Error while printing certain NamedTuple types from `@code_warntype`

[danielwe]

It's possible to write code such that @code_warntype infers things like NamedTuple{(:a, :b),Tuple{Vararg{T}}}, even though such a type can't be constructed directly. When the printing of this type reaches show_at_namedtuple, it breaks the assumption that length(types.parameters) == length(syms). The result is that @code_warntype throws in the middle of printing its output.

MWE:

julia> f(::Val{vars}, mat) where {vars} = (; zip(vars, eachrow(mat))...)
f (generic function with 1 method)

julia> f(Val((:a, :b)), rand(2, 1))
(a = [0.6820962296161969], b = [0.6568417470938385])

julia> @code_warntype f(Val((:a, :b)), rand(2, 1))
MethodInstance for f(::Val{(:a, :b)}, ::Matrix{Float64})
  from f(::Val{vars}, mat) where vars @ Main REPL[1]:1
Static Parameters
  vars = (:a, :b)
Arguments
  #self#::Core.Const(Main.f)
  _::Core.Const(Val{(:a, :b)}())
  mat::Matrix{Float64}
BodyERROR: BoundsError: attempt to access Core.SimpleVector at index [2]
Stacktrace:
  [1] getindex
    @ ./essentials.jl:932 [inlined]
  [2] show_at_namedtuple(io::IOContext{IOBuffer}, syms::Tuple{Symbol, Symbol}, types::DataType)
    @ Base ./show.jl:1191
  [3] show_datatype(io::IOContext{IOBuffer}, x::DataType, wheres::Vector{TypeVar})
    @ Base ./show.jl:1167
  [4] _show_type(io::IOBuffer, x::Type)
    @ Base ./show.jl:1007
  [5] show(io::IOBuffer, x::Type)
    @ Base ./show.jl:965
  [6] print(io::IOBuffer, x::Type)
    @ Base ./strings/io.jl:35
  [7] print_to_string(::String, ::Vararg{Any})
    @ Base ./strings/io.jl:148
  [8] string
    @ ./strings/io.jl:189 [inlined]
  [9] warntype_type_printer(io::Base.TTY; type::Any, used::Bool, show_type::Bool, ::@Kwargs{})
    @ InteractiveUtils ~/.julia/juliaup/julia-1.11.5+0.aarch64.apple.darwin14/share/julia/stdlib/v1.11/InteractiveUtils/src/codeview.jl:32
 [10] code_warntype(io::Base.TTY, f::Any, t::Any; debuginfo::Symbol, optimize::Bool, kwargs::@Kwargs{})
    @ InteractiveUtils ~/.julia/juliaup/julia-1.11.5+0.aarch64.apple.darwin14/share/julia/stdlib/v1.11/InteractiveUtils/src/codeview.jl:151
 [11] code_warntype(io::Base.TTY, f::Any, t::Any)
    @ InteractiveUtils ~/.julia/juliaup/julia-1.11.5+0.aarch64.apple.darwin14/share/julia/stdlib/v1.11/InteractiveUtils/src/codeview.jl:77
 [12] code_warntype(::Any, ::Vararg{Any}; kwargs...)
    @ InteractiveUtils ~/.julia/juliaup/julia-1.11.5+0.aarch64.apple.darwin14/share/julia/stdlib/v1.11/InteractiveUtils/src/codeview.jl:159
 [13] top-level scope
    @ REPL[3]:1

[danielwe]

I suppose the ideal solution would be to improve inference such that these malformed types never occur. Either the construction errors, or the value tuple has the same length as the syms tuple. If type inference understood that and returned NamedTuple{vars,Tuple{Vararg{T,length(vars)}}}, the issue would not occur.