TYPE-ERROR redefining class as structure

[Luke Gorrie]

I found a reproducible way to provoke a TYPE-ERROR from the compiler. First define a class and then compile (only) a structure definition with the same name.

$ echo "(defstruct x)" > x.lisp

$ sbcl --eval '(defclass x () ())' --eval '(compile-file "x.lisp")'
This is SBCL 2.1.2.nixos, an implementation of ANSI Common Lisp.
More information about SBCL is available at <http://www.sbcl.org/>.

SBCL is free software, provided as is, with absolutely no warranty.
It is mostly in the public domain; some portions are provided under
BSD-style licenses. See the CREDITS and COPYING files in the
distribution for more information.
; compiling file "/home/luke/x.lisp" (written 01 SEP 2021 12:10:08 PM):
; processing (DEFSTRUCT X)
; file: /home/luke/x.lisp
; in: DEFSTRUCT X
; (DEFSTRUCT X)
; --> PROGN EVAL-WHEN
; ==>
; (SB-KERNEL::%COMPILER-DEFSTRUCT
; '#<SB-KERNEL:DEFSTRUCT-DESCRIPTION X {10015B9AB3}>
; '#(#<SB-KERNEL:LAYOUT (ID=1) for T {50201003}>
; #<SB-KERNEL:LAYOUT (ID=2) for STRUCTURE-OBJECT {50201073}>))
;
; caught WARNING:
; change in superclasses of class #<SB-KERNEL:STRUCTURE-CLASSOID X>:
; The current superclasses: (T SB-PCL::SLOT-OBJECT STANDARD-OBJECT)
; the most recently loaded superclasses: (T STRUCTURE-OBJECT)

debugger invoked on a TYPE-ERROR in thread
#<THREAD "main thread" RUNNING {1001550103}>:
  The value
    NIL
  is not of type
    SB-KERNEL:LAYOUT

Type HELP for debugger help, or (SB-EXT:EXIT) to exit from SBCL.

restarts (invokable by number or by possibly-abbreviated name):
  0: [CONTINUE] Ignore runtime option --eval "(compile-file \"x.lisp\")".
  1: [ABORT ] Skip rest of --eval and --load options.
  2: Skip to toplevel READ/EVAL/PRINT loop.
  3: [EXIT ] Exit SBCL (calling #'EXIT, killing the process).

(SB-KERNEL:UNDECLARE-STRUCTURE #<SB-KERNEL:STRUCTURE-CLASSOID X> NIL)
0]

$ uname -a
Linux thinky 5.13.9 #1-NixOS SMP Sun Aug 8 07:06:57 UTC 2021 x86_64 GNU/Linux

$ sbcl --version
SBCL 2.1.2.nixos