Drop support for `Base.tail()` #703

JamesWrigley · 2026-01-28T21:29:57Z

This causes invalidations and should not be needed anymore by ChainRules.jl since the rule that uses it: #576 (comment)
Was deleted in JuliaDiff/ChainRules.jl#680.

I ran this on the ChainRules.jl tests and saw a ton of failures locally (also seen in CI), but none that seem related to this change. Disclaimer: I have almost no understanding of this kind of thing 🙈

Fixes #576.

Fixes these invalidations seen in CurveFit.jl:

 inserting tail(t::ChainRulesCore.Tangent{<:NamedTuple{<:Any, <:Tuple{}}}) @ ChainRulesCore ~/.julia/packages/ChainRulesCore/Vsbj9/src/tangent_types/structural_tangent.jl:378 invalidated:                                                                                                                                    
   mt_backedges:  1: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for Base.Iterators._zip_isdone(::Tuple{Tuple{Int64}, Vararg{Any}}, ::Any) (0 children)                                                                                                                                                   
                  2: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for Base.Iterators._zip_iterate_some(::Tuple{Tuple{Int64}, Vararg{Any}}, ::Any, ::Tuple{Missing, Vararg{Any}}, ::Missing) (0 children)                                                                                                   
                  3: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for Base.Iterators._zip_iterate_some(::Tuple{Tuple{Int64}, Vararg{Any}}, ::Any, ::Tuple{Missing, Vararg{Any}}, ::Bool) (0 children)                                                                                                      
                  4: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for Base.Iterators._zip_iterate_interleave(::NamedTuple, ::Any, ::Tuple{Bool, Vararg{Any}}) (0 children)                                                                                                                                 
                  5: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for Base.Iterators._zip_iterate_interleave(::Any, ::Any, ::Any) (0 children)                                                                                                                                                             
                  6: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for Base.Iterators._zip_isdone(::Tuple{NTuple{4, Symbol}, Vararg{Any}}, ::Any) (0 children)                                                                                                                                              
                  7: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for Base.Iterators._zip_iterate_some(::Tuple{NTuple{4, Symbol}, Vararg{Any}}, ::Any, ::Tuple{Missing, Vararg{Any}}, ::Missing) (0 children)                                                                                              
                  8: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for Base.Iterators._zip_iterate_some(::Any, ::Any, ::Tuple{Any, Vararg{Any}}, ::Bool) (0 children)                                                                                                                                       
                  9: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for Base.Iterators._zip_iterate_some(::Tuple{NTuple{4, Symbol}, Vararg{Any}}, ::Any, ::Tuple{Missing, Vararg{Any}}, ::Bool) (0 children)                                                                                                 
                 10: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for Base.Iterators._zip_isdone(::Tuple{Tuple{Symbol}, Vararg{Any}}, ::Any) (0 children)                                                                                                                                                  
                 11: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for Base.Iterators._zip_iterate_some(::Tuple{Tuple{Symbol}, Vararg{Any}}, ::Any, ::Tuple{Missing, Vararg{Any}}, ::Missing) (0 children)                                                                                                  
                 12: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for Base.Iterators._zip_iterate_some(::Any, ::Any, ::Tuple{Any, Vararg{Any}}, ::Bool) (0 children)                                                                                                                                       
                 13: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for Base.Iterators._zip_iterate_some(::Tuple{Tuple{Symbol}, Vararg{Any}}, ::Any, ::Tuple{Missing, Vararg{Any}}, ::Bool) (0 children)                                                                                                     
                 14: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for Base.Iterators._zip_isdone(::Tuple{Tuple{Symbol, Symbol, Symbol}, Vararg{Any}}, ::Any) (0 children)                                                                                                                                  
                 15: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for Base.Iterators._zip_iterate_some(::Tuple{Tuple{Symbol, Symbol, Symbol}, Vararg{Any}}, ::Any, ::Tuple{Missing, Vararg{Any}}, ::Missing) (0 children)                                                                                  
                 16: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for Base.Iterators._zip_iterate_some(::Any, ::Any, ::Tuple{Any, Vararg{Any}}, ::Bool) (0 children)                                                                                                                                       
