gcc 15 "array subscript 0" warning at level -O3

Whilst poking at Erik Rijkers' nearby report, I found that
Fedora 42's gcc 15.0.1 will produce this complaint if you
select optimization level -O3:

In file included from ../../../../src/include/access/htup_details.h:22,
from pl_exec.c:21:
In function 'assign_simple_var',
inlined from 'exec_set_found' at pl_exec.c:8609:2:
../../../../src/include/varatt.h:230:36: warning: array subscript 0 is outside array bounds of 'char[0]' [-Warray-bounds=]
230 | (((varattrib_1b_e *) (PTR))->va_tag)
| ^
../../../../src/include/varatt.h:94:12: note: in definition of macro 'VARTAG_IS_EXPANDED'
94 | (((tag) & ~1) == VARTAG_EXPANDED_RO)
| ^~~
../../../../src/include/varatt.h:284:57: note: in expansion of macro 'VARTAG_1B_E'
284 | #define VARTAG_EXTERNAL(PTR) VARTAG_1B_E(PTR)
| ^~~~~~~~~~~
../../../../src/include/varatt.h:301:57: note: in expansion of macro 'VARTAG_EXTERNAL'
301 | (VARATT_IS_EXTERNAL(PTR) && !VARTAG_IS_EXPANDED(VARTAG_EXTERNAL(PTR)))
| ^~~~~~~~~~~~~~~
pl_exec.c:8797:17: note: in expansion of macro 'VARATT_IS_EXTERNAL_NON_EXPANDED'
8797 | VARATT_IS_EXTERNAL_NON_EXPANDED(DatumGetPointer(newvalue)))
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In function 'exec_set_found':
cc1: note: source object is likely at address zero

Buildfarm member serinus has been producing the identical warning for
some time. I'd been ignoring that because it runs "experimental gcc",
but I guess the experiment has leaked out to production distros.

What seems to be happening here is that after inlining
assign_simple_var into exec_set_found, the compiler decides that
"newvalue" might be zero (since it's a BoolGetDatum result),
and then it warns -- in a rather strange way -- about the
potential null dereference. The dereference is not reachable
because of the preceding "var->datatype->typlen == -1" check,
but that's not stopping the optimizer from bitching.

I experimented with modifying exec_set_found thus:

var = (PLpgSQL_var *) (estate->datums[estate->found_varno]);
+ Assert(var->datatype->typlen == 1);
assign_simple_var(estate, var, BoolGetDatum(state), false, false);

which should be OK since we're expecting the "found" variable to
be boolean. That does silence the warning, but of course only
in --enable-cassert builds.

Anybody have an idea about how to silence it more effectively?
There are going to be more people seeing this as gcc 15 propagates.

regards, tom lane