https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105149
--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #5)
> 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":
include/stdarg.h:49:44: note: in definition of macro 'va_arg'
49 | #define va_arg(v,l) __builtin_va_arg(v,l)
| ^
t.c:6:15: note: (so you should pass 'D (*)()' not 'D()' to 'va_arg')
6 | va_arg (ap, D()) ();
include/stdarg.h:49:44: note: in definition of macro 'va_arg'
49 | #define va_arg(v,l) __builtin_va_arg(v,l)
| ^
t.c:6:15: note: if this code is reached, the program will abort
6 | va_arg (ap, D()) ();
include/stdarg.h:49:44: note: in definition of macro 'va_arg'
49 | #define va_arg(v,l) __builtin_va_arg(v,l)
| ^
because the C++ implementation of lang_hooks.types.type_promotes_to
says the function type doesn't promote to itself. The C frontend
seems to lack that.
>
>
> <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 >>> ();
