Internal error: encountered unexpected error in runtime

[goerch]

MRE:

function Base.iterate(::Nothing)
end

mutable struct TrieSet{K} <: AbstractSet{AbstractArray{K}}
    dict::Dict{K, TrieSet{K}}
    value::Bool
    function TrieSet{K}() where K
        new(Dict{K, TrieSet{K}}(), false)
    end
end

function walk(trie::TrieSet{K}, f, prefix=Vector{K}()) where K
    if trie.value
        f(prefix)
    end
    for (char, subtrie) in trie.dict
        walk(subtrie, f, vcat(prefix, char))
    end
end

function Base.push!(trie::TrieSet{K}, key) where K
    for char in key
        if trie.dict == nothing
            trie.dict = Dict{K, TrieSet{K}}(char => TrieSet{K}())
        elseif !haskey(trie.dict, char)
            trie.dict[char] = TrieSet{K}()
        end
        trie = trie.dict[char]
    end
    trie.value = true
end

function getsubtrie(trie::TrieSet{K}, key) where K
    prefix = Vector{K}()
    for char in key
        if trie.dict == nothing || !haskey(trie.dict, char)
            return nothing
        end
        push!(prefix, char)
        trie = trie.dict[char]
    end
    return trie, prefix
end

struct PrefixTrieSet{K} <: AbstractSet{AbstractArray{K}}
    subtrie::TrieSet{K}
    prefix::Vector{K}
    function PrefixTrieSet{K}(trie, prefix) where K
        new(getsubtrie(trie, prefix)...)
    end
end

function keys(trie::PrefixTrieSet{K}) where K
    result = Vector{Vector{K}}()
    walk(trie.subtrie, x -> push!(result, x), trie.prefix)
    return result
end

set = TrieSet{Char}()
push!(set, "a")
keys(PrefixTrieSet{Char}(set, "a"))

Trace back running the program on 1.6.3:

