`const` redefinition crashes with `illegal hardware instruction`

[adienes]

same MWE as #57565 but I noticed it crashes Julia so making that a separate issue to track

julia> versioninfo()
Julia Version 1.13.0-DEV.128
Commit c8cd8fdb832 (2025-02-27 16:26 UTC)
Build Info:
  Official https://julialang.org release
Platform Info:
  OS: macOS (arm64-apple-darwin24.0.0)
  CPU: 10 × Apple M1 Max
  WORD_SIZE: 64
  LLVM: libLLVM-19.1.7 (ORCJIT, apple-m1)
  GC: Built with stock GC
Threads: 1 default, 1 interactive, 1 GC (on 8 virtual cores)

julia> const x = 1
1

julia> function foo()
           while rand() < 0.99
               global x = (rand(1:2:11)^2) % 4
               println(x)
           end
       end
foo (generic function with 1 method)

julia> foo()

[89197] signal 4 (1): Illegal instruction: 4
in expression starting at REPL[4]:1
foo at ./REPL[3]:3
unknown function (ip: 0x113a70133) at (unknown file)
jl_apply at /Users/julia/.julia/scratchspaces/a66863c6-20e8-4ff4-8a62-49f30b1f605e/agent-cache/default-honeycrisp-R17H3W25T9.0/build/default-honeycrisp-R17H3W25T9-0/julialang/julia-master/src/./julia.h:2290 [inlined]
do_call at /Users/julia/.julia/scratchspaces/a66863c6-20e8-4ff4-8a62-49f30b1f605e/agent-cache/default-honeycrisp-R17H3W25T9.0/build/default-honeycrisp-R17H3W25T9-0/julialang/julia-master/src/interpreter.c:124
eval_stmt_value at /Users/julia/.julia/scratchspaces/a66863c6-20e8-4ff4-8a62-49f30b1f605e/agent-cache/default-honeycrisp-R17H3W25T9.0/build/default-honeycrisp-R17H3W25T9-0/julialang/julia-master/src/interpreter.c:193
eval_body at /Users/julia/.julia/scratchspaces/a66863c6-20e8-4ff4-8a62-49f30b1f605e/agent-cache/default-honeycrisp-R17H3W25T9.0/build/default-honeycrisp-R17H3W25T9-0/julialang/julia-master/src/interpreter.c:688
jl_interpret_toplevel_thunk at /Users/julia/.julia/scratchspaces/a66863c6-20e8-4ff4-8a62-49f30b1f605e/agent-cache/default-honeycrisp-R17H3W25T9.0/build/default-honeycrisp-R17H3W25T9-0/julialang/julia-master/src/interpreter.c:897
jl_toplevel_eval_flex at /Users/julia/.julia/scratchspaces/a66863c6-20e8-4ff4-8a62-49f30b1f605e/agent-cache/default-honeycrisp-R17H3W25T9.0/build/default-honeycrisp-R17H3W25T9-0/julialang/julia-master/src/toplevel.c:1032
__repl_entry_eval_expanded_with_loc at /Users/julia/.julia/scratchspaces/a66863c6-20e8-4ff4-8a62-49f30b1f605e/agent-cache/default-honeycrisp-R17H3W25T9.0/build/default-honeycrisp-R17H3W25T9-0/julialang/julia-master/usr/share/julia/stdlib/v1.13/REPL/src/REPL.jl:340
jl_apply at /Users/julia/.julia/scratchspaces/a66863c6-20e8-4ff4-8a62-49f30b1f605e/agent-cache/default-honeycrisp-R17H3W25T9.0/build/default-honeycrisp-R17H3W25T9-0/julialang/julia-master/src/./julia.h:2290 [inlined]
jl_f__call_latest at /Users/julia/.julia/scratchspaces/a66863c6-20e8-4ff4-8a62-49f30b1f605e/agent-cache/default-honeycrisp-R17H3W25T9.0/build/default-honeycrisp-R17H3W25T9-0/julialang/julia-master/src/builtins.c:883
#invokelatest#1 at ./essentials.jl:1090 [inlined]
invokelatest at ./essentials.jl:1086 [inlined]
toplevel_eval_with_hooks at /Users/julia/.julia/scratchspaces/a66863c6-20e8-4ff4-8a62-49f30b1f605e/agent-cache/default-honeycrisp-R17H3W25T9.0/build/default-honeycrisp-R17H3W25T9-0/julialang/julia-master/usr/share/julia/stdlib/v1.13/REPL/src/REPL.jl:347
toplevel_eval_with_hooks at /Users/julia/.julia/scratchspaces/a66863c6-20e8-4ff4-8a62-49f30b1f605e/agent-cache/default-honeycrisp-R17H3W25T9.0/build/default-honeycrisp-R17H3W25T9-0/julialang/julia-master/usr/share/julia/stdlib/v1.13/REPL/src/REPL.jl:351
toplevel_eval_with_hooks at /Users/julia/.julia/scratchspaces/a66863c6-20e8-4ff4-8a62-49f30b1f605e/agent-cache/default-honeycrisp-R17H3W25T9.0/build/default-honeycrisp-R17H3W25T9-0/julialang/julia-master/usr/share/julia/stdlib/v1.13/REPL/src/REPL.jl:344 [inlined]
