Stack overflow when defining some promote_rules on nightly

[KristofferC]

This stack overflows on nightly:

struct Identity{T}
  value::T
end

struct Const{T}
  value::T
end

const Either{L, R} = Union{Const{L}, Identity{R}}

Base.promote_rule(::Type{Const{L2}}, ::Type{Either{L1, R}}) where {L1, R, L2 <: L1} = Either{L1, R}
Base.promote_rule(::Type{Const{L}}, ::Type{Either{L, <:Any}}) where {L} = Either{L, <:Any}

Reduced from PkgEval failure in DataTypesBasic.jl (https://s3.amazonaws.com/julialang-reports/nanosoldier/pkgeval/by_hash/c3bb6df_vs_788b2c7/Continuables.1.6.0-DEV-a896693d33.log).