polymorphic types - enforce casting to most common type automatically

Hello

now a functions with more than one polymorphic arguments are relative
fragile due missing casting to most common type. Some our "functions" like
"coalesce" can do it, so it is surprising for our users.

our custom polymorphic function foo(anyelement, anyelement) working well for

foo(10,20) or foo(10.1, 20.1), but not for foo(10, 20.1)

I am thinking, so we can add a searching most common type stage without
breaking to backing compatibility.

What do you think about it?

Regards

Pavel