eval_user_input at /Users/julia/.julia/scratchspaces/a66863c6-20e8-4ff4-8a62-49f30b1f605e/agent-cache/default-honeycrisp-R17H3W25T9.0/build/default-honeycrisp-R17H3W25T9-0/julialang/julia-master/usr/share/julia/stdlib/v1.13/REPL/src/REPL.jl:369
repl_backend_loop at /Users/julia/.julia/scratchspaces/a66863c6-20e8-4ff4-8a62-49f30b1f605e/agent-cache/default-honeycrisp-R17H3W25T9.0/build/default-honeycrisp-R17H3W25T9-0/julialang/julia-master/usr/share/julia/stdlib/v1.13/REPL/src/REPL.jl:481
#start_repl_backend#41 at /Users/julia/.julia/scratchspaces/a66863c6-20e8-4ff4-8a62-49f30b1f605e/agent-cache/default-honeycrisp-R17H3W25T9.0/build/default-honeycrisp-R17H3W25T9-0/julialang/julia-master/usr/share/julia/stdlib/v1.13/REPL/src/REPL.jl:466
start_repl_backend at /Users/julia/.julia/scratchspaces/a66863c6-20e8-4ff4-8a62-49f30b1f605e/agent-cache/default-honeycrisp-R17H3W25T9.0/build/default-honeycrisp-R17H3W25T9-0/julialang/julia-master/usr/share/julia/stdlib/v1.13/REPL/src/REPL.jl:463 [inlined]
#run_repl#48 at /Users/julia/.julia/scratchspaces/a66863c6-20e8-4ff4-8a62-49f30b1f605e/agent-cache/default-honeycrisp-R17H3W25T9.0/build/default-honeycrisp-R17H3W25T9-0/julialang/julia-master/usr/share/julia/stdlib/v1.13/REPL/src/REPL.jl:689
run_repl at /Users/julia/.julia/scratchspaces/a66863c6-20e8-4ff4-8a62-49f30b1f605e/agent-cache/default-honeycrisp-R17H3W25T9.0/build/default-honeycrisp-R17H3W25T9-0/julialang/julia-master/usr/share/julia/stdlib/v1.13/REPL/src/REPL.jl:675
jfptr_run_repl_20745.1 at /Users/andy/.julia/juliaup/julia-nightly/share/julia/compiled/v1.13/REPL/u0gqU_j73Rz.dylib (unknown line)
run_std_repl at ./client.jl:488
jfptr_run_std_repl_60938.1 at /Users/andy/.julia/juliaup/julia-nightly/lib/julia/sys.dylib (unknown line)
jl_apply at /Users/julia/.julia/scratchspaces/a66863c6-20e8-4ff4-8a62-49f30b1f605e/agent-cache/default-honeycrisp-R17H3W25T9.0/build/default-honeycrisp-R17H3W25T9-0/julialang/julia-master/src/./julia.h:2290 [inlined]
jl_f__call_latest at /Users/julia/.julia/scratchspaces/a66863c6-20e8-4ff4-8a62-49f30b1f605e/agent-cache/default-honeycrisp-R17H3W25T9.0/build/default-honeycrisp-R17H3W25T9-0/julialang/julia-master/src/builtins.c:883
#invokelatest#1 at ./essentials.jl:1090 [inlined]
invokelatest at ./essentials.jl:1086 [inlined]
run_main_repl at ./client.jl:509
repl_main at ./client.jl:596 [inlined]
_start at ./client.jl:571
jfptr__start_60157.1 at /Users/andy/.julia/juliaup/julia-nightly/lib/julia/sys.dylib (unknown line)
jl_apply at /Users/julia/.julia/scratchspaces/a66863c6-20e8-4ff4-8a62-49f30b1f605e/agent-cache/default-honeycrisp-R17H3W25T9.0/build/default-honeycrisp-R17H3W25T9-0/julialang/julia-master/src/./julia.h:2290 [inlined]
true_main at /Users/julia/.julia/scratchspaces/a66863c6-20e8-4ff4-8a62-49f30b1f605e/agent-cache/default-honeycrisp-R17H3W25T9.0/build/default-honeycrisp-R17H3W25T9-0/julialang/julia-master/src/jlapi.c:964
jl_repl_entrypoint at /Users/julia/.julia/scratchspaces/a66863c6-20e8-4ff4-8a62-49f30b1f605e/agent-cache/default-honeycrisp-R17H3W25T9.0/build/default-honeycrisp-R17H3W25T9-0/julialang/julia-master/src/jlapi.c:1124
Allocations: 2088352 (Pool: 2088276; Big: 76); GC: 3
[1]    89197 trace trap  julia +nightly

[Keno]

A simpler reproducer is

julia> const x = 1
1

julia> f() = global x = 1
f (generic function with 1 method)

julia> f()
Unreachable reached at 0x7f6758b062ee

[577425] signal 4 (2): Illegal instruction

The compiler thinks assignment to const without the const keyword should always error, but it is in fact allowed if and only if the rhs is egal to the value that's already there. That said, I think we should consider removing the special case and just making assignments to const bindings universally an error (as the compiler assumes they are).

[adienes]

ah gotcha, thanks.

I think we should consider removing the special case and just making assignments to const bindings universally an error (as the compiler assumes they are).

not that anybody asked me, but I would 1000% agree