Internal error: encountered unexpected error in runtime:
BoundsError(a=svec(), i=0)
getindex at .\essentials.jl:589 [inlined]
unswitchtupleunion at .\compiler\typeutils.jl:219
getfield_elim_pass! at .\compiler/ssair\passes.jl:624
run_passes at .\compiler/ssair\driver.jl:133
optimize at .\compiler\optimize.jl:272 [inlined]
_typeinf at .\compiler\typeinfer.jl:244
typeinf at .\compiler\typeinfer.jl:209
typeinf_ext at .\compiler\typeinfer.jl:892
typeinf_ext_toplevel at .\compiler\typeinfer.jl:925
typeinf_ext_toplevel at .\compiler\typeinfer.jl:921
jfptr_typeinf_ext_toplevel_14355.clone_1 at C:\Users\Win10\AppData\Local\Programs\Julia-1.6.3\lib\julia\sys.dll (unknown line)
_jl_invoke at /cygdrive/c/buildbot/worker/package_win64/build/src\gf.c:2237 [inlined]
jl_apply_generic at /cygdrive/c/buildbot/worker/package_win64/build/src\gf.c:2419 [inlined]
jl_apply at /cygdrive/c/buildbot/worker/package_win64/build/src\julia.h:1703 [inlined]
jl_type_infer at /cygdrive/c/buildbot/worker/package_win64/build/src\gf.c:298
jl_generate_fptr at /cygdrive/c/buildbot/worker/package_win64/build/src\jitlayers.cpp:340
jl_compile_method_internal at /cygdrive/c/buildbot/worker/package_win64/build/src\gf.c:1970
jl_compile_method_internal at /cygdrive/c/buildbot/worker/package_win64/build/src\gf.c:1924 [inlined]
_jl_invoke at /cygdrive/c/buildbot/worker/package_win64/build/src\gf.c:2229 [inlined]
jl_apply_generic at /cygdrive/c/buildbot/worker/package_win64/build/src\gf.c:2419
jl_apply at /cygdrive/c/buildbot/worker/package_win64/build/src\julia.h:1703 [inlined]
do_call at /cygdrive/c/buildbot/worker/package_win64/build/src\interpreter.c:115
eval_value at /cygdrive/c/buildbot/worker/package_win64/build/src\interpreter.c:204
eval_stmt_value at /cygdrive/c/buildbot/worker/package_win64/build/src\interpreter.c:155 [inlined]
eval_body at /cygdrive/c/buildbot/worker/package_win64/build/src\interpreter.c:576
jl_interpret_toplevel_thunk at /cygdrive/c/buildbot/worker/package_win64/build/src\interpreter.c:670
top-level scope at C:\Users\Win10\source\repos\julia\reduce internal runtime error\src\problem.jl:61
jl_toplevel_eval_flex at /cygdrive/c/buildbot/worker/package_win64/build/src\toplevel.c:877
jl_toplevel_eval_flex at /cygdrive/c/buildbot/worker/package_win64/build/src\toplevel.c:825
jl_toplevel_eval at /cygdrive/c/buildbot/worker/package_win64/build/src\toplevel.c:886 [inlined]
jl_toplevel_eval_in at /cygdrive/c/buildbot/worker/package_win64/build/src\toplevel.c:929
eval at .\boot.jl:360 [inlined]
include_string at .\loading.jl:1116
_include at .\loading.jl:1170
include at .\client.jl:444
jl_apply at /cygdrive/c/buildbot/worker/package_win64/build/src\julia.h:1703 [inlined]
do_call at /cygdrive/c/buildbot/worker/package_win64/build/src\interpreter.c:115
eval_value at /cygdrive/c/buildbot/worker/package_win64/build/src\interpreter.c:204
eval_stmt_value at /cygdrive/c/buildbot/worker/package_win64/build/src\interpreter.c:155 [inlined]
eval_body at /cygdrive/c/buildbot/worker/package_win64/build/src\interpreter.c:576
jl_interpret_toplevel_thunk at /cygdrive/c/buildbot/worker/package_win64/build/src\interpreter.c:670
top-level scope at none:1
jl_toplevel_eval_flex at /cygdrive/c/buildbot/worker/package_win64/build/src\toplevel.c:877
jl_toplevel_eval at /cygdrive/c/buildbot/worker/package_win64/build/src\toplevel.c:886 [inlined]
jl_toplevel_eval_in at /cygdrive/c/buildbot/worker/package_win64/build/src\toplevel.c:929
eval at .\boot.jl:360 [inlined]
repleval at C:\Users\Win10\.julia\packages\Atom\bfwsW\src\repl.jl:198
#242 at C:\Users\Win10\.julia\packages\Atom\bfwsW\src\repl.jl:228
unknown function (ip: 000000005b418dd3)
with_logstate at .\logging.jl:491
with_logger at .\logging.jl:603 [inlined]
evalrepl at C:\Users\Win10\.julia\packages\Atom\bfwsW\src\repl.jl:216
jl_apply at /cygdrive/c/buildbot/worker/package_win64/build/src\julia.h:1703 [inlined]
do_call at /cygdrive/c/buildbot/worker/package_win64/build/src\interpreter.c:115
eval_value at /cygdrive/c/buildbot/worker/package_win64/build/src\interpreter.c:204
eval_stmt_value at /cygdrive/c/buildbot/worker/package_win64/build/src\interpreter.c:155 [inlined]
eval_body at /cygdrive/c/buildbot/worker/package_win64/build/src\interpreter.c:576
jl_interpret_toplevel_thunk at /cygdrive/c/buildbot/worker/package_win64/build/src\interpreter.c:670
top-level scope at C:\Users\Win10\.julia\packages\Atom\bfwsW\src\repl.jl:264
jl_toplevel_eval_flex at /cygdrive/c/buildbot/worker/package_win64/build/src\toplevel.c:877
jl_toplevel_eval at /cygdrive/c/buildbot/worker/package_win64/build/src\toplevel.c:886 [inlined]
jl_toplevel_eval_in at /cygdrive/c/buildbot/worker/package_win64/build/src\toplevel.c:929
eval at .\boot.jl:360 [inlined]
eval_user_input at C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:139
repl_backend_loop at C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:200
start_repl_backend at C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:185
#run_repl#42 at C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:317
run_repl at C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:305
#874 at .\client.jl:387
jfptr_YY.874_32319.clone_1 at C:\Users\Win10\AppData\Local\Programs\Julia-1.6.3\lib\julia\sys.dll (unknown line)
jl_apply at /cygdrive/c/buildbot/worker/package_win64/build/src\julia.h:1703 [inlined]
jl_f__call_latest at /cygdrive/c/buildbot/worker/package_win64/build/src\builtins.c:714
#invokelatest#2 at .\essentials.jl:708 [inlined]
invokelatest at .\essentials.jl:706 [inlined]
run_main_repl at .\client.jl:372
exec_options at .\client.jl:302
_start at .\client.jl:485
jfptr__start_22917.clone_1 at C:\Users\Win10\AppData\Local\Programs\Julia-1.6.3\lib\julia\sys.dll (unknown line)
jl_apply at /cygdrive/c/buildbot/worker/package_win64/build/src\julia.h:1703 [inlined]
true_main at /cygdrive/c/buildbot/worker/package_win64/build/src\jlapi.c:560
repl_entrypoint at /cygdrive/c/buildbot/worker/package_win64/build/src\jlapi.c:702
mainCRTStartup at /cygdrive/c/buildbot/worker/package_win64/build/cli\loader_exe.c:51
BaseThreadInitThunk at C:\Windows\System32\KERNEL32.DLL (unknown line)
RtlUserThreadStart at C:\Windows\SYSTEM32\ntdll.dll (unknown line)

