Generic NamedTuples break `fieldtypes`

[tkemmer]

Using named tuples with type variables sometimes leads to errors when the NamedTupleTools package is loaded. In particular, the pirated fieldtypes function causes errors, e.g., when printing the documentation (example code uses Julia 1.8 and NamedTupleTools 0.14.1):

w/o NamedTupleTools (expected behavior)

julia> const Foo{T} = @NamedTuple{a::T}
NamedTuple{(:a,), Tuple{T}} where T

help?> Foo
search: Foo floor pointer_from_objref OverflowError RoundFromZero unsafe_copyto! functionloc StackOverflowError @functionloc OutOfMemoryError UndefKeywordError unsafe_pointer_to_objref

  No documentation found.

  Summary
  ≡≡≡≡≡≡≡≡≡

  struct NamedTuple{(:a,), Tuple{T}}

  Fields
  ≡≡≡≡≡≡≡≡

  a :: T

w/ NamedTupleTools

julia> using NamedTupleTools

julia> const Foo{T} = @NamedTuple{a::T}
NamedTuple{(:a,), Tuple{T}} where T

help?> Foo
search: Foo FooBar floor pointer_from_objref OverflowError RoundFromZero unsafe_copyto! functionloc StackOverflowError @functionloc OutOfMemoryError UndefKeywordError unsafe_pointer_to_objref

ERROR: UndefVarError: T not defined
Stacktrace:
 [1] fieldtypes(#unused#::Type{NamedTuple{(:a,), Tuple{T}}})
   @ NamedTupleTools ~/.julia/packages/NamedTupleTools/2Wwof/src/NamedTupleTools.jl:66
 [2] summarize(io::IOBuffer, TT::Type, binding::Base.Docs.Binding)
   @ REPL /usr/lib64/julia-1.8.2/share/julia/stdlib/v1.8/REPL/src/docview.jl:286
 [3] summarize(binding::Base.Docs.Binding, sig::Type)
   @ REPL /usr/lib64/julia-1.8.2/share/julia/stdlib/v1.8/REPL/src/docview.jl:249
 [4] doc(binding::Base.Docs.Binding, sig::Type)
   @ REPL /usr/lib64/julia-1.8.2/share/julia/stdlib/v1.8/REPL/src/docview.jl:182
 [5] doc(binding::Base.Docs.Binding)
   @ REPL /usr/lib64/julia-1.8.2/share/julia/stdlib/v1.8/REPL/src/docview.jl:160
 [6] top-level scope
   @ /usr/lib64/julia-1.8.2/share/julia/stdlib/v1.8/REPL/src/docview.jl:474

Notably, this breaks otherwise valid Julia code by just using the NamedTupleTools package.