Possible Bug With TypeVars

General Usage
1 
   _       _ _(_)_     |  Documentation: https://docs.julialang.org
  (_)     | (_) (_)    |
   _ _   _| |_  __ _   |  Type "?" for help, "]?" for Pkg help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 1.2.0 (2019-08-20)
 _/ |\__'_|_|_|\__'_|  |  Official https://julialang.org/ release
|__/                   |

julia> struct MM{T}
       end

julia> with(tv1::TypeVar,tv2::TypeVar) = MM{Union{tv1,tv2}}
with (generic function with 1 method)

julia> struct Q{T,M}
           x::with(T,M)
       end
Unreachable reached at 0x12ad0be2a

signal (4): Illegal instruction: 4
in expression starting at REPL[3]:1
with at ./REPL[2]:1
do_call at /Users/sabae/buildbot/worker/package_macos64/build/src/interpreter.c:323
do_call at /Users/sabae/buildbot/worker/package_macos64/build/src/interpreter.c:322
eval_structtype at /Users/sabae/buildbot/worker/package_macos64/build/src/interpreter.c:256
eval_body at /Users/sabae/buildbot/worker/package_macos64/build/src/interpreter.c:740
jl_interpret_toplevel_thunk_callback at /Users/sabae/buildbot/worker/package_macos64/build/src/interpreter.c:884
unknown function (ip: 0xfffffffffffffffe)
unknown function (ip: 0x11a9fa28f)
unknown function (ip: 0x3)
jl_interpret_toplevel_thunk at /Users/sabae/buildbot/worker/package_macos64/build/src/interpreter.c:893
jl_toplevel_eval_flex at /Users/sabae/buildbot/worker/package_macos64/build/src/toplevel.c:815
jl_toplevel_eval_flex at /Users/sabae/buildbot/worker/package_macos64/build/src/toplevel.c:764
jl_toplevel_eval at /Users/sabae/buildbot/worker/package_macos64/build/src/toplevel.c:824 [inlined]
jl_toplevel_eval_in at /Users/sabae/buildbot/worker/package_macos64/build/src/toplevel.c:844
eval at ./boot.jl:330
eval_user_input at /Users/sabae/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.2/REPL/src/REPL.jl:86
macro expansion at /Users/sabae/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.2/REPL/src/REPL.jl:118 [inlined]
#26 at ./task.jl:268
jl_apply at /Users/sabae/buildbot/worker/package_macos64/build/src/./julia.h:1614 [inlined]
start_task at /Users/sabae/buildbot/worker/package_macos64/build/src/task.c:596
Allocations: 3095920 (Pool: 3095210; Big: 710); GC: 6
[1]    7610 illegal hardware instruction  julia
2

This may be fixed:

julia> struct MM{T} end

julia> with(tv1::TypeVar,tv2::TypeVar) = MM{Union{tv1,tv2}}
with (generic function with 1 method)

julia> struct Q{T,M}
           x::with(T,M)
       end

julia> VERSION
v"1.3.0-rc4.17"