https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105149
--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
The C++ frontend accepts this and diagnoses
In file included from t.c:1:
t.c: In function 'void foo(int, ...)':
t.c:6:15: warning: 'D()' is promoted to 'D (*)()' when passed through '...'
6 | va_arg (ap, D()) ();
and produces the following GENERIC, decaying the "loaded function":
<call_expr 0x7ffff6692c00
type <record_type 0x7ffff6681348 D cxx-odr-p type_5 QI
size <integer_cst 0x7ffff653a048 constant 8>
unit-size <integer_cst 0x7ffff653a060 constant 1>
align:8 warn_if_not_align:0 symtab:0 alias-set -1 canonical-type
0x7ffff6681348
fields <function_decl 0x7ffff6685600 __dt type <method_type
0x7ffff6698000>
public abstract external autoinline decl_3 QI t.c:2:19 align:16
warn_if_not_align:0 context <record_type 0x7ffff6681348 D>
full-name "D::~D() noexcept (<uninstantiated>)"
not-really-extern chain <function_decl 0x7ffff6685800 __dt_base >>
context <translation_unit_decl 0x7ffff6525168 t.c>
full-name "struct D"
X() X(constX&) this=(X&) n_parents=0 use_template=0 interface-unknown
pointer_to_this <pointer_type 0x7ffff66817e0> chain <type_decl
0x7ffff6547980 ._anon_0>>
side-effects
fn <addr_expr 0x7ffff6678980
type <pointer_type 0x7ffff6681690 type <function_type 0x7ffff66815e8>
unsigned DI
size <integer_cst 0x7ffff6517f48 constant 64>
unit-size <integer_cst 0x7ffff6517f60 constant 8>
align:64 warn_if_not_align:0 symtab:0 alias-set -1 canonical-type
0x7ffff6681690>
side-effects
arg:0 <va_arg_expr 0x7ffff6678960 type <function_type 0x7ffff66815e8>
side-effects
arg:0 <addr_expr 0x7ffff6678a00 type <pointer_type 0x7ffff6544dc8>
arg:0 <var_decl 0x7ffff6526c60 ap>>
t.c:6:3 start: t.c:6:3 finish: t.c:6:3>
t.c:6:3 start: t.c:6:3 finish: t.c:6:3>
t.c:6:3 start: t.c:6:3 finish: t.c:6:3>
which we then gimplify to
__builtin_trap ();
<<< Unknown tree: integer_cst >>> ();
