[Bug c++/125555] [16/17 Regression] error: mangling of ... conflicts with a previous mangle [import std]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125555

--- Comment #6 from Leander Schulten <leander.schulten at tetys dot de> ---
I asked claude back then when creating the issue and it came up with 

diff --git a/gcc/cp/module.cc b/gcc/cp/module.cc
index d81db520bab83..1329c6e646b43 100644
--- a/gcc/cp/module.cc
+++ b/gcc/cp/module.cc
@@ -15449,7 +15449,18 @@ depset::hash::add_specializations (bool decl_p)
              else if (is_friend)
                {
                  if (TI_TEMPLATE (ti) != entry->tmpl
-                     || !template_args_equal (TI_ARGS (ti), entry->tmpl))
+                     && template_args_equal (TI_ARGS (ti), entry->args))
+                   /* SPEC is an implicit instantiation of a hidden friend
+                      template.  It is keyed in the specialization table to
+                      the most general template (ENTRY->TMPL), but it was
+                      produced from the per-class pseudo-instantiation of the
+                      friend (TI_TEMPLATE).  This is a genuine specialization
+                      that importers must be able to merge with equivalent
+                      instantiations of their own, so record it as an ordinary
+                      specialization rather than dropping it as a friend clone
+                      (PR c++/125552).  */
+                   is_friend = false;
+                 else
                    goto template_friend;
                }
            }
with tests and commit msg:
https://github.com/autoantwort/gcc/commit/44074a193ddc506296b47c1fc1d05d772f8ff871