                 17: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for Base.Iterators._zip_iterate_some(::Tuple{Tuple{Symbol, Symbol, Symbol}, Vararg{Any}}, ::Any, ::Tuple{Missing, Vararg{Any}}, ::Bool) (0 children)                                                                                     
                 18: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for Base.Iterators._zip_isdone(::Tuple{Tuple{Symbol, Symbol}, Vararg{Any}}, ::Any) (0 children)                                                                                                                                          
                 19: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for Base.Iterators._zip_iterate_some(::Tuple{Tuple{Symbol, Symbol}, Vararg{Any}}, ::Any, ::Tuple{Missing, Vararg{Any}}, ::Missing) (0 children)                                                                                          
                 20: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for Base.Iterators._zip_iterate_some(::Any, ::Any, ::Tuple{Any, Vararg{Any}}, ::Bool) (0 children)                                                                                                                                       
                 21: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for Base.Iterators._zip_iterate_some(::Tuple{Tuple{Symbol, Symbol}, Vararg{Any}}, ::Any, ::Tuple{Missing, Vararg{Any}}, ::Bool) (0 children)                                                                                             
                 22: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for Base.Iterators._zip_isdone(::Tuple{Tuple{Int64, Int64}, Vararg{Any}}, ::Any) (0 children)                                                                                                                                            
                 23: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for Base.Iterators._zip_iterate_some(::Tuple{Tuple{Int64, Int64}, Vararg{Any}}, ::Any, ::Tuple{Missing, Vararg{Any}}, ::Missing) (0 children)                                                                                            
                 24: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for Base.Iterators._zip_iterate_some(::Any, ::Any, ::Tuple{Any, Vararg{Any}}, ::Bool) (0 children)                                                                                                                                       
                 25: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for Base.Iterators._zip_iterate_some(::Tuple{Tuple{Int64, Int64}, Vararg{Any}}, ::Any, ::Tuple{Missing, Vararg{Any}}, ::Bool) (0 children)                                                                                               
                 26: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for Base.Iterators._zip_isdone(::Tuple{Tuple{}, Vararg{Tuple{}}}, ::Any) (0 children)                                                                                                                                                    
                 27: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for iterate(::Base.LogicalIndex, ::Any) (0 children)                                                                                                                                                                                     
                 28: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for iterate(::AbstractRange{Float64}, ::Any) (0 children)                                                                                                                                                                                
                 29: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for iterate(::AbstractRange{String}, ::Any) (1 children)                                                                                                                                                                                 
                 30: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for Base.Iterators._zip_iterate_interleave(::NamedTuple, ::Any, ::Any) (1 children)                                                                                                                                                      
                 31: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for Base.Iterators._zip_iterate_interleave(::Any, ::Any, ::NamedTuple) (1 children)                                                                                                                                                      
                 32: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for iterate(::Base.Iterators.Enumerate{Vector{Any}}, ::Any) (2 children)                                                                                                                                                                 
                 33: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for iterate(::AbstractRange{Compiler.MethodMatchInfo}, ::Any) (4 children)                                                                                                                                                               
                 34: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for iterate(::AbstractRange{Compiler.ApplyCallInfo}, ::Any) (4 children)                                                                                                                                                                 
