`TypeErrror` when building cross-references

[matthias314]

When I try to create the documentation for my package SmallCollections.jl, I get the following error:

~/git/SmallCollections.jl$ (cd docs && julia --project=. make.jl)
[ Info: SetupBuildDirectory: setting up build directory.
[ Info: Doctest: running doctests.
[ Info: ExpandTemplates: expanding markdown templates.
[ Info: CrossReferences: building cross-references.
ERROR: LoadError: TypeError: in typeassert, expected DataType, got Type{Union{Tuple{N}, Tuple{MutableSmallVector{N}, Union{NTuple{N, T} where T, AbstractFixedVector{N}}}}}
Stacktrace:
  [1] to_tuple_type(t::Any)
    @ Base ./runtime_internals.jl:1207
  [2] signature_type(f::Any, argtypes::Any)
    @ Base ./runtime_internals.jl:1222
  [3] hasmethod(f::Any, t::Any)
    @ Base ./reflection.jl:1013
  [4] find_object(λ::Function, binding::Base.Docs.Binding, typesig::Type)
    @ Documenter /usr/local/julia-depot/packages/Documenter/HdXI4/src/cross_references.jl:510
[...]

The problem are the two lines
https://github.com/matthias314/SmallCollections.jl/blob/25e78c7c7a413f99254f90e03716bf37b9b7f0ff/docs/src/smallvector.md?plain=1#L15-L16

unsafe_copyto!(::MutableSmallVector{N}, ::AbstractSmallVector{N}) where N
unsafe_copyto!(::MutableSmallVector, ::Union{NTuple, AbstractFixedVector})

Deleting either of them makes the error go away. Both lines together trigger the error.

I've created the branch documenter-bug (matthias314/SmallCollections.jl@5afe3aa) for investigating this. I'm using Julia 1.12.1 and Documenter.jl 1.15.0.