subquery results bypassed

Anthony Wood (woody+postgresqlbugs(at)switchonline(dot)com(dot)au) reports a bug with a severity of 1
The lower the number the more severe it is.

Short Description
subquery results bypassed

Long Description
The second query in the example code should return:

a | b | c
---+---+---
(0 rows)

but actually returns

a | b | c
---+---+---
1 | 3 | 2
(1 row)

which is wrong, because this tuple is not in the subquery

Sample Code
CREATE TABLE "bug" ("a" TEXT, "b" TEXT, "c" TEXT);
INSERT INTO "bug" VALUES ('1','2','1');
INSERT INTO "bug" VALUES ('1','3','2');
SELECT DISTINCT ON ("a") * FROM "bug" ORDER BY "a","c";
SELECT * FROM (SELECT DISTINCT ON ("a") * FROM "bug" ORDER BY "a","c") bug WHERE "b"='3';
DROP TABLE "bug";

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