Type stable trait for checking field types

hasfields_trait.jl

julia> HasFields(::Type{T}) where {T} = Val{true}()
HasFields (generic function with 1 method)

julia> HasFields(::Type{T}, ::Type{U}, name) where {T, U} = Val{fieldtype(T, name) <: U}()
HasFields (generic function with 2 methods)

julia> HasFields(Some, Any, :value)
Val{true}()

julia> HasFields(::Type{T}, ::Type{U}, name, names...) where {T, U} = Val{fieldtype(T, name) <: U && HasFields(T, U, names...) isa Val{true}}()
HasFields (generic function with 3 methods)

julia> HasFields(Pair{Int, Int}, Int, :first, :second)
Val{true}()

julia> @code_warntype HasFields(Pair{Int, Int}, Int, :first, :second)
Variables
  #self#::Core.Const(HasFields)
  #unused#@_2::Core.Const(Pair{Int64, Int64})
  #unused#@_3::Core.Const(Int64)
  name::Symbol
  names::Tuple{Symbol}
  @_6::Bool

Body::Val{true}
1 ─ %1  = Main.fieldtype($(Expr(:static_parameter, 1)), name)::Core.Const(Int64)
│   %2  = (%1 <: $(Expr(:static_parameter, 2)))::Core.Const(true)
│         %2
│   %4  = Core.tuple($(Expr(:static_parameter, 1)), $(Expr(:static_parameter, 2)))::Core.Const((Pair{Int64, Int64}, Int64))
│   %5  = Core._apply_iterate(Base.iterate, Main.HasFields, %4, names)::Core.Const(Val{true}())
│   %6  = Core.apply_type(Main.Val, true)::Core.Const(Val{true})
│         (@_6 = %5 isa %6)
└──       goto #3
2 ─       Core.Const(:(@_6 = false))
3 ┄ %10 = @_6::Core.Const(true)::Core.Const(true)
│   %11 = Core.apply_type(Main.Val, %10)::Core.Const(Val{true})
│   %12 = (%11)()::Core.Const(Val{true}())
└──       return %12