
+                ),
                 AlterTableAction::AddTableConstraint { .. } => {
                     (Operation::AddConstraint, Some(table.as_str()), None)
                 }

src/db.rs

@@ -608,6 +608,17 @@ enum Request {
         source: Option<String>,
         reply: oneshot::Sender<Result<TableDescription, DbError>>,
     },
+    /// `ALTER TABLE … ALTER COLUMN <col> SET DEFAULT <expr>` — set a
+    /// column's default to raw SQL text (ADR-0035 Amendment 2). Distinct
+    /// from `AddConstraint(Default(Value))` because the advanced default
+    /// is raw `sql_expr` text with no typed `Value`.
+    SetColumnDefault {
+        table: String,
+        column: String,
+        default_sql: String,
+        source: Option<String>,
+        reply: oneshot::Sender<Result<TableDescription, DbError>>,
+    },
     /// `ALTER TABLE … ADD [CONSTRAINT <name>] CHECK (<expr>)` — a
     /// table-level CHECK, named or unnamed (ADR-0035 §4g).
     AlterAddTableCheck {
@@ -1131,6 +1142,27 @@ impl Database {
         recv.await.map_err(|_| DbError::WorkerGone)?
     }
 
+    /// `ALTER TABLE … ALTER COLUMN <col> SET DEFAULT <expr>` — set the
+    /// column's default to raw SQL text (ADR-0035 Amendment 2).
+    pub async fn set_column_default(
+        &self,
+        table: String,
+        column: String,
+        default_sql: String,
+        source: Option<String>,
+    ) -> Result<TableDescription, DbError> {
+        let (reply, recv) = oneshot::channel();
+        self.send(Request::SetColumnDefault {
+            table,
+            column,
+            default_sql,
+            source,
+            reply,
+        })
+        .await?;
+        recv.await.map_err(|_| DbError::WorkerGone)?
+    }
+
     /// `ALTER TABLE … ADD [CONSTRAINT <name>] CHECK (<expr>)` — a
     /// table-level CHECK (ADR-0035 §4g).
     pub async fn alter_add_table_check(
@@ -2366,6 +2398,24 @@ fn handle_request(
                 kind,
             ));
         }
+        Request::SetColumnDefault {
+            table,
+            column,
+            default_sql,
+            source,
+            reply,
+        } => {
+            snapshot_then(snap, batch, conn, source.as_deref(), reply, || {
+                do_set_column_default(
+                    conn,
+                    persistence,
+                    source.as_deref(),
+                    &table,
+                    &column,
+                    &default_sql,
+                )
+            });
+        }
         Request::AlterAddTableCheck {
             table,
             name,
@@ -4034,6 +4084,72 @@ fn do_drop_constraint(
     do_describe_table(conn, table)
 }
 
+/// `ALTER TABLE … ALTER COLUMN <col> SET DEFAULT <expr>` — set the
+/// column's default to raw SQL text (ADR-0035 Amendment 2). Mirrors
+/// `do_add_constraint`'s `Default` branch (the §6 `serial`/`shortid`
+/// refusal; no dry-run — a default never touches existing rows) but
+/// takes raw `sql_expr` text instead of a typed `Value` (advanced
+/// `SET DEFAULT` has no AST). DEFAULT is recoverable from the engine
+/// catalog, so there is no metadata write — the rebuilt table's DDL
+/// carries it (ADR-0029 §7).
+fn do_set_column_default(
+    conn: &Connection,
+    persistence: Option<&Persistence>,
+    source: Option<&str>,
+    table: &str,
+    column: &str,
+    default_sql: &str,
+) -> Result<TableDescription, DbError> {
+    let canonical_table = require_canonical_table(conn, table)?;
+    let table = canonical_table.as_str();
+    let old_schema = read_schema(conn, table)?;
+    let col_user_type = {
+        let col = old_schema
+            .columns
+            .iter()
+            .find(|c| c.name == column)
+            .ok_or_else(|| DbError::Sqlite {
+                message: format!("no such column: {table}.{column}"),
+                kind: SqliteErrorKind::NoSuchColumn,
+            })?;
+        col.user_type
+    };
+
+    // ADR-0029 §6 — an auto-generated column fills its own values, so a
+    // `default` would be a second, ambiguous source of "the value when
+    // none is given" (mirrors do_add_constraint).
+    if matches!(col_user_type, Some(Type::Serial | Type::ShortId)) {
+        return Err(DbError::Unsupported(format!(
+            "`{table}.{column}` is a {ty} column — it auto-fills its own \
+             values, so it cannot also carry a `default`.",
+            ty = col_user_type.expect("matched Some above").keyword(),
+        )));
+    }
+
+    let mut new_schema = old_schema.clone();
+    {
+        let target = new_schema
+            .columns
+            .iter_mut()
+            .find(|c| c.name == column)
+            .expect("column existence checked above");
+        target.default_sql = Some(default_sql.to_string());
+    }
+
+    let metadata_updates = |tx: &rusqlite::Transaction<'_>| -> Result<(), DbError> {
+        let changes = Changes {
+            schema_dirty: true,
+            rewritten_tables: vec![table.to_string()],
+            ..Changes::default()
+        };
+        finalize_persistence(tx, persistence, source, &changes)?;
+        Ok(())
+    };
+
+    rebuild_table(conn, table, &old_schema, &new_schema, metadata_updates)?;
+    do_describe_table(conn, table)
+}
+
 /// A row's primary-key cell values paired with its value in
 /// the column under test — the unit an ADR-0029 §5 dry-run
 /// scans.

