BigQuery→DuckDB: DATE_ADD with non-literal INTERVAL expression emits invalid SQL (missing parentheses, nested DATE_DIFF not transpiled)

[nikchha]

DATE_ADD(date, INTERVAL <expr> <part>) where <expr> is not a literal produces invalid DuckDB SQL: the interval expression is emitted without parentheses, and a nested function inside it is not transpiled.

Version: @polyglot-sql/sdk 0.12.1, DuckDB 1.5.5

Repro 1: column reference

transpile(
  "SELECT DATE_ADD(DATE '2026-01-01', INTERVAL n MONTH) AS d FROM (SELECT 2 AS n)",
  Dialect.BigQuery,
  Dialect.DuckDB,
).sql[0];

Actual:

SELECT CAST('2026-01-01' AS DATE) + INTERVAL n MONTH AS d FROM (SELECT 2 AS n)

DuckDB: Parser Error: syntax error at or near "MONTH"

Expected: ... + INTERVAL (n) MONTH ..., which DuckDB accepts and returns 2026-03-01.

Repro 2: nested DATE_DIFF

transpile(
  "SELECT DATE_ADD(DATE '2026-01-01', INTERVAL DATE_DIFF(DATE '2026-03-15', DATE '2026-01-01', MONTH) MONTH) AS d",
  Dialect.BigQuery,
  Dialect.DuckDB,
).sql[0];

Actual:

SELECT CAST('2026-01-01' AS DATE) + INTERVAL DATE_DIFF(CAST('2026-03-15' AS DATE), CAST('2026-01-01' AS DATE), MONTH) MONTH AS d

DuckDB: Parser Error: syntax error at or near "("

Besides the missing parentheses, the inner DATE_DIFF keeps BigQuery argument order and syntax. At top level the same call is transpiled correctly to DATE_DIFF('MONTH', CAST('2026-01-01' AS DATE), CAST('2026-03-15' AS DATE)), so the interval's inner expression seems to skip the dialect transforms.

Expected:

SELECT CAST('2026-01-01' AS DATE) + INTERVAL (DATE_DIFF('MONTH', CAST('2026-01-01' AS DATE), CAST('2026-03-15' AS DATE))) MONTH AS d

[tobilg]

Thanks for reporting, this will be fixed in the 0.13.0 release that will be done today. Please check and close if this fixed your issue.