Further observations: the program doesn't crash after the trace back. The trace back vanishes, if I remove Base.iterate(::Nothing).

[Seelengrab]

I can't reproduce this on

julia> versioninfo()
Julia Version 1.8.0-DEV.631
Commit 5a98032fa8 (2021-10-01 08:05 UTC)
Platform Info:
  OS: Linux (x86_64-pc-linux-gnu)
  CPU: Intel(R) Core(TM) i7-6600U CPU @ 2.60GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-12.0.1 (ORCJIT, skylake)

though I do get a MethodError for a missing iterate(::TrieSet{Char}) method.

[inkydragon]

Confirmed on julia 1.6.3

julia> versioninfo()
Julia Version 1.6.3
Commit ae8452a9e0 (2021-09-23 17:34 UTC)
Platform Info:
  OS: Windows (x86_64-w64-mingw32)
  CPU: Intel(R) Core(TM) i5-9400F CPU @ 2.90GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-11.0.1 (ORCJIT, skylake)

After the internal error, REPL does not crash.
If you continue executing keys(PrefixTrieSet{Char}(set, "a")) no error occurs.

error output

julia> function Base.iterate(::Nothing)
       end

julia> mutable struct TrieSet{K} <: AbstractSet{AbstractArray{K}}
           dict::Dict{K, TrieSet{K}}
           value::Bool
           function TrieSet{K}() where K
               new(Dict{K, TrieSet{K}}(), false)
           end
       end

julia> function walk(trie::TrieSet{K}, f, prefix=Vector{K}()) where K
           if trie.value
               f(prefix)
           end
           for (char, subtrie) in trie.dict
               walk(subtrie, f, vcat(prefix, char))
           end
       end
walk (generic function with 2 methods)

julia> function Base.push!(trie::TrieSet{K}, key) where K
           for char in key
               if trie.dict == nothing
                   trie.dict = Dict{K, TrieSet{K}}(char => TrieSet{K}())
               elseif !haskey(trie.dict, char)
                   trie.dict[char] = TrieSet{K}()
               end
               trie = trie.dict[char]
           end
           trie.value = true
       end

julia> function getsubtrie(trie::TrieSet{K}, key) where K
           prefix = Vector{K}()
           for char in key
               if trie.dict == nothing || !haskey(trie.dict, char)
                   return nothing
               end
               push!(prefix, char)
               trie = trie.dict[char]
           end
           return trie, prefix
       end
getsubtrie (generic function with 1 method)

julia> struct PrefixTrieSet{K} <: AbstractSet{AbstractArray{K}}
           subtrie::TrieSet{K}
           prefix::Vector{K}
           function PrefixTrieSet{K}(trie, prefix) where K
               new(getsubtrie(trie, prefix)...)
           end
       end

julia> function keys(trie::PrefixTrieSet{K}) where K
           result = Vector{Vector{K}}()
           walk(trie.subtrie, x -> push!(result, x), trie.prefix)
           return result
       end
keys (generic function with 1 method)

