Question about FUNCDETAIL_MULTIPLE

I was wondering what the philosophy is behind letting an "ambiguous"
function be created in the first place. Is this for backwards
compatibility or perhaps for historical reasons? Could someone clarify
this please?

Consider the following example:

(
FYI:
parse_func.c->ParseFuncOrColumn->164
parse_func.c->ParseFuncOrColumn->810
parse_func.c->ParseFuncOrColumn->836
namespace.c->FuncnameGetCandidates->607
namespace.c->FuncnameGetCandidates->826
parse_func.c->ParseFuncOrColumn->224
)

-----------------------------------------------------------------------------
create or replace function foo1(int,int default 1,int default 2) returns
int as
$$
select $1+$2+$3
$$
language sql;

create or replace function foo1(int,int default 1) returns int as
$$
select $1+$2
$$
language sql;

select * from foo1(10,10)

ERROR: function foo1(integer, integer) is not unique
LINE 15: select * from foo1(10,10)
^
HINT: Could not choose a best candidate function. You might need to add
explicit type casts.
-----------------------------------------------------------------------------

--
Regards,
Gevik