PL/pgSQL THEN binging in condition

postgres=# SELECT version();
version
------------------------------------------------------------
PostgreSQL 16.4, compiled by Visual C++ build 1940, 64-bit
(1 row)

postgres=# DO $$
postgres$# BEGIN
postgres$# IF
postgres$# CASE
postgres$# WHEN TRUE
postgres$# THEN TRUE
postgres$# END
postgres$# THEN
postgres$# NULL;
postgres$# END IF;
postgres$# END
postgres$# $$;
ERROR: syntax error at end of input
LINE 5: WHEN TRUE
^

It seems error here because first THEN bound to IF statement rather than
CASE expression. Workaround here would be using parenthesis:

postgres=# DO $$
postgres$# BEGIN
postgres$# IF
postgres$# (CASE
postgres$# WHEN TRUE
postgres$# THEN TRUE
postgres$# END)
postgres$# THEN
postgres$# NULL;
postgres$# END IF;
postgres$# END
postgres$# $$;
DO

If behavior can not be changed for compatibility reasons, it at least can
use better error message.