domains, case statements, functions: bug?

create domain m numeric(5,2);
create table t (c m);
create function f(t) returns m as $ select case when true then $1.c end $
language sql;

psql:/tmp/t1.sql:3: ERROR: return type mismatch in function declared to
return m
DETAIL: Actual return type is numeric.
CONTEXT: SQL function "f"