Type instability when using integers in pattern type that aren't statically known

[AntonOresten]

Non-constant-propagated integers will lead to type instability if used in a pattern.

julia> a = 3;

julia> reduce(sum, x, ((a, :b), ..) --> (:b, ..)); # allowed, but not type stable if `a` is calculated at runtime

julia> reduce(sum, x, ((:a, :b), ..) --> (:b, ..), a=a); # preferred, as the pattern type just has the name

I don't think this would need to happen in a certain macro-based interface (#2 (comment)).