Expression indexes ignore typmod of expression

A Salesforce colleague asked me why, for something like

regression=# create table foo1 (f1 char(15), f2 char(15));
CREATE TABLE
regression=# create index on foo1((case when f1>'z' then f1 else f2 end));
CREATE INDEX
regression=# \d foo1_f2_idx
Index "public.foo1_f2_idx"
Column | Type | Definition
--------+--------+-----------------------------------
f2 | bpchar | ( +
| | CASE +
| | WHEN f1 > 'z'::bpchar THEN f1+
| | ELSE f2 +
| | END)
btree, for table "public.foo1"

the index column ends up as "bpchar" and not "char(15)". The CASE
expression does get resolved as char(15), but it turns out that
index.c just ignores that. I think this is just a hangover from
before we paid much attention to expression typmods at all, and
propose the attached patch.

Comments?

regards, tom lane