Hi,

With the latest Vim version (patch 9.0.1093), when sourcing the
following script:

------------------------------------------------------------------------------
vim9script

class MyCar
  this.make: string

  def new(make_arg: string)
      this.make = make_arg
  enddef

  def GetMake()
      echomsg $"make = {this.make}"
  enddef
endclass

var c = MyCar.new("abc")
c.GetMake()

var c = MyCar.new("def")
c.GetMake()
------------------------------------------------------------------------------

the following ASAN log is generated:

=================================================================
==36972==ERROR: AddressSanitizer: heap-use-after-free on address
0x60600007cd08 at pc 0x55c5cf683dda bp 0x7ffe185805f0 sp
0x7ffe185805e0
READ of size 4 at 0x60600007cd08 thread T0
    #0 0x55c5cf683dd9 in object_unref vim/src/vim9class.c:863
    #1 0x55c5cf5dad0c in clear_tv vim/src/typval.c:167
    #2 0x55c5ced6fdac in ex_let vim/src/evalvars.c:1162
    #3 0x55c5ced6d5f3 in ex_var vim/src/evalvars.c:956
    #4 0x55c5ceddb840 in do_one_cmd vim/src/ex_docmd.c:2582
    #5 0x55c5cedcec2d in do_cmdline vim/src/ex_docmd.c:994
    #6 0x55c5cf3876d8 in do_source_ext vim/src/scriptfile.c:1667
    #7 0x55c5cf383b0e in cmd_source vim/src/scriptfile.c:1146
    #8 0x55c5cf383e9d in ex_source vim/src/scriptfile.c:1189
    #9 0x55c5ceddb840 in do_one_cmd vim/src/ex_docmd.c:2582
    #10 0x55c5cedcec2d in do_cmdline vim/src/ex_docmd.c:994
    #11 0x55c5cf0f1145 in nv_colon vim/src/normal.c:3191
    #12 0x55c5cf0db1ec in normal_cmd vim/src/normal.c:939
    #13 0x55c5cf90d694 in main_loop vim/src/main.c:1536
    #14 0x55c5cf90ba86 in vim_main2 vim/src/main.c:888
    #15 0x55c5cf90aec9 in main vim/src/main.c:434
    #16 0x7f48456d0d8f in __libc_start_call_main
../sysdeps/nptl/libc_start_call_main.h:58
    #17 0x7f48456d0e3f in __libc_start_main_impl ../csu/libc-start.c:392
    #18 0x55c5ceb03764 in _start (vim/src/vim+0x1271764)

0x60600007cd08 is located 8 bytes inside of 56-byte region
[0x60600007cd00,0x60600007cd38)
freed by thread T0 here:
    #0 0x7f48462d5517 in __interceptor_free
../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:127
    #1 0x55c5ceb03f37 in vim_free vim/src/alloc.c:615
    #2 0x55c5cf683d2b in object_clear vim/src/vim9class.c:853
    #3 0x55c5cf683eb6 in object_unref vim/src/vim9class.c:864
    #4 0x55c5cf5dad0c in clear_tv vim/src/typval.c:167
    #5 0x55c5ced87d43 in set_var_const vim/src/evalvars.c:3971
    #6 0x55c5cece8269 in set_var_lval vim/src/eval.c:1677
    #7 0x55c5ced74bc2 in ex_let_one vim/src/evalvars.c:1868
    #8 0x55c5ced70071 in ex_let_vars vim/src/evalvars.c:1200
    #9 0x55c5ced6fd99 in ex_let vim/src/evalvars.c:1160
    #10 0x55c5ced6d5f3 in ex_var vim/src/evalvars.c:956
    #11 0x55c5ceddb840 in do_one_cmd vim/src/ex_docmd.c:2582
    #12 0x55c5cedcec2d in do_cmdline vim/src/ex_docmd.c:994
    #13 0x55c5cf3876d8 in do_source_ext vim/src/scriptfile.c:1667
    #14 0x55c5cf383b0e in cmd_source vim/src/scriptfile.c:1146
    #15 0x55c5cf383e9d in ex_source vim/src/scriptfile.c:1189
    #16 0x55c5ceddb840 in do_one_cmd vim/src/ex_docmd.c:2582
    #17 0x55c5cedcec2d in do_cmdline vim/src/ex_docmd.c:994
    #18 0x55c5cf0f1145 in nv_colon vim/src/normal.c:3191
    #19 0x55c5cf0db1ec in normal_cmd vim/src/normal.c:939
    #20 0x55c5cf90d694 in main_loop vim/src/main.c:1536
    #21 0x55c5cf90ba86 in vim_main2 vim/src/main.c:888
    #22 0x55c5cf90aec9 in main vim/src/main.c:434
    #23 0x7f48456d0d8f in __libc_start_call_main
../sysdeps/nptl/libc_start_call_main.h:58

