Regression in inferred return type for `join` on simple iterator.

[KristofferC]

julia> itr = ("foo" for _ in 1:100)
Base.Generator{UnitRange{Int64}, var"#1#2"}(var"#1#2"(), 1:100)

julia> @code_warntype join(itr)
MethodInstance for join(::Base.Generator{UnitRange{Int64}, var"#1#2"})
  from join(iterator) @ Base strings/io.jl:374
Arguments
  #self#::Core.Const(join)
  iterator::Base.Generator{UnitRange{Int64}, var"#1#2"}
Body::Union{Base.AnnotatedString{String}, String}

This used to infer to String.

Ref #54919.

[simeonschaub]

Possible duplicate of #55279?

[KristofferC]

I don't think so because in #55279, the issue is that seekstart itself is all of a sudden much slower.

julia> io = IOBuffer()

julia> @btime seekstart($io)
# master  
85.537 ns (0 allocations: 0 bytes)

# 1.11
 3.125 ns (0 allocations: 0 bytes)

As I wrote in #55279 (comment), I think there is some "inference poisoning" going on.