coalesce with all nulls can only be assigned to text

Here is sample code demonstrating the issue:

test=# create table test_coalesce(f1 int not null, f2 date);
CREATE TABLE
test=# insert into test_coalesce values (1, null);
INSERT 0 1
test=# insert into test_coalesce values (2, coalesce(null, null));
ERROR:  column "f2" is of type date but expression is of type text
HINT:  You will need to rewrite or cast the expression.
test=# create cast (text as date) with function date(text) as
assignment;
ERROR:  cast from type text to type date already exists
test=# create cast (text as date) with function date(text) as
implicit;
ERROR:  cast from type text to type date already exists

The last statement is not something which would make sense to hand
code, but we have a framework which is plugging in the arguments for the
coalesce function at run time.  One solution to this is to modify the
framework to wrap any null with a cast to a type.  Due to the effort and
risk of that approach, I'm looking for alternatives.  Besides, the above
just doesn't make sense to me.

Any suggestions?

-Kevin