src/dsl/command.rs

@@ -756,8 +756,26 @@ pub enum AlterTableAction {
     /// with `ChangeColumnMode::ForceConversion`, which is the ADR-0035 §7
     /// advanced-mode policy (lossy cells are *performed* with a note, no
     /// force flag; static-refused / incompatible still refuse). One undo
-    /// step (the executor's rebuild). ADR-0035 §4f.
+    /// step (the executor's rebuild). ADR-0035 §4f. The ISO synonym
+    /// `SET DATA TYPE` (canonical) and the PostgreSQL `TYPE` shorthand
+    /// both build this action (ADR-0035 Amendment 2).
     AlterColumnType { column: String, ty: Type },
+    /// `ALTER COLUMN <name> SET NOT NULL` (ADR-0035 Amendment 2) — a
+    /// documented PostgreSQL extension (ISO has no in-place NOT-NULL
+    /// verb). Decomposes to `do_add_constraint(NotNull)` (ADR-0029).
+    SetColumnNotNull { column: String },
+    /// `ALTER COLUMN <name> DROP NOT NULL` (ADR-0035 Amendment 2).
+    /// Decomposes to `do_drop_constraint(NotNull)`.
+    DropColumnNotNull { column: String },
+    /// `ALTER COLUMN <name> SET DEFAULT <expr>` (ADR-0035 Amendment 2,
+    /// ISO). `default_sql` is the raw `sql_expr` text (the §4a.2 / §4e
+    /// mechanism — `sql_expr` builds no AST, so the default cannot be a
+    /// typed `Value`). Decomposes to the dedicated `do_set_column_default`
+    /// executor (not `do_add_constraint`, which is `Value`-based).
+    SetColumnDefault { column: String, default_sql: String },
+    /// `ALTER COLUMN <name> DROP DEFAULT` (ADR-0035 Amendment 2, ISO).
+    /// Decomposes to `do_drop_constraint(Default)`.
+    DropColumnDefault { column: String },
     /// `ADD [CONSTRAINT <name>] (CHECK (…) | UNIQUE (…) | FOREIGN KEY
     /// (…) REFERENCES …)` — a table-level constraint (ADR-0035 §4g). The
     /// `name` is the `CONSTRAINT <name>` prefix (the FK carries its own

src/dsl/grammar/ddl.rs

@@ -1948,16 +1948,55 @@ const AT_RENAME_TAIL: Node = Node::Choice(AT_RENAME_TAIL_CHOICES);
 static AT_RENAME_NODES: &[Node] = &[Node::Word(Word::keyword("rename")), AT_RENAME_TAIL];
 const AT_RENAME: Node = Node::Seq(AT_RENAME_NODES);
 