                 35: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for iterate(::AbstractRange{Union{Nothing, Compiler.ConstResult}}, ::Any) (4 children)                                                                                                                                                   
                 36: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for iterate(::AbstractRange{Union{Nothing, Compiler.AbstractIterationInfo}}, ::Any) (4 children)                                                                                                                                         
                 37: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for iterate(::AbstractRange{Compiler.CallMeta}, ::Any) (4 children)                                                                                                                                                                      
                 38: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for iterate(::AbstractRange{Compiler.NewPhiCNode2}, ::Any) (4 children)                                                                                                                                                                  
                 39: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for iterate(::AbstractRange{Tuple{LineNumberNode, Expr}}, ::Any) (4 children)                                                                                                                                                            
                 40: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for Base.Iterators._zip_isdone(::Tuple{Vararg{Tuple{}}}, ::Any) (4 children)                                                                                                                                                             
                 41: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for iterate(::Base.Iterators.Enumerate, ::Any) (5 children)                                                                                              
                 42: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for Base.Iterators._zip_iterate_interleave(::NamedTuple, ::NamedTuple, ::Any) (5 children)                                                               
                 43: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for Base.Iterators._zip_iterate_some(::NamedTuple, ::Any, ::Tuple{Bool, Vararg{Any}}, ::Bool) (5 children)                                               
                 44: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for Base.Iterators._zip_iterate_some(::NamedTuple, ::Any, ::Tuple{Any, Vararg{Any}}, ::Bool) (5 children)                                                
                 45: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for Base.Broadcast.check_broadcast_shape(::Any, ::Tuple{Base.OneTo{Int64}}) (5 children)                                                                 
                 46: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for axes(::Base.ReinterpretArray{Float64, 1, S, A, true} where A<:Union{SubArray{S, N, A, I, true} where {N, A<:DenseArray, I<:Union{Tuple{Vararg{Real}}, Tuple{AbstractUnitRange, Vararg{Any}}}}, DenseArray{S}}) where S (5 children)
                 47: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for iterate(::SubArray{T, 1} where T, ::Tuple{Any, Vararg{Any}}) (6 children)                                                                            
                 48: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for Base.Iterators._zip_iterate_some(::Any, ::NamedTuple, ::Tuple{Bool, Vararg{Any}}, ::Bool) (10 children)                                              
                 49: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for size(::Base.ReinterpretArray{Float64, N, S, A, true} where {N, A<:Union{SubArray{S, N, A, I, true} where {N, A<:DenseArray, I<:Union{Tuple{Vararg{Real}}, Tuple{AbstractUnitRange, Vararg{Any}}}}, DenseArray{S}}}) where S (10 child
ren)                                                                                                                                                                                                                                                                              
                 50: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for Base.Iterators._zip_iterate_some(::Tuple, ::Any, ::Tuple{Missing, Vararg{Any}}, ::Missing) (12 children)                                             
                 51: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for iterate(::SubArray{T, 1} where T, ::Tuple{Any}) (15 children)                                                                                        
                 52: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for Base._overflowind(::Any, ::Any, ::NTuple{N, Int64} where N) (15 children)                                                                            
                 53: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for iterate(::AbstractRange{Any}, ::Any) (25 children)                                                                                                   
                 54: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for Base.Iterators._zip_isdone(::Tuple, ::Any) (38 children)                                                                                             
                 55: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for Base.Iterators._zip_iterate_some(::Any, ::NamedTuple, ::Tuple{Any, Vararg{Any}}, ::Bool) (50 children)                                               
                 56: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for Base._cshp(::Int64, ::Tuple{Bool}, ::Tuple{Int64}, ::Any) (66 children)

