test$ nasm -v
NASM version 2.16.02rc2 compiled on Oct 12 2023
test$ cat test.asm
%assign slot 26
%push

%ifdef INIT
%define %$entry26
%define entry26
%endif

%define foo bar
%ifdef A
 %define %$entry%[slot] foo
%elifdef B
 %xdefine %$entry%[slot] foo
%elifdef C
 %define entry%[slot] foo
%elifdef D
 %xdefine entry%[slot] foo
%endif
%define foo qux
%fatal %$entry26 entry26
test$ nasm test.asm -DA
test.asm:20: fatal: qux entry26
test$ nasm test.asm -DB
test.asm:20: fatal: bar entry26
test$ nasm test.asm -DC
test.asm:20: fatal: %$entry26 qux
test$ nasm test.asm -DD
test.asm:20: fatal: %$entry26 bar
test$ nasm test.asm -DINIT -DA
test.asm:11: error: `%define' expects a macro identifier
test.asm:20: fatal:
test$ nasm test.asm -DINIT -DB
test.asm:13: error: `%xdefine' expects a macro identifier
test.asm:20: fatal:
test$ nasm test.asm -DINIT -DC
test.asm:20: fatal: qux
test$ nasm test.asm -DINIT -DD
test.asm:20: fatal: bar
test$