julia>

julia> set = TrieSet{Char}()
Error showing value of type TrieSet{Char}:
ERROR: MethodError: no method matching iterate(::TrieSet{Char})
Closest candidates are:
  iterate(::Union{LinRange, StepRangeLen}) at range.jl:664
  iterate(::Union{LinRange, StepRangeLen}, ::Int64) at range.jl:664
  iterate(::T) where T<:Union{Base.KeySet{var"#s77", var"#s76"} where {var"#s77", var"#s76"<:Dict}, Base.ValueIterator{var"#s75"} where var"#s75"<:Dict} at dict.jl:693
  ...
Stacktrace:
  [1] isempty(itr::TrieSet{Char})
    @ Base .\essentials.jl:767
  [2] show(io::IOContext{Base.TTY}, #unused#::MIME{Symbol("text/plain")}, t::TrieSet{Char})
    @ Base .\show.jl:153
  [3] (::REPL.var"#38#39"{REPL.REPLDisplay{REPL.LineEditREPL}, MIME{Symbol("text/plain")}, Base.RefValue{Any}})(io::Any)
    @ REPL C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:220
  [4] with_repl_linfo(f::Any, repl::REPL.LineEditREPL)
    @ REPL C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:462
  [5] display(d::REPL.REPLDisplay, mime::MIME{Symbol("text/plain")}, x::Any)
    @ REPL C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:213
  [6] display(d::REPL.REPLDisplay, x::Any)
    @ REPL C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:225
  [7] display(x::Any)
    @ Base.Multimedia .\multimedia.jl:328
  [8] #invokelatest#2
    @ .\essentials.jl:708 [inlined]
  [9] invokelatest
    @ .\essentials.jl:706 [inlined]
 [10] print_response(errio::IO, response::Any, show_value::Bool, have_color::Bool, specialdisplay::Union{Nothing, AbstractDisplay})
    @ REPL C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:247
 [11] (::REPL.var"#40#41"{REPL.LineEditREPL, Pair{Any, Bool}, Bool, Bool})(io::Any)
    @ REPL C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:231
 [12] with_repl_linfo(f::Any, repl::REPL.LineEditREPL)
    @ REPL C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:462
 [13] print_response(repl::REPL.AbstractREPL, response::Any, show_value::Bool, have_color::Bool)
    @ REPL C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:229
 [14] (::REPL.var"#do_respond#61"{Bool, Bool, REPL.var"#72#82"{REPL.LineEditREPL, REPL.REPLHistoryProvider}, REPL.LineEditREPL, REPL.LineEdit.Prompt})(s::REPL.LineEdit.MIState, buf::Any, ok::Bool)
    @ REPL C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:798
 [15] #invokelatest#2
    @ .\essentials.jl:708 [inlined]
 [16] invokelatest
    @ .\essentials.jl:706 [inlined]
 [17] run_interface(terminal::REPL.Terminals.TextTerminal, m::REPL.LineEdit.ModalInterface, s::REPL.LineEdit.MIState)
    @ REPL.LineEdit C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\LineEdit.jl:2441
 [18] run_frontend(repl::REPL.LineEditREPL, backend::REPL.REPLBackendRef)
    @ REPL C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:1126
 [19] (::REPL.var"#44#49"{REPL.LineEditREPL, REPL.REPLBackendRef})()
    @ REPL .\task.jl:411

