`findnext(A, start)` and `findnext(testf::Function, A, start)` behave differently

[goretkin]

e.g.

using InfiniteArrays

# works
findfirst(>(5), 1:InfiniteArrays.Infinity())

# does not
findfirst(1:InfiniteArrays.Infinity() .> 5)

Divergence at #35965

[dlfivefifty]

Just needs

julia/base/array.jl

Line 1725 in dffc889

i = oftype(l, start)

To become

i = oftype(first(keys(A)), start)

Shall I make a PR?

[nalimilan]

Probably better convert to eltype(keys(A)) instead, to avoid special-casing the first index.

[goretkin]

@dlfivefifty I would prefer to get #36965 in as a solution. And then we could follow @nalimilan's suggestion, and we'll only have to fix it in half as many places.

[vtjnash]

Addressed by #36965, IIUC