c++: Fix auto return type deduction with expansion statements [PR121583]

c++: Fix auto return type deduction with expansion statements [PR121583]

The following testcase ICEs during expansion, because cfun->returns_struct
wasn't cleared, despite auto being deduced to int.

The problem is that check_return_type -> apply_deduced_return_type
is called when parsing the expansion stmt body, at that time
processing_template_decl is non-zero and apply_deduced_return_type
in that case doesn't do the
     if (function *fun = DECL_STRUCT_FUNCTION (fco))
       {
         bool aggr = aggregate_value_p (result, fco);
 #ifdef PCC_STATIC_STRUCT_RETURN
         fun->returns_pcc_struct = aggr;
 #endif
         fun->returns_struct = aggr;
       }
My assumption is that !processing_template_decl in that case
is used in the sense "the fco function is not a function template",
for function templates no reason to bother with fun->returns*struct,
nothing will care about that.
When returning a type dependent expression in the expansion stmt
body, apply_deduced_return_type just won't be called during parsing,
but when instantiating the body and all will be fine.  But when
returning a non-type-dependent expression, while check_return_type
will be called again during instantiation of the body, as the return
type is no longer auto in that case apply_deduced_return_type will not
be called again and so nothing will fix up fun->returns*struct.

The following patch fixes that by using !uses_template_parms (fco)
check instead of !processing_template_decl.

2025-08-28  Jakub Jelinek  <jakub@redhat.com>

	PR c++/121583
	* semantics.cc (apply_deduced_return_type): Adjust
	fun->returns*_struct when !uses_template_parms (fco) instead of
	when !processing_template_decl.

	* g++.dg/cpp26/expansion-stmt23.C: New test.
	* g++.dg/cpp26/expansion-stmt24.C: New test.