Re: [PATCH] c++/modules: Provide definitions of synthesized methods outside their defining module [PR120499]

[Jason Merrill]

On 8/22/25 2:02 AM, Nathaniel Shead wrote:

Bootstrapped and regtested on x86_64-pc-linux-gnu, OK for trunk?

OK.

-- >8 --

In the PR, we're getting a linker error from _Vector_impl's destructor
never getting emitted.  This is because of a combination of factors:

1. in imp-member-4_a, the destructor is not used and so there is no
    definition generated.

2. in imp-member-4_b, the destructor gets synthesized (as part of the
    synthesis for Coll's destructor) but is not ODR-used and so does not
    get emitted.  Despite there being a definition provided in this TU,
    the destructor is still considered imported and so isn't streamed
    into the module body.

3. in imp-member-4_c, we need to ODR-use the destructor but we only got
    a forward declaration from imp-member-4_b, so we cannot emit a body.

The point of failure here is step 2; this function has effectively been
declared in the imp-member-4_b module, and so we shouldn't treat it as
imported.  This way we'll properly stream the body so that importers can
emit it.

        PR c++/120499

gcc/cp/ChangeLog:

        * method.cc (synthesize_method): Set the instantiating module.

gcc/testsuite/ChangeLog:

        * g++.dg/modules/imp-member-4_a.C: New test.
        * g++.dg/modules/imp-member-4_b.C: New test.
        * g++.dg/modules/imp-member-4_c.C: New test.

Signed-off-by: Nathaniel Shead <nathanielosh...@gmail.com>
---
  gcc/cp/method.cc                              |  3 +++
  gcc/testsuite/g++.dg/modules/imp-member-4_a.C | 25 +++++++++++++++++++
  gcc/testsuite/g++.dg/modules/imp-member-4_b.C | 17 +++++++++++++
  gcc/testsuite/g++.dg/modules/imp-member-4_c.C | 14 +++++++++++
  4 files changed, 59 insertions(+)
  create mode 100644 gcc/testsuite/g++.dg/modules/imp-member-4_a.C
  create mode 100644 gcc/testsuite/g++.dg/modules/imp-member-4_b.C
  create mode 100644 gcc/testsuite/g++.dg/modules/imp-member-4_c.C

diff --git a/gcc/cp/method.cc b/gcc/cp/method.cc
index 397e496ed47..ef8370fac5b 100644
--- a/gcc/cp/method.cc
+++ b/gcc/cp/method.cc
@@ -1851,6 +1851,9 @@ synthesize_method (tree fndecl)
    finish_function_body (stmt);
    finish_function (/*inline_p=*/false);

+ /* Remember that we were defined in this module. */

+  set_instantiating_module (fndecl);
+
    if (!DECL_DELETED_FN (fndecl))
      expand_or_defer_fn (fndecl);

diff --git a/gcc/testsuite/g++.dg/modules/imp-member-4_a.C b/gcc/testsuite/g++.dg/modules/imp-member-4_a.C

new file mode 100644
index 00000000000..f0699eb314c
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/imp-member-4_a.C
@@ -0,0 +1,25 @@
+// PR c++/120499
+// { dg-additional-options "-fmodules -fdump-lang-module-blocks" }
+// { dg-module-cmi A }
+
+export module A;
+
+struct allocator {
+  ~allocator() {}
+};
+
+export template <typename _Tp>
+struct vector {
+  struct _Vector_impl : public allocator {};
+  _Vector_impl _M_impl;
+  vector() = default;
+};
+
+template <typename T>
+struct regex_token_iterator {
+  vector<int> _M_subs;
+};
+template struct regex_token_iterator<const char*>;
+
+// No definition of _Vector_impl::~_Vector_impl here (not synthesized)
+// { dg-final { scan-lang-dump-not 
{'::vector@A:1<int>::_Vector_impl@A:1<int>::__dt '} module } }
diff --git a/gcc/testsuite/g++.dg/modules/imp-member-4_b.C 
b/gcc/testsuite/g++.dg/modules/imp-member-4_b.C
new file mode 100644
index 00000000000..db485095e77
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/imp-member-4_b.C
@@ -0,0 +1,17 @@
+// PR c++/120499
+// { dg-additional-options "-fmodules -fdump-lang-module-blocks" }
+// { dg-module-cmi B }
+
+export module B;
+import A;
+
+struct Coll {
+  vector<int> vals;
+};
+
+export Coll createColl() {
+  return Coll{};
+}
+
+// But the definition of _Vector_impl::~_Vector_impl has been synthesized here
+// { dg-final { scan-lang-dump-times {\[0\]=decl definition 
'::vector@A:1<int>::_Vector_impl@A:1<int>::__dt '} 1 module } }
diff --git a/gcc/testsuite/g++.dg/modules/imp-member-4_c.C 
b/gcc/testsuite/g++.dg/modules/imp-member-4_c.C
new file mode 100644
index 00000000000..d405ce28bf7
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/imp-member-4_c.C
@@ -0,0 +1,14 @@
+// PR c++/120499
+// { dg-module-do link }
+// { dg-additional-options "-fmodules -fdump-lang-module" }
+
+import B;
+
+int main() {
+  createColl();
+}
+
+// And we should use the definition of _Vector_impl::~_Vector_impl now.
+// In this case we got the seeded import from A of the declaration
+// so let's just ensure we stream the definition.
+// { dg-final { scan-lang-dump {Reading function definition 
'::vector@A:1<int>::_Vector_impl@A:1<int>::__dt @A:1'} module } }