-// `ALTER COLUMN <col> TYPE <type>` (ADR-0035 §4f). The type slot reuses
-// SQL_TYPE (the same alias map + `double precision` pair the CREATE
-// TABLE / ADD COLUMN forms use). The builder keys on the `type` keyword
-// — unique to this action (ADD COLUMN's type is a `col_type` ident).
+// `ALTER COLUMN <col> <action>` (ADR-0035 §4f + Amendment 2). The action
+// tail is a Choice on distinct concrete keywords — `type` / `set` /
+// `drop` — trap-safe. The type slot reuses SQL_TYPE (the same alias map +
+// `double precision` pair the CREATE TABLE / ADD COLUMN forms use).
+//
+//   TYPE <ty>                 — PostgreSQL shorthand (§4f)
+//   SET DATA TYPE <ty>        — ISO canonical synonym (Amendment 2)
+//   SET NOT NULL              — documented extension (Amendment 2)
+//   SET DEFAULT <expr>        — ISO (Amendment 2), raw sql_expr text
+//   DROP NOT NULL             — ISO-ish (Amendment 2)
+//   DROP DEFAULT              — ISO (Amendment 2)
+//
+// `NOT NULL` reused by both SET and DROP tails (distinct sibling leads in
+// each). `DEFAULT <expr>` reuses the CREATE TABLE `DEFAULT_NODES` so a
+// default is one syntax, captured as raw text (sql_expr builds no AST).
+static AT_AC_TYPE_NODES: &[Node] = &[
+    Node::Word(Word::keyword("type")),
+    super::sql_create_table::SQL_TYPE,
+];
+const AT_AC_TYPE: Node = Node::Seq(AT_AC_TYPE_NODES);
+static AT_AC_NOT_NULL_NODES: &[Node] =
+    &[Node::Word(Word::keyword("not")), Node::Word(Word::keyword("null"))];
+const AT_AC_NOT_NULL: Node = Node::Seq(AT_AC_NOT_NULL_NODES);
+static AT_AC_SET_DATA_TYPE_NODES: &[Node] = &[
+    Node::Word(Word::keyword("data")),
+    Node::Word(Word::keyword("type")),
+    super::sql_create_table::SQL_TYPE,
+];
+const AT_AC_SET_DATA_TYPE: Node = Node::Seq(AT_AC_SET_DATA_TYPE_NODES);
+static AT_AC_SET_TAIL_CHOICES: &[Node] = &[
+    AT_AC_SET_DATA_TYPE,
+    AT_AC_NOT_NULL,
+    Node::Seq(super::sql_create_table::DEFAULT_NODES),
+];
+const AT_AC_SET_TAIL: Node = Node::Choice(AT_AC_SET_TAIL_CHOICES);
+static AT_AC_SET_NODES: &[Node] = &[Node::Word(Word::keyword("set")), AT_AC_SET_TAIL];
+const AT_AC_SET: Node = Node::Seq(AT_AC_SET_NODES);
+static AT_AC_DROP_TAIL_CHOICES: &[Node] =
+    &[AT_AC_NOT_NULL, Node::Word(Word::keyword("default"))];
+const AT_AC_DROP_TAIL: Node = Node::Choice(AT_AC_DROP_TAIL_CHOICES);
+static AT_AC_DROP_NODES: &[Node] = &[Node::Word(Word::keyword("drop")), AT_AC_DROP_TAIL];
+const AT_AC_DROP: Node = Node::Seq(AT_AC_DROP_NODES);
+static AT_ALTER_COLUMN_TAIL_CHOICES: &[Node] = &[AT_AC_TYPE, AT_AC_SET, AT_AC_DROP];
+const AT_ALTER_COLUMN_TAIL: Node = Node::Choice(AT_ALTER_COLUMN_TAIL_CHOICES);
 static AT_ALTER_COLUMN_NODES: &[Node] = &[
     Node::Word(Word::keyword("alter")),
     Node::Word(Word::keyword("column")),
     COLUMN_NAME,
-    Node::Word(Word::keyword("type")),
-    super::sql_create_table::SQL_TYPE,
+    AT_ALTER_COLUMN_TAIL,
 ];
 const AT_ALTER_COLUMN: Node = Node::Seq(AT_ALTER_COLUMN_NODES);
 
@@ -2148,6 +2187,42 @@ fn build_alter_column_type(path: &MatchedPath) -> Result<AlterTableAction, Valid
     Ok(AlterTableAction::AlterColumnType { column, ty })
 }
 
