Add test case for collation mismatch in recursive query

This isn't very important by itself, but was left on my list of things
without test coverage for the collation feature.

src/test/regress/expected/collate.linux.utf8.out

@@ -630,6 +630,16 @@ HINT:  You can override the collation by applying the COLLATE clause to one or b
 CREATE TABLE test_u AS SELECT a, b FROM collate_test1 UNION ALL SELECT a, b FROM collate_test3; -- fail
 ERROR:  no collation was derived for column "b" with collatable type text
 HINT:  Use the COLLATE clause to set the collation explicitly.
+-- collation mismatch between recursive and non-recursive term
+WITH RECURSIVE foo(x) AS
+   (SELECT x FROM (VALUES('a' COLLATE "en_US"),('b')) t(x)
+   UNION ALL
+   SELECT (x || 'c') COLLATE "de_DE" FROM foo WHERE length(x) < 10)
+SELECT * FROM foo;
+ERROR:  recursive query "foo" column 1 has collation "en_US" in non-recursive term but collation "de_DE" overall
+LINE 2:    (SELECT x FROM (VALUES('a' COLLATE "en_US"),('b')) t(x)
+                   ^
+HINT:  Use the COLLATE clause to set the collation of the non-recursive term.
 -- casting
 SELECT CAST('42' AS text COLLATE "C");
 ERROR:  syntax error at or near "COLLATE"

src/test/regress/sql/collate.linux.utf8.sql

@@ -190,6 +190,13 @@ SELECT a, b FROM collate_test1 EXCEPT SELECT a, b FROM collate_test3 ORDER BY 2;
 
 CREATE TABLE test_u AS SELECT a, b FROM collate_test1 UNION ALL SELECT a, b FROM collate_test3; -- fail
 
+-- collation mismatch between recursive and non-recursive term
+WITH RECURSIVE foo(x) AS
+   (SELECT x FROM (VALUES('a' COLLATE "en_US"),('b')) t(x)
+   UNION ALL
+   SELECT (x || 'c') COLLATE "de_DE" FROM foo WHERE length(x) < 10)
+SELECT * FROM foo;
+
 
 -- casting