[gcc r16-9532] c++/modules: ICE with GMF variable later defined inline [PR126783]

https://gcc.gnu.org/g:61f16b5960e4a53d7ac5881f4db74ccfb7cb6df7

commit r16-9532-g61f16b5960e4a53d7ac5881f4db74ccfb7cb6df7
Author: Patrick Palka <[email protected]>
Date:   Thu Aug 13 18:07:25 2026 -0400

    c++/modules: ICE with GMF variable later defined inline [PR126783]
    
    Here transfer_defining_module (added in r16-5213) assumes that if
    a redeclaration has DECL_LANG_SPECIFIC allocated, then so must the
    original declaration, but this isn't true for q below.  Instead we
    should call retrofit_lang_decl to allocate DECL_LANG_SPECIFIC as
    needed; other parts of duplicate_decls do the same.
    
            PR c++/126783
    
    gcc/cp/ChangeLog:
    
            * module.cc (transfer_defining_module): Call retrofit_lang_decl
            instead of assuming the old declaration already has
            DECL_LANG_SPECIFIC allocated.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/modules/gmf-6.C: New test.
    
    Reviewed-by: Jason Merrill <[email protected]>
    (cherry picked from commit a03b3ab4b0a6a446fe9a1d7e6e82967a8dd3edcf)

Diff:
---
 gcc/cp/module.cc                     | 2 +-
 gcc/testsuite/g++.dg/modules/gmf-6.C | 7 +++++++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/module.cc b/gcc/cp/module.cc
index 83d3929415cb..b7c1eae2df61 100644
--- a/gcc/cp/module.cc
+++ b/gcc/cp/module.cc
@@ -22399,7 +22399,7 @@ transfer_defining_module (tree olddecl, tree newdecl)
 
   if (DECL_LANG_SPECIFIC (new_inner))
     {
-      gcc_checking_assert (DECL_LANG_SPECIFIC (old_inner));
+      retrofit_lang_decl (old_inner);
       if (DECL_MODULE_PURVIEW_P (new_inner))
 	DECL_MODULE_PURVIEW_P (old_inner) = true;
       if (!DECL_MODULE_IMPORT_P (new_inner))
diff --git a/gcc/testsuite/g++.dg/modules/gmf-6.C b/gcc/testsuite/g++.dg/modules/gmf-6.C
new file mode 100644
index 000000000000..201650917a32
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/gmf-6.C
@@ -0,0 +1,7 @@
+// PR c++/126783
+// { dg-additional-options "-fmodules -Wno-global-module" }
+// { dg-module-cmi M }
+module;
+extern int const q;
+inline constexpr int q = 1;
+export module M;