+/// Build an `ALTER COLUMN <c> SET/DROP NOT NULL` / `SET/DROP DEFAULT`
+/// action (ADR-0035 Amendment 2). `SET DATA TYPE` is handled by the
+/// `type`-keyword branch, so this only sees the four constraint forms.
+/// The 2×2 is decided by `set` (vs drop) and `default` (vs not null);
+/// `SET DEFAULT`'s value is captured as raw `sql_expr` text by span,
+/// reusing `build_alter_add_column_spec`'s mechanism (no AST — 4a.2).
+fn build_alter_column_attr(
+    path: &MatchedPath,
+    source: &str,
+) -> Result<AlterTableAction, ValidationError> {
+    let column = require_ident(path, "column_name")?;
+    let is_set = path.contains_word("set");
+    let is_default = path.contains_word("default");
+    Ok(match (is_set, is_default) {
+        (true, true) => {
+            let mut items = path.items.iter().peekable();
+            let mut default_sql: Option<String> = None;
+            while let Some(item) = items.next() {
+                if matches!(&item.kind, MatchedKind::Word("default"))
+                    && let Some((start, end)) = capture_expr_span(&mut items)
+                {
+                    default_sql = Some(source[start..end].trim().to_string());
+                }
+            }
+            let default_sql = default_sql.ok_or_else(|| ValidationError {
+                message_key: "parse.error_wrapper",
+                args: vec![("detail", "set default needs a value".to_string())],
+            })?;
+            AlterTableAction::SetColumnDefault { column, default_sql }
+        }
+        (false, true) => AlterTableAction::DropColumnDefault { column },
+        (true, false) => AlterTableAction::SetColumnNotNull { column },
+        (false, false) => AlterTableAction::DropColumnNotNull { column },
+    })
+}
+
 /// Build `Command::SqlAlterTable` (ADR-0035 §4e/§4f/§4g). Exactly one
 /// action `Choice` branch matched; the builder recovers which from the
 /// matched words. Discrimination order matters:
@@ -2171,7 +2246,19 @@ fn build_alter_column_type(path: &MatchedPath) -> Result<AlterTableAction, Valid
 fn build_sql_alter_table(path: &MatchedPath, source: &str) -> Result<Command, ValidationError> {
     let table = require_ident(path, "table_name")?;
     let action = if path.contains_word("type") {
+        // covers `TYPE <ty>` and the ISO synonym `SET DATA TYPE <ty>`
         build_alter_column_type(path)?
+    } else if path.contains_word("column")
+        && !path.contains_word("add")
+        && !path.contains_word("rename")
+        && (path.contains_word("set")
+            || path.contains_word("default")
+            || (path.contains_word("not") && path.contains_word("null")))
+    {
+        // ADR-0035 Amendment 2: `ALTER COLUMN <c> SET/DROP NOT NULL` and
+        // `SET/DROP DEFAULT`. `add column … not null/default` is excluded
+        // by `!add`; plain `drop column` lacks the attribute markers.
+        build_alter_column_attr(path, source)?
     } else if path.contains_word("column") {
         if path.contains_word("add") {
             AlterTableAction::AddColumn(Box::new(build_alter_add_column_spec(path, source)?))
@@ -2934,6 +3021,92 @@ mod sql_alter_table_tests {
         ));
     }
 