julia> push!(set, "a")
true

julia> keys(PrefixTrieSet{Char}(set, "a"))
Internal error: encountered unexpected error in runtime:
BoundsError(a=svec(), i=0)
getindex at .\essentials.jl:589 [inlined]
unswitchtupleunion at .\compiler\typeutils.jl:219
getfield_elim_pass! at .\compiler/ssair\passes.jl:624
run_passes at .\compiler/ssair\driver.jl:133
optimize at .\compiler\optimize.jl:272 [inlined]
_typeinf at .\compiler\typeinfer.jl:244
typeinf at .\compiler\typeinfer.jl:209
typeinf_ext at .\compiler\typeinfer.jl:892
typeinf_ext_toplevel at .\compiler\typeinfer.jl:925
typeinf_ext_toplevel at .\compiler\typeinfer.jl:921
jfptr_typeinf_ext_toplevel_14355.clone_1 at C:\Users\woclass\AppData\Local\Programs\Julia-1.6.3\lib\julia\sys.dll (unknown line)
_jl_invoke at /cygdrive/c/buildbot/worker/package_win64/build/src\gf.c:2237 [inlined]
jl_apply_generic at /cygdrive/c/buildbot/worker/package_win64/build/src\gf.c:2419 [inlined]
jl_apply at /cygdrive/c/buildbot/worker/package_win64/build/src\julia.h:1703 [inlined]
jl_type_infer at /cygdrive/c/buildbot/worker/package_win64/build/src\gf.c:298
jl_generate_fptr at /cygdrive/c/buildbot/worker/package_win64/build/src\jitlayers.cpp:340
jl_compile_method_internal at /cygdrive/c/buildbot/worker/package_win64/build/src\gf.c:1970
jl_compile_method_internal at /cygdrive/c/buildbot/worker/package_win64/build/src\gf.c:1924 [inlined]
_jl_invoke at /cygdrive/c/buildbot/worker/package_win64/build/src\gf.c:2229 [inlined]
jl_apply_generic at /cygdrive/c/buildbot/worker/package_win64/build/src\gf.c:2419
jl_apply at /cygdrive/c/buildbot/worker/package_win64/build/src\julia.h:1703 [inlined]
do_call at /cygdrive/c/buildbot/worker/package_win64/build/src\interpreter.c:115
eval_value at /cygdrive/c/buildbot/worker/package_win64/build/src\interpreter.c:204
eval_stmt_value at /cygdrive/c/buildbot/worker/package_win64/build/src\interpreter.c:155 [inlined]
eval_body at /cygdrive/c/buildbot/worker/package_win64/build/src\interpreter.c:576
jl_interpret_toplevel_thunk at /cygdrive/c/buildbot/worker/package_win64/build/src\interpreter.c:670
top-level scope at REPL[10]:1
jl_toplevel_eval_flex at /cygdrive/c/buildbot/worker/package_win64/build/src\toplevel.c:877
jl_toplevel_eval_flex at /cygdrive/c/buildbot/worker/package_win64/build/src\toplevel.c:825
jl_toplevel_eval at /cygdrive/c/buildbot/worker/package_win64/build/src\toplevel.c:886 [inlined]
jl_toplevel_eval_in at /cygdrive/c/buildbot/worker/package_win64/build/src\toplevel.c:929
eval at .\boot.jl:360 [inlined]
eval_user_input at C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:139
repl_backend_loop at C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:200
start_repl_backend at C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:185
#run_repl#42 at C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:317
run_repl at C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:305
#874 at .\client.jl:387
jfptr_YY.874_32319.clone_1 at C:\Users\woclass\AppData\Local\Programs\Julia-1.6.3\lib\julia\sys.dll (unknown line)
jl_apply at /cygdrive/c/buildbot/worker/package_win64/build/src\julia.h:1703 [inlined]
jl_f__call_latest at /cygdrive/c/buildbot/worker/package_win64/build/src\builtins.c:714
#invokelatest#2 at .\essentials.jl:708 [inlined]
invokelatest at .\essentials.jl:706 [inlined]
run_main_repl at .\client.jl:372
exec_options at .\client.jl:302
_start at .\client.jl:485
jfptr__start_22917.clone_1 at C:\Users\woclass\AppData\Local\Programs\Julia-1.6.3\lib\julia\sys.dll (unknown line)
jl_apply at /cygdrive/c/buildbot/worker/package_win64/build/src\julia.h:1703 [inlined]
true_main at /cygdrive/c/buildbot/worker/package_win64/build/src\jlapi.c:560
repl_entrypoint at /cygdrive/c/buildbot/worker/package_win64/build/src\jlapi.c:702
mainCRTStartup at /cygdrive/c/buildbot/worker/package_win64/build/cli\loader_exe.c:51
BaseThreadInitThunk at C:\WINDOWS\System32\KERNEL32.DLL (unknown line)
RtlUserThreadStart at C:\WINDOWS\SYSTEM32\ntdll.dll (unknown line)
1-element Vector{Vector{Char}}:
 ['a']

