"default" keyword as function attribute

[Jakob Ovrum via Digitalmars-d]

We often repeat advice to put `@safe:`, or some other function 
attribute, at the top of your modules to apply the attribute en 
masse.

In practice this quickly becomes infeasible. Sooner or later, a 
generic function is introduced to idiomatic D modules, and many 
of them, perhaps most of them, are more general when attribute 
inference is used. Any explicit attribute application disables 
inference of that attribute. In my experience this is by far the 
most common reason that blanket application of attributes is 
impractical.

A recent example is this Phobos PR[1] which cannot apply `@safe` 
module-wide because the module contains generic functions where 
attribute inference is desirable. In this particular case it 
could be avoided by defining those generic functions before 
declaring `@safe:`, but this is at best unintuitive organization 
of code, and at worst highly intrusive when the generic function 
is a member function as it affects the whole type. It would also 
affect generated documentation with the current state of our 
tools.

DIP 79[2] suggests a number of possible language amendments that 
would help with this problem. The @attribute(<boolean 
expression>) syntax has support from (and was originally proposed 
by?) Andrei, but it doesn't really help with the attribute 
inference problem so I think we should consider it a separate 
issue.

It suggests "default" as a possible function attribute which 
negates previously applied attributes, which seems like it would 
solve the problem with minimal change to the language. Maybe 
there is a better way, or maybe we should move forward with it. 
What do you think?

Fixing this problem would encourage attribute proliferation while 
facilitating correct use of attribute inference.

[1] https://github.com/D-Programming-Language/phobos/pull/4082
[2] http://wiki.dlang.org/DIP79