internal error when running JuMP's tests

[mlubin]

julia> versioninfo()
Julia Version 0.7.0-rc2.0
Commit 78540cba4c (2018-08-02 19:14 UTC)
Platform Info:
  OS: Linux (x86_64-pc-linux-gnu)
  CPU: Intel(R) Core(TM) i5-7300U CPU @ 2.60GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-6.0.0 (ORCJIT, skylake)

When running JuMP's tests (current master, commit cd6457b6c0c82de339b30bbd747a3f1bac135b8b), the following internal error is printed:

Internal error: encountered unexpected error in runtime:
ErrorException("type Nothing has no field args")
rec_backtrace at /buildworker/worker/package_linux64/build/src/stackwalk.c:94
record_backtrace at /buildworker/worker/package_linux64/build/src/task.c:246
jl_throw at /buildworker/worker/package_linux64/build/src/task.c:577
jl_errorf at /buildworker/worker/package_linux64/build/src/rtutils.c:77
jl_field_index at /buildworker/worker/package_linux64/build/src/datatype.c:821
jl_f_getfield at /buildworker/worker/package_linux64/build/src/builtins.c:755
try_compute_fieldidx_expr at ./compiler/ssair/passes.jl:23
getfield_elim_pass! at ./compiler/ssair/passes.jl:720
run_passes at ./compiler/ssair/driver.jl:123
optimize at ./compiler/optimize.jl:162
typeinf at ./compiler/typeinfer.jl:35
typeinf_ext at ./compiler/typeinfer.jl:567
typeinf_ext at ./compiler/typeinfer.jl:604
jfptr_typeinf_ext_1.clone_1 at /home/mlubin/julia/julia-0.7.0-rc2/lib/julia/sys.so (unknown line)
jl_apply_generic at /buildworker/worker/package_linux64/build/src/gf.c:2165
jl_apply at /buildworker/worker/package_linux64/build/src/julia.h:1538 [inlined]
jl_apply_with_saved_exception_state at /buildworker/worker/package_linux64/build/src/rtutils.c:257
jl_type_infer at /buildworker/worker/package_linux64/build/src/gf.c:275
jl_compile_method_internal at /buildworker/worker/package_linux64/build/src/gf.c:1767 [inlined]
jl_fptr_trampoline at /buildworker/worker/package_linux64/build/src/gf.c:1811
jl_apply_generic at /buildworker/worker/package_linux64/build/src/gf.c:2165
macro expansion at /home/mlubin/.julia/dev/JuMP/test/derivatives.jl:40 [inlined]
macro expansion at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v0.7/Test/src/Test.jl:1079 [inlined]
top-level scope at /home/mlubin/.julia/dev/JuMP/test/derivatives.jl:11
jl_fptr_trampoline at /buildworker/worker/package_linux64/build/src/gf.c:1812
jl_toplevel_eval_flex at /buildworker/worker/package_linux64/build/src/toplevel.c:825
jl_parse_eval_all at /buildworker/worker/package_linux64/build/src/ast.c:841
jl_load at /buildworker/worker/package_linux64/build/src/toplevel.c:865
include at ./boot.jl:317 [inlined]
include_relative at ./loading.jl:1038
include at ./sysimg.jl:29
jl_apply_generic at /buildworker/worker/package_linux64/build/src/gf.c:2165
include at ./client.jl:398
jl_apply_generic at /buildworker/worker/package_linux64/build/src/gf.c:2165
do_call at /buildworker/worker/package_linux64/build/src/interpreter.c:324
eval_value at /buildworker/worker/package_linux64/build/src/interpreter.c:428
eval_stmt_value at /buildworker/worker/package_linux64/build/src/interpreter.c:363 [inlined]
eval_body at /buildworker/worker/package_linux64/build/src/interpreter.c:686
jl_interpret_toplevel_thunk_callback at /buildworker/worker/package_linux64/build/src/interpreter.c:799
unknown function (ip: 0xfffffffffffffffe)
unknown function (ip: 0x7f32773291df)
unknown function (ip: 0xffffffffffffffff)