julia>


julia> set2 = TrieSet{Char}()
Error showing value of type TrieSet{Char}:
ERROR: MethodError: no method matching iterate(::TrieSet{Char})
Closest candidates are:
  iterate(::Union{LinRange, StepRangeLen}) at range.jl:664
  iterate(::Union{LinRange, StepRangeLen}, ::Int64) at range.jl:664
  iterate(::T) where T<:Union{Base.KeySet{var"#s77", var"#s76"} where {var"#s77", var"#s76"<:Dict}, Base.ValueIterator{var"#s75"} where var"#s75"<:Dict} at dict.jl:693
  ...
Stacktrace:
  [1] isempty(itr::TrieSet{Char})
    @ Base .\essentials.jl:767
  [2] show(io::IOContext{Base.TTY}, #unused#::MIME{Symbol("text/plain")}, t::TrieSet{Char})
    @ Base .\show.jl:153
  [3] (::REPL.var"#38#39"{REPL.REPLDisplay{REPL.LineEditREPL}, MIME{Symbol("text/plain")}, Base.RefValue{Any}})(io::Any)
    @ REPL C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:220
  [4] with_repl_linfo(f::Any, repl::REPL.LineEditREPL)
    @ REPL C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:462
  [5] display(d::REPL.REPLDisplay, mime::MIME{Symbol("text/plain")}, x::Any)
    @ REPL C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:213
  [6] display(d::REPL.REPLDisplay, x::Any)
    @ REPL C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:225
  [7] display(x::Any)
    @ Base.Multimedia .\multimedia.jl:328
  [8] #invokelatest#2
    @ .\essentials.jl:708 [inlined]
  [9] invokelatest
    @ .\essentials.jl:706 [inlined]
 [10] print_response(errio::IO, response::Any, show_value::Bool, have_color::Bool, specialdisplay::Union{Nothing, AbstractDisplay})
    @ REPL C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:247
 [11] (::REPL.var"#40#41"{REPL.LineEditREPL, Pair{Any, Bool}, Bool, Bool})(io::Any)
    @ REPL C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:231
 [12] with_repl_linfo(f::Any, repl::REPL.LineEditREPL)
    @ REPL C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:462
 [13] print_response(repl::REPL.AbstractREPL, response::Any, show_value::Bool, have_color::Bool)
    @ REPL C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:229
 [14] (::REPL.var"#do_respond#61"{Bool, Bool, REPL.var"#72#82"{REPL.LineEditREPL, REPL.REPLHistoryProvider}, REPL.LineEditREPL, REPL.LineEdit.Prompt})(s::REPL.LineEdit.MIState, buf::Any, ok::Bool)
    @ REPL C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:798
 [15] #invokelatest#2
    @ .\essentials.jl:708 [inlined]
 [16] invokelatest
    @ .\essentials.jl:706 [inlined]
 [17] run_interface(terminal::REPL.Terminals.TextTerminal, m::REPL.LineEdit.ModalInterface, s::REPL.LineEdit.MIState)
    @ REPL.LineEdit C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\LineEdit.jl:2441
 [18] run_frontend(repl::REPL.LineEditREPL, backend::REPL.REPLBackendRef)
    @ REPL C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:1126
 [19] (::REPL.var"#44#49"{REPL.LineEditREPL, REPL.REPLBackendRef})()
    @ REPL .\task.jl:411

julia> push!(set2, "a")
true

julia> keys(PrefixTrieSet{Char}(set2, "a"))
1-element Vector{Vector{Char}}:
 ['a']

julia> set === set2
false


julia> set
Error showing value of type TrieSet{Char}:
ERROR: MethodError: no method matching iterate(::TrieSet{Char})
Closest candidates are:
  iterate(::Union{LinRange, StepRangeLen}) at range.jl:664
  iterate(::Union{LinRange, StepRangeLen}, ::Int64) at range.jl:664
  iterate(::T) where T<:Union{Base.KeySet{var"#s77", var"#s76"} where {var"#s77", var"#s76"<:Dict}, Base.ValueIterator{var"#s75"} where var"#s75"<:Dict} at dict.jl:693
  ...
Stacktrace:
  [1] isempty(itr::TrieSet{Char})
    @ Base .\essentials.jl:767
  [2] show(io::IOContext{Base.TTY}, #unused#::MIME{Symbol("text/plain")}, t::TrieSet{Char})
    @ Base .\show.jl:153
  [3] (::REPL.var"#38#39"{REPL.REPLDisplay{REPL.LineEditREPL}, MIME{Symbol("text/plain")}, Base.RefValue{Any}})(io::Any)
    @ REPL C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:220
  [4] with_repl_linfo(f::Any, repl::REPL.LineEditREPL)
    @ REPL C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:462
  [5] display(d::REPL.REPLDisplay, mime::MIME{Symbol("text/plain")}, x::Any)
    @ REPL C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:213
  [6] display(d::REPL.REPLDisplay, x::Any)
    @ REPL C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:225
  [7] display(x::Any)
    @ Base.Multimedia .\multimedia.jl:328
  [8] #invokelatest#2
    @ .\essentials.jl:708 [inlined]
  [9] invokelatest
    @ .\essentials.jl:706 [inlined]
 [10] print_response(errio::IO, response::Any, show_value::Bool, have_color::Bool, specialdisplay::Union{Nothing, AbstractDisplay})
    @ REPL C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:247
 [11] (::REPL.var"#40#41"{REPL.LineEditREPL, Pair{Any, Bool}, Bool, Bool})(io::Any)
    @ REPL C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:231
 [12] with_repl_linfo(f::Any, repl::REPL.LineEditREPL)
    @ REPL C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:462
 [13] print_response(repl::REPL.AbstractREPL, response::Any, show_value::Bool, have_color::Bool)
    @ REPL C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:229
 [14] (::REPL.var"#do_respond#61"{Bool, Bool, REPL.var"#72#82"{REPL.LineEditREPL, REPL.REPLHistoryProvider}, REPL.LineEditREPL, REPL.LineEdit.Prompt})(s::REPL.LineEdit.MIState, buf::Any, ok::Bool)
    @ REPL C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:798
 [15] #invokelatest#2
    @ .\essentials.jl:708 [inlined]
 [16] invokelatest
    @ .\essentials.jl:706 [inlined]
 [17] run_interface(terminal::REPL.Terminals.TextTerminal, m::REPL.LineEdit.ModalInterface, s::REPL.LineEdit.MIState)
    @ REPL.LineEdit C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\LineEdit.jl:2441
 [18] run_frontend(repl::REPL.LineEditREPL, backend::REPL.REPLBackendRef)
    @ REPL C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:1126
 [19] (::REPL.var"#44#49"{REPL.LineEditREPL, REPL.REPLBackendRef})()
    @ REPL .\task.jl:411

