searchsorted "by" method should not be applied to key

[StephenVavasis]

This would be a breaking change, so probably it will be rejected, but anyway:

The function call searchsorted(v, key, by=mytransform) applies mytransform to key, which isn't helpful. It would be more useful if the searchsorted function assumes that the key is already of the type equal to the output of mytransform. The common use for searchsorted is to search the sorted array v for a particular key that is a field or is computed from the fields of the array entries. Typically, the user would know the key but would not want to construct a dummy record whose type is eltype(v) just to hold that key.

Just to be clear, here is a trace of what I mean. I think the first invocation sequence (that gives the error) is more useful than the second.

julia> v = [(2,5), (3,9), (12,15)]
3-element Array{Tuple{Int64,Int64},1}:
 (2, 5)
 (3, 9)
 (12, 15)

julia> searchsortedfirst(v, 7, by = a-> a[2])
ERROR: BoundsError
Stacktrace:
 [1] getindex at .\number.jl:78 [inlined]
 [2] #5 at .\REPL[4]:1 [inlined]
 [3] lt at .\ordering.jl:51 [inlined]
 [4] searchsortedfirst at .\sort.jl:178 [inlined]
 [5] searchsortedfirst at .\sort.jl:291 [inlined]
 [6] #searchsortedfirst#4 at .\sort.jl:293 [inlined]
 [7] (::getfield(Base.Sort, Symbol("#kw##searchsortedfirst")))(::NamedTuple{(:by,),Tuple{getfield(Main, Symbol("##5#6"))}}, ::typeof(searchsortedfirst), ::Array{Tuple{Int64,Int64},1}, ::Int64) at .\none:0
 [8] top-level scope at REPL[4]:1

julia> searchsortedfirst(v, (99999,7), by = a-> a[2])
2

[JeffBezanson]

Duplicate of #9429. I do think this would be useful. It would be nice to add it as a new function or keyword argument of some sort.