On Thu, Oct 30, 2025 at 07:20:22PM +0200, Jason Merrill wrote:
> On 10/28/25 12:06 AM, Nathaniel Shead wrote:
> > 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.
>
> Makes sense, but how is that different from a variable template
> specialization that we've instantiated the declaration of, but not the
> definition, e.g.
>
> template <class T> T v = T();
> decltype(v<int>) v2 = 0;
>
> ?
In 'instantiate_decl' we have the following hunk
set_instantiating_module (d);
if (variable_template_p (gen_tmpl))
note_vague_linkage_variable (d);
instantiate_body (td, args, d, false);
A variable template will then always have 'note_vague_linkage_variable'
called once its body needs to be instantiated, but 'arr' in my example
does not because PRIMARY_TEMPLATE_P doesn't hold. In a non-modules
scenario this is fine because we'll have already done this in
finish_static_data_member_decl, but not in this case.
> > 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:} } }
> > >
> >
>
