On Mon, Oct 27, 2025 at 05:04:18PM +0200, Jason Merrill wrote:
> On 10/26/25 2:40 PM, Nathaniel Shead wrote:
> > Bootstrapped and regtested on x86_64-pc-linux-gnu, OK for trunk/15?
> >
> > -- >8 --
> >
> > The linker error in the PR is caused because when a static is defined
> > out of the class body, it doesn't yet have a definition and so
> > read_var_def (which would otherwise have noted it) never gets called.
>
> I don't object to this change, but there's certainly a definition in
> inst6_a, why wouldn't we call read_var_def? I imagine it's because we never
> write_var_def for some reason?
>
Right. We've instantiated Type<int> so a declaration exists for
Type<int>::var, but because of this hunk in instantiate_class_template
no definition has been instantiated yet, so DECL_INITIAL is null, and so
has_definition returns false and we never call write_var_def.
if (VAR_P (r))
{
/* In [temp.inst]:
[t]he initialization (and any associated
side-effects) of a static data member does
not occur unless the static data member is
itself used in a way that requires the
definition of the static data member to
exist.
Therefore, we do not substitute into the
initialized for the static data member here. */
finish_static_data_member_decl
(r,
/*init=*/NULL_TREE,
/*init_const_expr_p=*/false,
/*asmspec_tree=*/NULL_TREE,
/*flags=*/0);
/* Instantiate members marked with attribute used. */
if (r != error_mark_node && DECL_PRESERVE_P (r))
used.safe_push (r);
}
Nathaniel
> > This instead moves the responsibility for noting class-scope variables
> > to read_class_def.
> >
> > PR c++/122421
> >
> > gcc/cp/ChangeLog:
> >
> > * module.cc (trees_in::read_var_def): Don't handle class-scope
> > variables anymore.
> > (trees_in::read_class_def): Handle them here instead.
> >
> > gcc/testsuite/ChangeLog:
> >
> > * g++.dg/modules/inst-6_a.C: New test.
> > * g++.dg/modules/inst-6_b.C: New test.
> >
> > Signed-off-by: Nathaniel Shead <[email protected]>
> > ---
> > gcc/cp/module.cc | 15 +++++++++------
> > gcc/testsuite/g++.dg/modules/inst-6_a.C | 14 ++++++++++++++
> > gcc/testsuite/g++.dg/modules/inst-6_b.C | 12 ++++++++++++
> > 3 files changed, 35 insertions(+), 6 deletions(-)
> > create mode 100644 gcc/testsuite/g++.dg/modules/inst-6_a.C
> > create mode 100644 gcc/testsuite/g++.dg/modules/inst-6_b.C
> >
> > diff --git a/gcc/cp/module.cc b/gcc/cp/module.cc
> > index ed0d69cead4..048aec598dd 100644
> > --- a/gcc/cp/module.cc
> > +++ b/gcc/cp/module.cc
> > @@ -13059,12 +13059,11 @@ trees_in::read_var_def (tree decl, tree
> > maybe_template)
> > if (DECL_EXPLICIT_INSTANTIATION (decl)
> > && !DECL_EXTERNAL (decl))
> > setup_explicit_instantiation_definition_linkage (decl);
> > - if (DECL_IMPLICIT_INSTANTIATION (decl)
> > - || (DECL_EXPLICIT_INSTANTIATION (decl)
> > - && !DECL_EXTERNAL (decl))
> > - || (DECL_CLASS_SCOPE_P (decl)
> > - && !DECL_VTABLE_OR_VTT_P (decl)
> > - && !DECL_TEMPLATE_INFO (decl)))
> > + /* Class static data members are handled in read_class_def. */
> > + if (!DECL_CLASS_SCOPE_P (decl)
> > + && (DECL_IMPLICIT_INSTANTIATION (decl)
> > + || (DECL_EXPLICIT_INSTANTIATION (decl)
> > + && !DECL_EXTERNAL (decl))))
> > note_vague_linkage_variable (decl);
> > }
> > if (!dyn_init)
> > @@ -13478,6 +13477,10 @@ trees_in::read_class_def (tree defn, tree
> > maybe_template)
> > DECL_ACCESS (d) = tree_cons (type, access, list);
> > }
> > }
> > +
> > + if (TREE_CODE (decl) == VAR_DECL
> > + && TREE_CODE (maybe_template) != TEMPLATE_DECL)
> > + note_vague_linkage_variable (decl);
> > }
> > }
> > diff --git a/gcc/testsuite/g++.dg/modules/inst-6_a.C
> > b/gcc/testsuite/g++.dg/modules/inst-6_a.C
> > new file mode 100644
> > index 00000000000..7f35cc161bf
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/modules/inst-6_a.C
> > @@ -0,0 +1,14 @@
> > +// PR c++/122421
> > +// { dg-additional-options "-fmodules" }
> > +// { dg-module-cmi M }
> > +
> > +export module M;
> > +
> > +export template <typename T> struct Type {
> > + static const int arr[3];
> > +};
> > +
> > +extern template const int Type<double>::arr[3];
> > +template <typename T> const int Type<T>::arr[] = { 42, 43, 44 };
> > +
> > +export Type<int> ti;
> > diff --git a/gcc/testsuite/g++.dg/modules/inst-6_b.C
> > b/gcc/testsuite/g++.dg/modules/inst-6_b.C
> > new file mode 100644
> > index 00000000000..5a8092ccb14
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/modules/inst-6_b.C
> > @@ -0,0 +1,12 @@
> > +// PR c++/122421
> > +// { dg-additional-options "-fmodules" }
> > +
> > +import M;
> > +
> > +int main() {
> > + const int& a = Type<int>::arr[0];
> > + const int& b = Type<double>::arr[0];
> > +}
> > +
> > +// { dg-final { scan-assembler {_ZNW1M4TypeIiE3arrE:} } }
> > +// { dg-final { scan-assembler-not {_ZNW1M4TypeIdE3arrE:} } }
>