julia> keys(PrefixTrieSet{Char}(set, "a"))
1-element Vector{Vector{Char}}:
 ['a']

[inkydragon]

Simplified test code:

function Base.iterate(::Nothing) end

function retnothing()
    always_true = isempty(Dict())
    if always_true
        return  # Always returns nothing 
    end
    return nothing, nothing
end

struct Foo
    f::Any
    _  # unused
    Foo() = new(retnothing()...)
end

Foo()

Only throw error on Julia 1.6.3, 1.7.0 works fine.

looong output

julia> function Base.iterate(::Nothing) end

julia> function retnothing()
           always_true = isempty(Dict())
           if always_true
               return  # Always returns nothing
           end
           return nothing, nothing
       end
retnothing (generic function with 1 method)


julia> @code_warntype retnothing()
Variables
  #self#::Core.Const(retnothing)
  always_true::Bool

Body::Union{Nothing, Tuple{Nothing, Nothing}}
1 ─ %1 = Main.Dict()::Dict{Any, Any}
│        (always_true = Main.isempty(%1))
└──      goto #3 if not always_true
2 ─      return nothing
3 ─ %5 = Core.tuple(Main.nothing, Main.nothing)::Core.Const((nothing, nothing))
└──      return %5

julia> @code_typed retnothing()
CodeInfo(
1 ─ %1 = invoke Dict{Any, Any}()::Dict{Any, Any}
│   %2 = Base.getfield(%1, :count)::Int64
│   %3 = (%2 === 0)::Bool
└──      goto #3 if not %3
2 ─      return nothing
3 ─      return (nothing, nothing)
) => Union{Nothing, Tuple{Nothing, Nothing}}

julia> @code_llvm retnothing()
;  @ REPL[2]:1 within `retnothing'
; Function Attrs: uwtable
define i8 @julia_retnothing_389() #0 {
top:
;  @ REPL[2]:2 within `retnothing'
; ┌ @ dict.jl:118 within `Dict'
   %0 = call nonnull {}* @j1_Dict_391({}* inttoptr (i64 1802257408 to {}*), {}** null, i32 0)
; └
; ┌ @ dict.jl:690 within `isempty'
; │┌ @ Base.jl:33 within `getproperty'
    %1 = bitcast {}* %0 to i8*
    %2 = getelementptr inbounds i8, i8* %1, i64 32
    %3 = bitcast i8* %2 to i64*
    %4 = load i64, i64* %3, align 8
; │└
; │┌ @ promotion.jl:410 within `=='
    %.not = icmp eq i64 %4, 0
; └└
;  @ REPL[2]:3 within `retnothing'
  %spec.select = select i1 %.not, i8 1, i8 2
;  @ REPL[2]:4 within `retnothing'
  ret i8 %spec.select
}


julia> struct Foo
           f::Any
           _  # unused
           Foo() = new(retnothing()...)
       end

