`Unreachable reached` in DocStringExtensions test suite

[nsajko]

Bisected to #56496. Running the DocStringExtensions v0.9.3 test suite produces the following error:

Unreachable reached at 0x76b156f7cfdf

[21255] signal 4 (2): Illegal instruction
in expression starting at /home/nsajko/.julia/packages/DocStringExtensions/JVu77/test/tests.jl:20
getproperty at ./Base_compiler.jl:48 [inlined]
#find_tuples##0 at ./none (unknown line)
iterate at ./generator.jl:48 [inlined]
collect at ./array.jl:789
unknown function (ip: 0x76b156f7cf92) at (unknown file)
_jl_invoke at /cache/build/tester-demeter6-14/julialang/julia-master/src/gf.c:3313 [inlined]
ijl_apply_generic at /cache/build/tester-demeter6-14/julialang/julia-master/src/gf.c:3501
find_tuples at /home/nsajko/.julia/packages/DocStringExtensions/JVu77/src/utilities.jl:280
unknown function (ip: 0x76b156f785b2) at (unknown file)
_jl_invoke at /cache/build/tester-demeter6-14/julialang/julia-master/src/gf.c:3313 [inlined]
ijl_apply_generic at /cache/build/tester-demeter6-14/julialang/julia-master/src/gf.c:3501
format at /home/nsajko/.julia/packages/DocStringExtensions/JVu77/src/abbreviations.jl:379
_jl_invoke at /cache/build/tester-demeter6-14/julialang/julia-master/src/gf.c:3313 [inlined]
ijl_apply_generic at /cache/build/tester-demeter6-14/julialang/julia-master/src/gf.c:3501
jl_apply at /cache/build/tester-demeter6-14/julialang/julia-master/src/julia.h:2240 [inlined]
do_call at /cache/build/tester-demeter6-14/julialang/julia-master/src/interpreter.c:125
eval_value at /cache/build/tester-demeter6-14/julialang/julia-master/src/interpreter.c:222
eval_stmt_value at /cache/build/tester-demeter6-14/julialang/julia-master/src/interpreter.c:173 [inlined]
eval_body at /cache/build/tester-demeter6-14/julialang/julia-master/src/interpreter.c:688
eval_body at /cache/build/tester-demeter6-14/julialang/julia-master/src/interpreter.c:545
eval_body at /cache/build/tester-demeter6-14/julialang/julia-master/src/interpreter.c:545
eval_body at /cache/build/tester-demeter6-14/julialang/julia-master/src/interpreter.c:545
eval_body at /cache/build/tester-demeter6-14/julialang/julia-master/src/interpreter.c:545
eval_body at /cache/build/tester-demeter6-14/julialang/julia-master/src/interpreter.c:545
eval_body at /cache/build/tester-demeter6-14/julialang/julia-master/src/interpreter.c:545
jl_interpret_toplevel_thunk at /cache/build/tester-demeter6-14/julialang/julia-master/src/interpreter.c:895
jl_toplevel_eval_flex at /cache/build/tester-demeter6-14/julialang/julia-master/src/toplevel.c:1065
jl_toplevel_eval_flex at /cache/build/tester-demeter6-14/julialang/julia-master/src/toplevel.c:1005
ijl_toplevel_eval at /cache/build/tester-demeter6-14/julialang/julia-master/src/toplevel.c:1076
ijl_toplevel_eval_in at /cache/build/tester-demeter6-14/julialang/julia-master/src/toplevel.c:1118
eval at ./boot.jl:460
include_string at ./loading.jl:2839
_jl_invoke at /cache/build/tester-demeter6-14/julialang/julia-master/src/gf.c:3313 [inlined]
ijl_apply_generic at /cache/build/tester-demeter6-14/julialang/julia-master/src/gf.c:3501
_include at ./loading.jl:2899
include at ./Base.jl:301
IncludeInto at ./Base.jl:302
jfptr_IncludeInto_69592.1 at /home/nsajko/tmp/jl/jl/nightly_assert/julia-aa05c98998/lib/julia/sys.so (unknown line)
_jl_invoke at /cache/build/tester-demeter6-14/julialang/julia-master/src/gf.c:3313 [inlined]
ijl_apply_generic at /cache/build/tester-demeter6-14/julialang/julia-master/src/gf.c:3501
jl_apply at /cache/build/tester-demeter6-14/julialang/julia-master/src/julia.h:2240 [inlined]
do_call at /cache/build/tester-demeter6-14/julialang/julia-master/src/interpreter.c:125
eval_value at /cache/build/tester-demeter6-14/julialang/julia-master/src/interpreter.c:222
eval_stmt_value at /cache/build/tester-demeter6-14/julialang/julia-master/src/interpreter.c:173 [inlined]
eval_body at /cache/build/tester-demeter6-14/julialang/julia-master/src/interpreter.c:688
jl_interpret_toplevel_thunk at /cache/build/tester-demeter6-14/julialang/julia-master/src/interpreter.c:895
jl_toplevel_eval_flex at /cache/build/tester-demeter6-14/julialang/julia-master/src/toplevel.c:1065
jl_toplevel_eval_flex at /cache/build/tester-demeter6-14/julialang/julia-master/src/toplevel.c:1005
ijl_toplevel_eval at /cache/build/tester-demeter6-14/julialang/julia-master/src/toplevel.c:1076
ijl_toplevel_eval_in at /cache/build/tester-demeter6-14/julialang/julia-master/src/toplevel.c:1118
eval at ./boot.jl:460
include_string at ./loading.jl:2839
_jl_invoke at /cache/build/tester-demeter6-14/julialang/julia-master/src/gf.c:3313 [inlined]
ijl_apply_generic at /cache/build/tester-demeter6-14/julialang/julia-master/src/gf.c:3501
_include at ./loading.jl:2899
include at ./Base.jl:301
IncludeInto at ./Base.jl:302
jfptr_IncludeInto_69592.1 at /home/nsajko/tmp/jl/jl/nightly_assert/julia-aa05c98998/lib/julia/sys.so (unknown line)
_jl_invoke at /cache/build/tester-demeter6-14/julialang/julia-master/src/gf.c:3313 [inlined]
ijl_apply_generic at /cache/build/tester-demeter6-14/julialang/julia-master/src/gf.c:3501
jl_apply at /cache/build/tester-demeter6-14/julialang/julia-master/src/julia.h:2240 [inlined]
do_call at /cache/build/tester-demeter6-14/julialang/julia-master/src/interpreter.c:125
eval_value at /cache/build/tester-demeter6-14/julialang/julia-master/src/interpreter.c:222
eval_stmt_value at /cache/build/tester-demeter6-14/julialang/julia-master/src/interpreter.c:173 [inlined]
eval_body at /cache/build/tester-demeter6-14/julialang/julia-master/src/interpreter.c:688
jl_interpret_toplevel_thunk at /cache/build/tester-demeter6-14/julialang/julia-master/src/interpreter.c:895
jl_toplevel_eval_flex at /cache/build/tester-demeter6-14/julialang/julia-master/src/toplevel.c:1065
jl_toplevel_eval_flex at /cache/build/tester-demeter6-14/julialang/julia-master/src/toplevel.c:1005
ijl_toplevel_eval at /cache/build/tester-demeter6-14/julialang/julia-master/src/toplevel.c:1076
ijl_toplevel_eval_in at /cache/build/tester-demeter6-14/julialang/julia-master/src/toplevel.c:1118
eval at ./boot.jl:460
exec_options at ./client.jl:295
_start at ./client.jl:558
jfptr__start_108729.1 at /home/nsajko/tmp/jl/jl/nightly_assert/julia-aa05c98998/lib/julia/sys.so (unknown line)
_jl_invoke at /cache/build/tester-demeter6-14/julialang/julia-master/src/gf.c:3313 [inlined]
ijl_apply_generic at /cache/build/tester-demeter6-14/julialang/julia-master/src/gf.c:3501
jl_apply at /cache/build/tester-demeter6-14/julialang/julia-master/src/julia.h:2240 [inlined]
true_main at /cache/build/tester-demeter6-14/julialang/julia-master/src/jlapi.c:900
jl_repl_entrypoint at /cache/build/tester-demeter6-14/julialang/julia-master/src/jlapi.c:1059
main at /cache/build/tester-demeter6-14/julialang/julia-master/cli/loader_exe.c:58
unknown function (ip: 0x76b158437e07) at /usr/lib/libc.so.6
__libc_start_main at /usr/lib/libc.so.6 (unknown line)
unknown function (ip: 0x4010b8) at /workspace/srcdir/glibc-2.17/csu/../sysdeps/x86_64/start.S
Allocations: 9287866 (Pool: 9287698; Big: 168); GC: 8
ERROR: Package DocStringExtensions errored during testing (received signal: 4)

