Fix internal error from CollateExpr in SQL/JSON DEFAULT expressions

SQL/JSON functions such as JSON_VALUE could fail with "unrecognized
node type" errors when a DEFAULT clause contained an explicit COLLATE
expression. That happened because assign_collations_walker() could
invoke exprSetCollation() on a JsonBehavior expression whose DEFAULT
still contained a CollateExpr, which exprSetCollation() does not
handle.

For example:

  SELECT JSON_VALUE('{"a":1}', '$.c' RETURNING text
                    DEFAULT 'A' COLLATE "C" ON EMPTY);

Fix by validating in transformJsonBehavior() that the DEFAULT
expression's collation matches the enclosing JSON expression’s
collation. In exprSetCollation(), replace the recursive call on the
JsonBehavior expression with an assertion that its collation already
matches the target, since the parser now enforces that condition.

Reported-by: Jian He <jian.universality@gmail.com>
Author: Jian He <jian.universality@gmail.com>
Reviewed-by: Amit Langote <amitlangote09@gmail.com>
Discussion: https://postgr.es/m/CACJufxHVwYYSyiVQ6o+PsRX6zQ7rAFinh_fv1kCfTsT1xG4Zeg@mail.gmail.com
Backpatch-through: 17

2 files changed, 62 insertions, 0 deletions

diff --git a/src/test/regress/expected/collate.icu.utf8.out b/src/test/regress/expected/collate.icu.utf8.out
index ef79d6f1ded..05d8b3b369e 100644
--- a/src/test/regress/expected/collate.icu.utf8.out
+++ b/src/test/regress/expected/collate.icu.utf8.out
@@ -2694,6 +2694,55 @@ SELECT * FROM t5 ORDER BY c ASC, a ASC;
  3 | d1 | d1
 (3 rows)
 
+-- Check that DEFAULT expressions in SQL/JSON functions use the same collation
+-- as the RETURNING type.  Mismatched collations should raise an error.
+CREATE DOMAIN d1 AS text COLLATE case_insensitive;
+CREATE DOMAIN d2 AS text COLLATE "C";
+SELECT JSON_VALUE('{"a": "A"}', '$.a' RETURNING d1 DEFAULT ('C' COLLATE "C") COLLATE case_insensitive ON EMPTY) = 'a'; -- true
+ ?column? 
+----------
+ t
+(1 row)
+
+SELECT JSON_VALUE('{"a": "A"}', '$.a' RETURNING d1 DEFAULT 'C' ON EMPTY) = 'a'; -- true
+ ?column? 
+----------
+ t
+(1 row)
+
+SELECT JSON_VALUE('{"a": "A"}', '$.a' RETURNING d1 DEFAULT 'C'::d2 ON EMPTY) = 'a'; -- error
+ERROR:  the collation of DEFAULT expression conflicts with RETURNING clause
+LINE 1: ...ON_VALUE('{"a": "A"}', '$.a' RETURNING d1 DEFAULT 'C'::d2 ON...
+                                                             ^
+DETAIL:  "C" versus "case_insensitive"
+SELECT JSON_VALUE('{"a": "A"}', '$.a' RETURNING d1 DEFAULT 'C' COLLATE "C" ON EMPTY) = 'a'; -- error
+ERROR:  the collation of DEFAULT expression conflicts with RETURNING clause
+LINE 1: ...ON_VALUE('{"a": "A"}', '$.a' RETURNING d1 DEFAULT 'C' COLLAT...
+                                                             ^
+DETAIL:  "C" versus "case_insensitive"
+SELECT JSON_VALUE('{"a": "A"}', '$.c' RETURNING d1 DEFAULT 'A' ON EMPTY) = 'a'; -- true
+ ?column? 
+----------
+ t
+(1 row)
+
+SELECT JSON_VALUE('{"a": "A"}', '$.c' RETURNING d1 DEFAULT 'A' COLLATE case_insensitive ON EMPTY) = 'a'; -- true
+ ?column? 
+----------
+ t
+(1 row)
+
+SELECT JSON_VALUE('{"a": "A"}', '$.c' RETURNING d1 DEFAULT 'A'::d2 ON EMPTY) = 'a'; -- error
+ERROR:  the collation of DEFAULT expression conflicts with RETURNING clause
+LINE 1: ...ON_VALUE('{"a": "A"}', '$.c' RETURNING d1 DEFAULT 'A'::d2 ON...
+                                                             ^
+DETAIL:  "C" versus "case_insensitive"
+SELECT JSON_VALUE('{"a": "A"}', '$.c' RETURNING d1 DEFAULT 'A' COLLATE "C" ON EMPTY) = 'a'; -- error
+ERROR:  the collation of DEFAULT expression conflicts with RETURNING clause
+LINE 1: ...ON_VALUE('{"a": "A"}', '$.c' RETURNING d1 DEFAULT 'A' COLLAT...
+                                                             ^
+DETAIL:  "C" versus "case_insensitive"
+DROP DOMAIN d1, d2;
 -- cleanup
 RESET search_path;
 SET client_min_messages TO warning;
diff --git a/src/test/regress/sql/collate.icu.utf8.sql b/src/test/regress/sql/collate.icu.utf8.sql
index dbc190227d0..6f5abac0dc0 100644
--- a/src/test/regress/sql/collate.icu.utf8.sql
+++ b/src/test/regress/sql/collate.icu.utf8.sql
@@ -997,6 +997,19 @@ INSERT INTO t5 (a, b) values (1, 'D1'), (2, 'D2'), (3, 'd1');
 -- rewriting.)
 SELECT * FROM t5 ORDER BY c ASC, a ASC;
 
+-- Check that DEFAULT expressions in SQL/JSON functions use the same collation
+-- as the RETURNING type.  Mismatched collations should raise an error.
+CREATE DOMAIN d1 AS text COLLATE case_insensitive;
+CREATE DOMAIN d2 AS text COLLATE "C";
+SELECT JSON_VALUE('{"a": "A"}', '$.a' RETURNING d1 DEFAULT ('C' COLLATE "C") COLLATE case_insensitive ON EMPTY) = 'a'; -- true
+SELECT JSON_VALUE('{"a": "A"}', '$.a' RETURNING d1 DEFAULT 'C' ON EMPTY) = 'a'; -- true
+SELECT JSON_VALUE('{"a": "A"}', '$.a' RETURNING d1 DEFAULT 'C'::d2 ON EMPTY) = 'a'; -- error
+SELECT JSON_VALUE('{"a": "A"}', '$.a' RETURNING d1 DEFAULT 'C' COLLATE "C" ON EMPTY) = 'a'; -- error
+SELECT JSON_VALUE('{"a": "A"}', '$.c' RETURNING d1 DEFAULT 'A' ON EMPTY) = 'a'; -- true
+SELECT JSON_VALUE('{"a": "A"}', '$.c' RETURNING d1 DEFAULT 'A' COLLATE case_insensitive ON EMPTY) = 'a'; -- true
+SELECT JSON_VALUE('{"a": "A"}', '$.c' RETURNING d1 DEFAULT 'A'::d2 ON EMPTY) = 'a'; -- error
+SELECT JSON_VALUE('{"a": "A"}', '$.c' RETURNING d1 DEFAULT 'A' COLLATE "C" ON EMPTY) = 'a'; -- error
+DROP DOMAIN d1, d2;
 
 -- cleanup
 RESET search_path;