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

codecov · 2026-01-28T21:32:19Z

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.52%. Comparing base (004e43e) to head (10e0ec5).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #703      +/-   ##
==========================================
+ Coverage   93.49%   93.52%   +0.03%     
==========================================
  Files          15       15              
  Lines         999      989      -10     
==========================================
- Hits          934      925       -9     
+ Misses         65       64       -1

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JamesWrigley · 2026-01-28T22:03:39Z

The number of failures is a bit disturbing, but they don't seem related to this PR (and they also show up on other PRs).

devmotion · 2026-01-28T23:53:07Z

Sorry, I mis-clicked. IMO it would be great if we could remove these definitions. There are unfortunately preexisting test failures IIRC. I'll have a closer look tomorrow.

devmotion · 2026-01-29T10:38:25Z

This looks fine to me. The test errors seem unrelated, in many cases caused by Aqua tests and in particular without any reference to tail.

It would be good to fix the doctest failures though, such that we're able to build docs for the upcoming release. Can you update the doctests (see https://documenter.juliadocs.org/stable/man/doctests/#Fixing-Outdated-Doctests)?

JamesWrigley · 2026-01-29T12:17:51Z

Ah yeah sure, fixed them in eb83113. BTW one of them was using rand(), I replaced that with a constant to avoid the tests breaking again if the RNG changes.

devmotion · 2026-01-29T13:10:39Z

src/tangent_types/thunks.jl

 julia> t = @thunk(3)
-Thunk(var"#4#5"())
+Thunk(var"#2#3"())


I'd suggest

Suggested change

julia> t = @thunk(3)

Thunk(var"#4#5"())

Thunk(var"#2#3"())

julia> t = @thunk(3);

or a doctest filter. But I think removing the output is simpler.

Yeah good idea, fixed in c2e51fd.

devmotion · 2026-01-30T23:44:47Z

Can you update the version number?

JamesWrigley · 2026-01-31T08:55:48Z

Sure, done in 10e0ec5.

devmotion · 2026-01-31T09:44:12Z

Project.toml

 name = "ChainRulesCore"
 uuid = "d360d2e6-b24c-11e9-a2a3-2a2ae2dbcce4"
-version = "1.26.0"
+version = "1.26.1"


This is a breaking change - it will definitely break older ChainRules releases and it might break packages that define rules using Base.tail.

Suggested change

version = "1.26.1"

version = "2.0.0"

Ah ok, fixed in 09cb407.

I also removed the compat bound in docs/Project.toml so that the docs will build. There's quite a few more failing tests now, I guess because downstream packages aren't compatible anymore.

Making this a breaking change is really annoying though.
Can we see if anyone is using it?
I suspect they were not, since Base.tail isn't part of Base's public API.

Easy way to see if people are using it is to just release it with this as a nonbreaking change,
and then if people complain tag a bug fix that reverts the change.

Like it will break old ChainRules.jl releases, but only pretty old ones that there is no particular reason to be using, unless they are locked in a Manifest.toml.
But in that case the version of ChainRulesCore.jl is also going to be locked to an old version.

It's annoying but it's the only way to avoid breaking existing releases, of ChainRules and possibly other packages.

SciML is making non-breaking releases that break existing versions of SciML and non-SciML packages quite regularly in my experience, and I think that's far more annoying than making a breaking release. Even if people would complain and a subsequent bug fix release would be made, there would still be a combination of package versions in the registry that is officially declared to be compatible but in fact isn't.

Even if people would complain and a subsequent bug fix release would be made, there would still be a combination of package versions in the registry that is officially declared to be compatible but in fact isn't.

This is why Pkg should refuse to install things other than the latest patch release of a package (at least post 1.0.0)
But can imitate that by yanking prior ones.

I guess arguably Base.tail is part of the public API.
It wasn't when I wrote those overloads.
But it is now documented as public.

At the moment there is 283 direct reverse dependencies.
That is a lot to make do a update.
I guess dependabot will catch them though.

Am I correct in thinking that all those direct dependents could use package extensions on ChainRules.jl instead?

JamesWrigley · 2026-02-08T14:47:53Z

(bump)

Drop support for Base.tail()

f9afcb4

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

JamesWrigley mentioned this pull request Jan 28, 2026

Invalidations from ChainRulesCore Tangent overload on Tail #576

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devmotion requested a review from Copilot January 28, 2026 23:45

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JuliaDiff deleted a comment from Copilot AI Jan 28, 2026

devmotion reviewed Jan 29, 2026

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Fix the doctests

c2e51fd

JamesWrigley force-pushed the tail branch from eb83113 to c2e51fd Compare January 29, 2026 14:00

devmotion reviewed Jan 31, 2026

View reviewed changes

JamesWrigley force-pushed the tail branch from 10e0ec5 to 09cb407 Compare January 31, 2026 09:57

Bump version

23fb04c

JamesWrigley force-pushed the tail branch from 09cb407 to 23fb04c Compare January 31, 2026 10:38

Drop support for Base.tail() #703

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Drop support for `Base.tail()` #703

Drop support for `Base.tail()` #703

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