@nospecialize could use argument validation

[jakobnissen]

I recently discovered a few footguns with @nospecialize:

julia> @nospecialize f(x) = x + 1

julia> f
ERROR: UndefVarError: `f` not defined

This happens because the function definition gets parsed as a kwarg to @nospecialize. This makes no sense of course, but no error is thrown.

Similarly

julia> function f(a, b)
           @nospecialize a, c
           a + b
       end
f (generic function with 1 method)

julia> f(1, 2)
3

Here, the compiler doesn't complain that c is not even an argument. This looks like a mistake that someone will 100% make eventually.