[vtjnash]

Apparently this is a call to x.var where x is Tuple{Any}, except that it turns out to be actually Tuple{T} where T, so we abort (ud2). We'll need to look further into

(rr) f 0
#0  jl_f_getfield (F=0x0, args=0x7fff36bbf510, nargs=2) at /home/vtjnash/julia1/src/builtins.c:1026
(rr) p jl_(args[1])
:var
(rr) p jl_(args[0])
Tuple{T} where T
(rr) p (void) jl_gdblookup($pc)
getproperty at ./Base_compiler.jl:48 [inlined]
#find_tuples##0 at ./none (unknown line)
$3 = void

This is while running the MethodInstance:

(::DocStringExtensions.var"#find_tuples##0#find_tuples##1"{Type{Tuple{T} where T}})(Type) from (::DocStringExtensions.var"#find_tuples##0#find_tuples##1"{typesig})(Any)

[vtjnash]

MWE is pretty simple:

julia> function find_tuples(typesig)
           if typesig isa UnionAll
               return [UnionAll(typesig.var, x) for x in find_tuples(typesig.body)]
           elseif typesig isa Union
               return [typesig.a, find_tuples(typesig.b)...]
           else
               return [typesig,]
           end
       end
find_tuples (generic function with 1 method)

julia> 

julia> find_tuples(Tuple{T} where T)

[44976] signal 4: Illegal instruction: 4

If we look at the closure involved, we see this may be one of the things we expected to become unsounded by #40985

[vtjnash]

Better MWE:

julia> f(t) = getproperty(t, :var)
f (generic function with 3 methods)

julia> code_typed(f, (Type{Tuple{T} where T},))
1-element Vector{Any}:
 CodeInfo(
    @ REPL[10]:1 within `f`
   ┌ @ Base_compiler.jl:48 within `getproperty`
1 ─│       builtin Base.getfield(t, :var)::Union{}
└──│     unreachable
   └
) => Union{}

julia> f(t) = getfield(t, :var)
f (generic function with 4 methods)

julia> code_typed(f, (Type{Tuple{T} where T},))
1-element Vector{Any}:
 CodeInfo(
    @ REPL[12]:1 within `f`
1 ─ %1 =   builtin Main.getfield(t, :var)::Any
└──      return %1
) => Any

This unsoundness appears to have been introduced by #46693 / #46723 with the introduction of isTypeDataType, since that function returning true is unsound in the presence of all covariant types (aka Tuple) and not just Union.

[vtjnash]

Fixed by #57275