julia> Foo()
Internal error: encountered unexpected error in runtime:
BoundsError(a=svec(), i=0)
getindex at .\essentials.jl:589 [inlined]
unswitchtupleunion at .\compiler\typeutils.jl:219
getfield_elim_pass! at .\compiler/ssair\passes.jl:624
run_passes at .\compiler/ssair\driver.jl:133
optimize at .\compiler\optimize.jl:272 [inlined]
_typeinf at .\compiler\typeinfer.jl:244
typeinf at .\compiler\typeinfer.jl:209
typeinf_ext at .\compiler\typeinfer.jl:892
typeinf_ext_toplevel at .\compiler\typeinfer.jl:925
typeinf_ext_toplevel at .\compiler\typeinfer.jl:921
jfptr_typeinf_ext_toplevel_14355.clone_1 at C:\Users\woclass\AppData\Local\Programs\Julia-1.6.3\lib\julia\sys.dll (unknown line)
_jl_invoke at /cygdrive/c/buildbot/worker/package_win64/build/src\gf.c:2237 [inlined]
jl_apply_generic at /cygdrive/c/buildbot/worker/package_win64/build/src\gf.c:2419 [inlined]
jl_apply at /cygdrive/c/buildbot/worker/package_win64/build/src\julia.h:1703 [inlined]
jl_type_infer at /cygdrive/c/buildbot/worker/package_win64/build/src\gf.c:298
jl_generate_fptr at /cygdrive/c/buildbot/worker/package_win64/build/src\jitlayers.cpp:340
jl_compile_method_internal at /cygdrive/c/buildbot/worker/package_win64/build/src\gf.c:1970
jl_compile_method_internal at /cygdrive/c/buildbot/worker/package_win64/build/src\gf.c:1924 [inlined]
_jl_invoke at /cygdrive/c/buildbot/worker/package_win64/build/src\gf.c:2229 [inlined]
jl_apply_generic at /cygdrive/c/buildbot/worker/package_win64/build/src\gf.c:2419
jl_apply at /cygdrive/c/buildbot/worker/package_win64/build/src\julia.h:1703 [inlined]
do_call at /cygdrive/c/buildbot/worker/package_win64/build/src\interpreter.c:115
eval_value at /cygdrive/c/buildbot/worker/package_win64/build/src\interpreter.c:204
eval_stmt_value at /cygdrive/c/buildbot/worker/package_win64/build/src\interpreter.c:155 [inlined]
eval_body at /cygdrive/c/buildbot/worker/package_win64/build/src\interpreter.c:576
jl_interpret_toplevel_thunk at /cygdrive/c/buildbot/worker/package_win64/build/src\interpreter.c:670
top-level scope at REPL[4]:1
jl_toplevel_eval_flex at /cygdrive/c/buildbot/worker/package_win64/build/src\toplevel.c:877
jl_toplevel_eval_flex at /cygdrive/c/buildbot/worker/package_win64/build/src\toplevel.c:825
jl_toplevel_eval at /cygdrive/c/buildbot/worker/package_win64/build/src\toplevel.c:886 [inlined]
jl_toplevel_eval_in at /cygdrive/c/buildbot/worker/package_win64/build/src\toplevel.c:929
eval at .\boot.jl:360 [inlined]
eval_user_input at C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:139
repl_backend_loop at C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:200
start_repl_backend at C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:185
#run_repl#42 at C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:317
run_repl at C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:305
#874 at .\client.jl:387
jfptr_YY.874_32319.clone_1 at C:\Users\woclass\AppData\Local\Programs\Julia-1.6.3\lib\julia\sys.dll (unknown line)
jl_apply at /cygdrive/c/buildbot/worker/package_win64/build/src\julia.h:1703 [inlined]
jl_f__call_latest at /cygdrive/c/buildbot/worker/package_win64/build/src\builtins.c:714
#invokelatest#2 at .\essentials.jl:708 [inlined]
invokelatest at .\essentials.jl:706 [inlined]
run_main_repl at .\client.jl:372
exec_options at .\client.jl:302
_start at .\client.jl:485
jfptr__start_22917.clone_1 at C:\Users\woclass\AppData\Local\Programs\Julia-1.6.3\lib\julia\sys.dll (unknown line)
jl_apply at /cygdrive/c/buildbot/worker/package_win64/build/src\julia.h:1703 [inlined]
true_main at /cygdrive/c/buildbot/worker/package_win64/build/src\jlapi.c:560
repl_entrypoint at /cygdrive/c/buildbot/worker/package_win64/build/src\jlapi.c:702
mainCRTStartup at /cygdrive/c/buildbot/worker/package_win64/build/cli\loader_exe.c:51
BaseThreadInitThunk at C:\WINDOWS\System32\KERNEL32.DLL (unknown line)
RtlUserThreadStart at C:\WINDOWS\SYSTEM32\ntdll.dll (unknown line)
ERROR: ArgumentError: new: too few arguments (expected 2)
Stacktrace:
 [1] Foo()
   @ Main .\REPL[3]:4
 [2] top-level scope
   @ REPL[4]:1

julia> versioninfo()
Julia Version 1.6.3
Commit ae8452a9e0 (2021-09-23 17:34 UTC)
Platform Info:
  OS: Windows (x86_64-w64-mingw32)
  CPU: Intel(R) Core(TM) i5-9400F CPU @ 2.90GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-11.0.1 (ORCJIT, skylake)

[vtjnash]

Looks like the fix for this was hidden in #39754. I'll push a cherry-pick to the backports-release-1.6 branch