previously allocated by thread T0 here:
    #0 0x7f48462d5867 in __interceptor_malloc
../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:145
    #1 0x55c5ceb03c1c in lalloc vim/src/alloc.c:246
    #2 0x55c5ceb03a83 in alloc_clear vim/src/alloc.c:177
    #3 0x55c5cf6f0522 in exec_instructions vim/src/vim9execute.c:3018
    #4 0x55c5cf7230e7 in call_def_function vim/src/vim9execute.c:5940
    #5 0x55c5cf64e23e in call_user_func vim/src/userfunc.c:2771
    #6 0x55c5cf6542a6 in call_user_func_check vim/src/userfunc.c:3189
    #7 0x55c5cf681f65 in class_object_index vim/src/vim9class.c:699
    #8 0x55c5ced152c3 in handle_subscript vim/src/eval.c:6793
    #9 0x55c5ced000c0 in eval9 vim/src/eval.c:4268
    #10 0x55c5cecfc72f in eval8 vim/src/eval.c:3798
    #11 0x55c5cecfad52 in eval7 vim/src/eval.c:3602
    #12 0x55c5cecf8d9b in eval6 vim/src/eval.c:3381
    #13 0x55c5cecf7d43 in eval5 vim/src/eval.c:3270
    #14 0x55c5cecf6705 in eval4 vim/src/eval.c:3121
    #15 0x55c5cecf5544 in eval3 vim/src/eval.c:2982
    #16 0x55c5cecf43ed in eval2 vim/src/eval.c:2856
    #17 0x55c5cecf2895 in eval1 vim/src/eval.c:2702
    #18 0x55c5cecf1f42 in eval0_retarg vim/src/eval.c:2613
    #19 0x55c5cecf1b19 in eval0 vim/src/eval.c:2547
    #20 0x55c5ced6f983 in ex_let vim/src/evalvars.c:1144
    #21 0x55c5ced6d5f3 in ex_var vim/src/evalvars.c:956
    #22 0x55c5ceddb840 in do_one_cmd vim/src/ex_docmd.c:2582
    #23 0x55c5cedcec2d in do_cmdline vim/src/ex_docmd.c:994
    #24 0x55c5cf3876d8 in do_source_ext vim/src/scriptfile.c:1667
    #25 0x55c5cf383b0e in cmd_source vim/src/scriptfile.c:1146
    #26 0x55c5cf383e9d in ex_source vim/src/scriptfile.c:1189
    #27 0x55c5ceddb840 in do_one_cmd vim/src/ex_docmd.c:2582
    #28 0x55c5cedcec2d in do_cmdline vim/src/ex_docmd.c:994
    #29 0x55c5cf0f1145 in nv_colon vim/src/normal.c:3191

SUMMARY: AddressSanitizer: heap-use-after-free vim/src/vim9class.c:863
in object_unref
Shadow bytes around the buggy address:
  0x0c0c80007950: fd fd fd fa fa fa fa fa fd fd fd fd fd fd fd fd
  0x0c0c80007960: fa fa fa fa fd fd fd fd fd fd fd fa fa fa fa fa
  0x0c0c80007970: fd fd fd fd fd fd fd fa fa fa fa fa 00 00 00 00
  0x0c0c80007980: 00 00 05 fa fa fa fa fa 00 00 00 00 00 00 00 fa
  0x0c0c80007990: fa fa fa fa 00 00 00 00 00 00 01 fa fa fa fa fa
=>0x0c0c800079a0: fd[fd]fd fd fd fd fd fa fa fa fa fa fd fd fd fd
  0x0c0c800079b0: fd fd fd fa fa fa fa fa 00 00 00 00 00 00 02 fa
  0x0c0c800079c0: fa fa fa fa fd fd fd fd fd fd fd fa fa fa fa fa
  0x0c0c800079d0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c0c800079e0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c0c800079f0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
  Shadow gap:              cc
==36972==ABORTING

- Yegappan

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