c++: Handle argument merging push_local_extern_decl_alias [PR123825]

The first testcase comes directly from
https://eel.is/c++draft/meta.reflection#names-1.8.2
and shows that we don't handle for -freflection the function
argument name difference handling isn't performed for local externs,
which doesn't go the duplicate_decls route but directly remembers
the alias.
The following patch handles outlines the DECL_ARGUMENTS handling
from duplicate_decls and uses it in push_local_extern_decl_alias
(with some minor differences and for now not propagating attributes
back for the push_local_extern_decl_alias case unless -freflection).

ALso, I found that the addition of a new alias created completely broken
DECL_ARGUMENTS (copied at most the first PARM_DECL, never more than that).
That is because copy_decl clears DECL_CHAIN, so the loop always stopped
after the first iteration.

2026-01-31  Jakub Jelinek  <jakub@redhat.com>

	PR c++/123825
	* cp-tree.h (merge_decl_arguments): Declare.
	* decl.cc (duplicate_decls): Outline DECL_ARGUMENTS handling
	into ...
	(merge_decl_arguments): ... new function.
	* name-lookup.cc (push_local_extern_decl_alias): Call
	merge_decl_arguments.  Don't copy just the first PARM_DECL when
	creating a new alias FUNCTION_DECL.

	* g++.dg/reflect/has_identifier3.C: New test.
	* g++.dg/reflect/identifier_of3.C: New test.
	* g++.dg/cpp26/attr-indeterminate5.C: New test.

0 files changed, 0 insertions, 0 deletions