Drop support for Base.tail()

This causes invalidations and should not be needed anymore by the upstream
ChainRules.

Author: JamesWrigley

src/tangent_types/abstract_zero.jl

@@ -17,7 +17,6 @@ Base.iterate(x::AbstractZero) = (x, nothing)
 Base.iterate(::AbstractZero, ::Any) = nothing
 
 Base.first(x::AbstractZero) = x
-Base.tail(x::AbstractZero) = x
 Base.last(x::AbstractZero) = x
 
 Base.Broadcast.broadcastable(x::AbstractZero) = Ref(x)

src/tangent_types/structural_tangent.jl

@@ -360,16 +360,6 @@ Base.iszero(::Tangent{<:,Tuple{}}) = true
 Base.first(tangent::Tangent{P,T}) where {P,T<:Union{Tuple,NamedTuple}} = first(backing(canonicalize(tangent)))
 Base.last(tangent::Tangent{P,T}) where {P,T<:Union{Tuple,NamedTuple}} = last(backing(canonicalize(tangent)))
 
-Base.tail(t::Tangent{P}) where {P<:Tuple} = Tangent{_tailtype(P)}(Base.tail(backing(canonicalize(t)))...)
-@generated _tailtype(::Type{P}) where {P<:Tuple} = Tuple{P.parameters[2:end]...}
-Base.tail(t::Tangent{<:Tuple{Any}}) = NoTangent()
-Base.tail(t::Tangent{<:Tuple{}}) = NoTangent()
-
-Base.tail(t::Tangent{P}) where {P<:NamedTuple} = Tangent{_tailtype(P)}(; Base.tail(backing(canonicalize(t)))...)
-_tailtype(::Type{NamedTuple{S,P}}) where {S,P} = NamedTuple{Base.tail(S), _tailtype(P)}
-Base.tail(t::Tangent{<:NamedTuple{<:Any, <:Tuple{Any}}}) = NoTangent()
-Base.tail(t::Tangent{<:NamedTuple{<:Any, <:Tuple{}}}) = NoTangent()
-
 function Base.getindex(tangent::Tangent{P,T}, idx::Int) where {P,T<:Union{Tuple,NamedTuple}}
     back = backing(canonicalize(tangent))
     return unthunk(getfield(back, idx))

src/tangent_types/thunks.jl

@@ -30,7 +30,6 @@ end
 
 Base.first(x::AbstractThunk) = first(unthunk(x))
 Base.last(x::AbstractThunk) = last(unthunk(x))
-Base.tail(x::AbstractThunk) = Base.tail(unthunk(x))
 
 Base.:(==)(a::AbstractThunk, b::AbstractThunk) = unthunk(a) == unthunk(b)
 

test/tangent_types/abstract_zero.jl

@@ -100,7 +100,6 @@
 
         @test first(z) === z
         @test last(z) === z
-        @test Base.tail(z) === z
     end
 
     @testset "NoTangent" begin

test/tangent_types/structural_tangent.jl

@@ -84,15 +84,11 @@ end
             @test getindex(Tangent{Tuple{Float64}}(@thunk 2.0^2), 1) == 4.0
             @test getproperty(Tangent{Tuple{Float64}}(2.0), 1) == 2.0
             @test getproperty(Tangent{Tuple{Float64}}(@thunk 2.0^2), 1) == 4.0
-            @test NoTangent() === @inferred Base.tail(tang1)
-            @test NoTangent() === @inferred Base.tail(Tangent{Tuple{}}())
 
             tang3 = Tangent{Tuple{Float64, String, Vector{Float64}}}(1.0, NoTangent(), @thunk [3.0] .+ 4)
             @test @inferred(first(tang3)) === tang3[1] === 1.0
             @test @inferred(last(tang3)) isa Thunk
             @test unthunk(last(tang3)) == [7.0]
-            @test Tuple(@inferred Base.tail(tang3))[1] === NoTangent()
-            @test Tuple(Base.tail(tang3))[end] isa Thunk
 
             NT = NamedTuple{(:a, :b),Tuple{Float64,Float64}}
             @test getindex(Tangent{NT}(; a=(@thunk 2.0^2)), :a) == 4.0
@@ -109,10 +105,6 @@ end
             @test unthunk(first(Tangent{NT}(; a=(@thunk 2.0^2)))) == 4.0
             @test last(Tangent{NT}(; a=(@thunk 2.0^2))) isa ZeroTangent
 
-            ntang1 = @inferred Base.tail(Tangent{NT}(; b=(@thunk 2.0^2)))
-            @test ntang1 isa Tangent{<:NamedTuple{(:b,)}}
-            @test NoTangent() === @inferred Base.tail(ntang1)
-
             # TODO: uncomment this once https://github.com/JuliaLang/julia/issues/35516
             # if VERSION >= v"1.8-"
             #     @test haskey(Tangent{Tuple{Float64}}(2.0), 1) == true

test/tangent_types/thunks.jl

@@ -17,11 +17,9 @@
         @test nothing === iterate(@thunk ()) == iterate(())
     end
 
-    @testset "first, last, tail" begin
+    @testset "first, last" begin
         @test first(@thunk (1,2,3) .+ 4) === 5
         @test last(@thunk (1,2,3) .+ 4) === 7
-        @test Base.tail(@thunk (1,2,3) .+ 4) === (6, 7)
-        @test Base.tail(@thunk NoTangent() * 5) === NoTangent()
     end
 
     @testset "show" begin