+    // --- ADR-0035 Amendment 2: ALTER COLUMN constraint gap-fill ---
+
+    #[test]
+    fn alter_column_set_data_type_is_a_type_synonym() {
+        // ISO `SET DATA TYPE` is the canonical form; it yields the same
+        // AlterColumnType action as the PostgreSQL `TYPE` shorthand.
+        match alter("alter table T alter column qty set data type int").1 {
+            AlterTableAction::AlterColumnType { column, ty } => {
+                assert_eq!(column, "qty");
+                assert_eq!(ty, crate::dsl::types::Type::Int);
+            }
+            other => panic!("expected AlterColumnType, got {other:?}"),
+        }
+        // alias map still applies through the synonym
+        assert!(matches!(
+            alter("alter table T alter column n set data type double precision").1,
+            AlterTableAction::AlterColumnType { ty: crate::dsl::types::Type::Real, .. }
+        ));
+    }
+
+    #[test]
+    fn alter_column_set_not_null_parses() {
+        let (table, action) = alter("alter table T alter column email set not null");
+        assert_eq!(table, "T");
+        match action {
+            AlterTableAction::SetColumnNotNull { column } => assert_eq!(column, "email"),
+            other => panic!("expected SetColumnNotNull, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn alter_column_drop_not_null_parses() {
+        match alter("alter table T alter column email drop not null").1 {
+            AlterTableAction::DropColumnNotNull { column } => assert_eq!(column, "email"),
+            other => panic!("expected DropColumnNotNull, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn alter_column_set_default_captures_raw_expr() {
+        match alter("alter table T alter column qty set default 0").1 {
+            AlterTableAction::SetColumnDefault { column, default_sql } => {
+                assert_eq!(column, "qty");
+                assert_eq!(default_sql, "0");
+            }
+            other => panic!("expected SetColumnDefault, got {other:?}"),
+        }
+        // a parenthesised expression default round-trips as raw text
+        match alter("alter table T alter column qty set default (1 + 1)").1 {
+            AlterTableAction::SetColumnDefault { default_sql, .. } => {
+                assert_eq!(default_sql, "(1 + 1)");
+            }
+            other => panic!("expected SetColumnDefault, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn alter_column_drop_default_parses() {
+        match alter("alter table T alter column qty drop default").1 {
+            AlterTableAction::DropColumnDefault { column } => assert_eq!(column, "qty"),
+            other => panic!("expected DropColumnDefault, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn alter_column_gap_fill_does_not_steal_the_existing_actions() {
+        // The new set/drop column-attribute forms must not misroute the
+        // top-level add/drop/rename-column or the bare `type` form.
+        assert!(matches!(
+            alter("alter table T add column note text not null").1,
+            AlterTableAction::AddColumn(_)
+        ));
+        assert!(matches!(
+            alter("alter table T drop column note").1,
+            AlterTableAction::DropColumn { .. }
+        ));
+        assert!(matches!(
+            alter("alter table T alter column a type text").1,
+            AlterTableAction::AlterColumnType { .. }
+        ));
+        assert!(matches!(
+            alter("alter table T drop constraint c_chk").1,
+            AlterTableAction::DropConstraint { .. }
+        ));
+    }
+
     #[test]
     fn type_discriminator_probe_column_named_type() {
         // PROBE (DA): the `type`-keyword discriminator keys on the literal

src/runtime.rs

@@ -33,7 +33,7 @@ use crate::db::{
     Database, DbError, DeleteResult, DropColumnResult, DropIndexOutcome, DropOutcome, InsertResult,
     QueryPlan, TableDescription, UpdateResult,
 };
-use crate::dsl::command::TableConstraint;
+use crate::dsl::command::{Constraint, ConstraintKind, TableConstraint};
 use crate::dsl::{AlterTableAction, ChangeColumnMode, Command, ColumnSpec};
 use crate::dsl::walker::Severity;
 use crate::event::AppEvent;
@@ -2185,6 +2185,26 @@ async fn execute_command_typed(
                 .change_column_type(table, column, ty, ChangeColumnMode::ForceConversion, src)
                 .await
                 .map(CommandOutcome::ChangeColumn),
+            // ADR-0035 Amendment 2: ALTER COLUMN constraint gap-fill.
+            // SET/DROP NOT NULL and DROP DEFAULT reuse the ADR-0029
+            // executors; SET DEFAULT needs the raw-SQL path (sql_expr has
+            // no typed Value).
+            AlterTableAction::SetColumnNotNull { column } => database
+                .add_constraint(table, column, Constraint::NotNull, src)
+                .await
+                .map(|d| CommandOutcome::Schema(Some(d))),
+            AlterTableAction::DropColumnNotNull { column } => database
+                .drop_constraint(table, column, ConstraintKind::NotNull, src)
+                .await
+                .map(|d| CommandOutcome::Schema(Some(d))),
+            AlterTableAction::SetColumnDefault { column, default_sql } => database
+                .set_column_default(table, column, default_sql, src)
+                .await
+                .map(|d| CommandOutcome::Schema(Some(d))),
+            AlterTableAction::DropColumnDefault { column } => database
+                .drop_constraint(table, column, ConstraintKind::Default, src)
+                .await
+                .map(|d| CommandOutcome::Schema(Some(d))),
             // `ADD [CONSTRAINT <name>] (CHECK | UNIQUE | FOREIGN KEY)`
             // (ADR-0035 §4g) — each reuses an existing low-level executor
             // (the FK via the relationship machinery `add 1:n