jl_interpret_toplevel_thunk at /buildworker/worker/package_linux64/build/src/interpreter.c:808
jl_toplevel_eval_flex at /buildworker/worker/package_linux64/build/src/toplevel.c:831
jl_parse_eval_all at /buildworker/worker/package_linux64/build/src/ast.c:841
jl_load at /buildworker/worker/package_linux64/build/src/toplevel.c:865
include at ./boot.jl:317 [inlined]
include_relative at ./loading.jl:1038
include at ./sysimg.jl:29
jl_apply_generic at /buildworker/worker/package_linux64/build/src/gf.c:2165
include at ./client.jl:398
jl_fptr_trampoline at /buildworker/worker/package_linux64/build/src/gf.c:1812
jl_apply_generic at /buildworker/worker/package_linux64/build/src/gf.c:2165
do_call at /buildworker/worker/package_linux64/build/src/interpreter.c:324
eval_value at /buildworker/worker/package_linux64/build/src/interpreter.c:428
eval_stmt_value at /buildworker/worker/package_linux64/build/src/interpreter.c:363 [inlined]
eval_body at /buildworker/worker/package_linux64/build/src/interpreter.c:686
jl_interpret_toplevel_thunk_callback at /buildworker/worker/package_linux64/build/src/interpreter.c:799
unknown function (ip: 0xfffffffffffffffe)
unknown function (ip: 0x7f32734cc64f)
unknown function (ip: 0xffffffffffffffff)
jl_interpret_toplevel_thunk at /buildworker/worker/package_linux64/build/src/interpreter.c:808
jl_toplevel_eval_flex at /buildworker/worker/package_linux64/build/src/toplevel.c:831
jl_toplevel_eval_flex at /buildworker/worker/package_linux64/build/src/toplevel.c:778
jl_toplevel_eval_in at /buildworker/worker/package_linux64/build/src/builtins.c:633
eval at ./boot.jl:319
jl_apply_generic at /buildworker/worker/package_linux64/build/src/gf.c:2165
macro expansion at ./logging.jl:317 [inlined]
exec_options at ./client.jl:229
_start at ./client.jl:432
jl_apply_generic at /buildworker/worker/package_linux64/build/src/gf.c:2165
jl_apply at /buildworker/worker/package_linux64/build/ui/../src/julia.h:1538 [inlined]
true_main at /buildworker/worker/package_linux64/build/ui/repl.c:112
main at /buildworker/worker/package_linux64/build/ui/repl.c:233
__libc_start_main at /build/glibc-OTsEL5/glibc-2.27/csu/../csu/libc-start.c:310
_start at /home/mlubin/julia/julia-0.7.0-rc2/bin/julia (unknown line)

This happens locally and also on travis (https://travis-ci.org/JuliaOpt/JuMP.jl/jobs/412077553#L796). The error appears to be ignored, it doesn't crash julia. (The tests proceed to fail at a later point.)

[Keno]

This would happen if a use is discovered to be unused during the pass. I think it would be correct just to ignore the statement in that case:

        for use in defuse.uses
-            field = try_compute_fieldidx_expr(typ, )
+           stmt = ir[SSAValue(use)]
+           stmt === nothing && continue
+           field = try_compute_fieldidx_expr(typ, stmt)
            field === nothing && (ok = false; break)
            push!(fielddefuse[field].uses, use)
        end

and similarly for the loop before that. Does need a properly reduced test case though.

[Keno]

This is a fun one. There's a second bug here that makes this transformation apply even though it's not legal in the JuMP example. After fixing that, the following simple example reproduces the reported behavior:

mutable struct foo
    a::Int
    b::Int
end
function bar()
    a = foo(1, 2)
    c, d = a.a, a.b
    e = (c, d)
    e[1]
end

(doesn't fail on master though, because of the same legality check bug working in the opposite direction).