Julia: Error displaying unions

[johnmyleswhite]

Working on 8eca27df422089a8c21301764c6485fb86d0fee8, I believe the following is a bug in the display logic for unions, although the error message's reference to Tuple{Char} makes me wonder if there's a type system issue as well:

julia> tmp = Array{Union{Tuple{Char}, Tuple{Char, Char}}}(undef, 1)
1-element Array{Union{Tuple{Char}, Tuple{Char,Char}},1}:
 ('\0',)

julia> tmp[1] = ('a', )
('a',)

julia> tmp
1-element Array{Union{Tuple{Char}, Tuple{Char,Char}},1}:
 ('a',)

julia> tmp[1] = ('a', 'b')
('a', 'b')

julia> tmp
1-element Array{Union{Tuple{Char}, Tuple{Char,Char}},1}:
Error showing value of type Array{Union{Tuple{Char}, Tuple{Char,Char}},1}:
ERROR: BoundsError: attempt to access Tuple{Char}
  at index [2]
Stacktrace:
 [1] show_delim_array(::IOContext{Base.GenericIOBuffer{Array{UInt8,1}}}, ::Tuple{Char,Char}, ::Char, ::Char, ::Char, ::Bool, ::Int64, ::Int64) at ./show.jl:692
 [2] show_delim_array at ./show.jl:677 [inlined]
 [3] show(::IOContext{Base.GenericIOBuffer{Array{UInt8,1}}}, ::Tuple{Char,Char}) at ./show.jl:710
 [4] #sprint#340(::IOContext{REPL.Terminals.TTYTerminal}, ::Int64, ::Function, ::Function, ::Tuple{Char,Char}) at ./strings/io.jl:99
 [5] #sprint at ./none:0 [inlined]
 [6] alignment at ./show.jl:1757 [inlined]
 [7] alignment(::IOContext{REPL.Terminals.TTYTerminal}, ::Array{Union{Tuple{Char}, Tuple{Char,Char}},1}, ::UnitRange{Int64}, ::UnitRange{Int64}, ::Int64, ::Int64, ::Int64) at ./arrayshow.jl:68
 [8] print_matrix(::IOContext{REPL.Terminals.TTYTerminal}, ::Array{Union{Tuple{Char}, Tuple{Char,Char}},1}, ::String, ::String, ::String, ::String, ::String, ::String, ::Int64, ::Int64) at ./arrayshow.jl:186
 [9] print_matrix at ./arrayshow.jl:159 [inlined]
 [10] print_array at ./arrayshow.jl:308 [inlined]
 [11] show(::IOContext{REPL.Terminals.TTYTerminal}, ::MIME{Symbol("text/plain")}, ::Array{Union{Tuple{Char}, Tuple{Char,Char}},1}) at ./arrayshow.jl:345
 [12] display(::REPL.REPLDisplay, ::MIME{Symbol("text/plain")}, ::Any) at /Users/johnmyleswhite/julia/usr/share/julia/stdlib/v1.2/REPL/src/REPL.jl:131
 [13] display(::REPL.REPLDisplay, ::Any) at /Users/johnmyleswhite/julia/usr/share/julia/stdlib/v1.2/REPL/src/REPL.jl:135
 [14] display(::Any) at ./multimedia.jl:287
 [15] #invokelatest#1 at ./essentials.jl:742 [inlined]
 [16] invokelatest at ./essentials.jl:741 [inlined]
 [17] print_response(::IO, ::Any, ::Any, ::Bool, ::Bool, ::Any) at /Users/johnmyleswhite/julia/usr/share/julia/stdlib/v1.2/REPL/src/REPL.jl:155
 [18] print_response(::REPL.AbstractREPL, ::Any, ::Any, ::Bool, ::Bool) at /Users/johnmyleswhite/julia/usr/share/julia/stdlib/v1.2/REPL/src/REPL.jl:140
 [19] (::getfield(REPL, Symbol("#do_respond#38")){Bool,getfield(REPL, Symbol("##48#57")){REPL.LineEditREPL,REPL.REPLHistoryProvider},REPL.LineEditREPL,REPL.LineEdit.Prompt})(::Any, ::Any, ::Any) at /Users/johnmyleswhite/julia/usr/share/julia/stdlib/v1.2/REPL/src/REPL.jl:714
 [20] #invokelatest#1 at ./essentials.jl:742 [inlined]
 [21] invokelatest at ./essentials.jl:741 [inlined]
 [22] run_interface(::REPL.Terminals.TextTerminal, ::REPL.LineEdit.ModalInterface, ::REPL.LineEdit.MIState) at /Users/johnmyleswhite/julia/usr/share/julia/stdlib/v1.2/REPL/src/LineEdit.jl:2273
 [23] run_frontend(::REPL.LineEditREPL, ::REPL.REPLBackendRef) at /Users/johnmyleswhite/julia/usr/share/julia/stdlib/v1.2/REPL/src/REPL.jl:1035
 [24] run_repl(::REPL.AbstractREPL, ::Any) at /Users/johnmyleswhite/julia/usr/share/julia/stdlib/v1.2/REPL/src/REPL.jl:192
 [25] (::getfield(Base, Symbol("##735#737")){Bool,Bool,Bool,Bool})(::Module) at ./client.jl:362
 [26] #invokelatest#1 at ./essentials.jl:742 [inlined]
 [27] invokelatest at ./essentials.jl:741 [inlined]
 [28] run_main_repl(::Bool, ::Bool, ::Bool, ::Bool, ::Bool) at ./client.jl:346
 [29] exec_options(::Base.JLOptions) at ./client.jl:284
 [30] _start() at ./client.jl:436

[nalimilan]

Might be due to https://github.com/JuliaLang/julia/pull/30442#issuecomment-449759082? It was noted that CI fails since that PR has been merged.

[johnmyleswhite]

Undoing those changes locally doesn't seem to fix the problem.

[johnmyleswhite]

To expand on that, this issue shows up on the v1.0.3 tag.

[JeffBezanson]

The underlying issue is with fieldtype; fieldtype(Union{Tuple{Char},Tuple{Char,Char}},2) throws a bounds error. I would be ok with that returning Char, but it is a bit subtle --- we probably want fieldtype(Tuple{Char},2) to continue to throw an error instead of returning Union{}.

[JeffBezanson]

Hmm, that raises the question of what fieldtype(Union{Tuple{Char},Tuple{Char,Char}}, 3) does... throw an error if all types in the union would give an error?

[vtjnash]

Maybe we should add a keyword arg throw/raise/error/strict/etc that selects for the alternate behavior for all types and indexes (basically the getfield tfunc)?