[gcc r16-7204] c++: defer DECL_ONE_ONLY vs consteval-only [PR122785]

[Jason Merrill]

https://gcc.gnu.org/g:35b17c7713d4bd7a675ee8908724325e957d1482

commit r16-7204-g35b17c7713d4bd7a675ee8908724325e957d1482
Author: Jason Merrill <jason@redhat.com>
Date:   Wed Jan 28 14:17:13 2026 +0800

    c++: defer DECL_ONE_ONLY vs consteval-only [PR122785]
    
    The failure in the given PR occurs because when setting up an
    imported vague-linkage variable, we currently call 'maybe_commonize_var'
    which for -freflection checks 'consteval_only_p'.  Unfortunately this
    latter function needs to call 'complete_type_p' which can perform
    recursive loading of the (possibly yet-to-be-streamed) class type,
    breaking modules assumptions.
    
    If we just remove the consteval_only_p early exit from maybe_commonize_var,
    we end up crashing at EOF while trying to mangle its comdat group, so we
    need to undo maybe_commonize_var along with setting DECL_EXTERN.
    
            PR c++/122785
    
    gcc/cp/ChangeLog:
    
            * decl.cc (maybe_commonize_var): Don't check consteval_only_p.
            (make_rtl_for_nonlocal_decl): Undo make_decl_one_only for
            consteval-only variables.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/modules/reflect-1_a.H: New test.
            * g++.dg/modules/reflect-1_b.C: New test.
    
    Co-authored-by: Nathaniel Shead <nathanieloshead@gmail.com>

Diff:
---
 gcc/cp/decl.cc                             | 11 +++++++----
 gcc/testsuite/g++.dg/modules/reflect-1_b.C |  6 ++++++
 gcc/testsuite/g++.dg/modules/reflect-1_a.H | 11 +++++++++++
 3 files changed, 24 insertions(+), 4 deletions(-)

diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index 270158510dff..c24bbc889ed7 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -7263,10 +7263,6 @@ maybe_commonize_var (tree decl)
   if (DECL_ARTIFICIAL (decl) && !DECL_DECOMPOSITION_P (decl))
     return;
 
-  /* These are not output at all.  */
-  if (consteval_only_p (decl))
-    return;
-
   /* Static data in a function with comdat linkage also has comdat
      linkage.  */
   if ((TREE_STATIC (decl)
@@ -8758,6 +8754,13 @@ make_rtl_for_nonlocal_decl (tree decl, tree init, const char* asmspec)
     {
       /* Disable assemble_variable.  */
       DECL_EXTERNAL (decl) = true;
+      /* Undo make_decl_one_only.  */
+      if (DECL_COMDAT_GROUP (decl))
+	{
+	  symtab_node *node = symtab_node::get (decl);
+	  node->set_comdat_group (NULL);
+	  node->dissolve_same_comdat_group_list ();
+	}
       return;
     }
 
diff --git a/gcc/testsuite/g++.dg/modules/reflect-1_b.C b/gcc/testsuite/g++.dg/modules/reflect-1_b.C
new file mode 100644
index 000000000000..a3c5b67d09af
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/reflect-1_b.C
@@ -0,0 +1,6 @@
+// PR c++/122785
+// { dg-do compile { target c++26 } }
+// { dg-additional-options "-fmodules -freflection" }
+
+import "reflect-1_a.H";
+using ::S;
diff --git a/gcc/testsuite/g++.dg/modules/reflect-1_a.H b/gcc/testsuite/g++.dg/modules/reflect-1_a.H
new file mode 100644
index 000000000000..025f2e27ea1a
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/reflect-1_a.H
@@ -0,0 +1,11 @@
+// PR c++/122785
+// { dg-do compile { target c++26 } }
+// { dg-additional-options "-fmodule-header -freflection" }
+// { dg-module-cmi {} }
+
+struct S {
+  friend S foo();
+  S bar(int);
+};
+inline S s;
+template <typename T> decltype(s.bar(T{})) foo(T);