[PATCH v3] c++: Fix handling of no-linkage decls for modules

[Nathaniel Shead <nathanieloshead@gmail.com>]

On Mon, Mar 11, 2024 at 02:13:34PM -0400, Jason Merrill wrote:
> On 3/8/24 18:18, Nathaniel Shead wrote:
> > On Fri, Mar 08, 2024 at 10:19:52AM -0500, Jason Merrill wrote:
> > > On 3/7/24 21:55, Nathaniel Shead wrote:
> > > > On Mon, Nov 27, 2023 at 03:59:39PM +1100, Nathaniel Shead wrote:
> > > > > On Thu, Nov 23, 2023 at 03:03:37PM -0500, Nathan Sidwell wrote:
> > > > > > On 11/20/23 04:47, Nathaniel Shead wrote:
> > > > > > > Bootstrapped and regtested on x86_64-pc-linux-gnu. I don't have write
> > > > > > > access.
> > > > > > > 
> > > > > > > -- >8 --
> > > > > > > 
> > > > > > > Block-scope declarations of functions or extern values are not allowed
> > > > > > > when attached to a named module. Similarly, class member functions are
> > > > > > > not inline if attached to a named module. However, in both these cases
> > > > > > > we currently only check if the declaration is within the module purview;
> > > > > > > it is possible for such a declaration to occur within the module purview
> > > > > > > but not be attached to a named module (e.g. in an 'extern "C++"' block).
> > > > > > > This patch makes the required adjustments.
> > > > > > 
> > > > > > 
> > > > > > Ah I'd been puzzling over the default inlinedness of  member-fns of
> > > > > > block-scope structs.  Could you augment the testcase to make sure that's
> > > > > > right too?
> > > > > > 
> > > > > > Something like:
> > > > > > 
> > > > > > // dg-module-do link
> > > > > > export module Mod;
> > > > > > 
> > > > > > export auto Get () {
> > > > > >     struct X { void Fn () {} };
> > > > > >     return X();
> > > > > > }
> > > > > > 
> > > > > > 
> > > > > > ///
> > > > > > import Mod
> > > > > > void Frob () { Get().Fn(); }
> > > > > > 
> > > > > 
> > > > > I gave this a try and it indeed doesn't work correctly; 'Fn' needs to be
> > > > > marked 'inline' for this to link (whether or not 'Get' itself is
> > > > > inline). I've tried tracing the code to work out what's going on but
> > > > > I've been struggling to work out how all the different flags (e.g.
> > > > > TREE_PUBLIC, TREE_EXTERNAL, TREE_COMDAT, DECL_NOT_REALLY_EXTERN)
> > > > > interact, which flags we want to be set where, and where the decision of
> > > > > what function definitions to emit is actually made.
> > > > > 
> > > > > I did find that doing 'mark_used(decl)' on all member functions in
> > > > > block-scope structs seems to work however, but I wonder if that's maybe
> > > > > too aggressive or if there's something else we should be doing?
> > > > 
> > > > I got around to looking at this again, here's an updated version of this
> > > > patch. Bootstrapped and regtested on x86_64-pc-linux-gnu, OK for trunk?
> > > > 
> > > > (I'm not sure if 'start_preparsed_function' is the right place to be
> > > > putting this kind of logic or if it should instead be going in
> > > > 'grokfndecl', e.g. decl.cc:10761 where the rules for making local
> > > > functions have no linkage are initially determined, but I found this
> > > > easier to implement: happy to move around though if preferred.)
> > > > 
> > > > -- >8 --
> > > > 
> > > > Block-scope declarations of functions or extern values are not allowed
> > > > when attached to a named module. Similarly, class member functions are
> > > > not inline if attached to a named module. However, in both these cases
> > > > we currently only check if the declaration is within the module purview;
> > > > it is possible for such a declaration to occur within the module purview
> > > > but not be attached to a named module (e.g. in an 'extern "C++"' block).
> > > > This patch makes the required adjustments.
> > > > 
> > > > While implementing this we discovered that block-scope local functions
> > > > are not correctly emitted, causing link failures; this patch also
> > > > corrects some assumptions here and ensures that they are emitted when
> > > > needed.
> > > > 
> > > > 	PR c++/112631
> > > > 
> > > > gcc/cp/ChangeLog:
> > > > 
> > > > 	* cp-tree.h (named_module_attach_p): New function.
> > > > 	* decl.cc (start_decl): Check for attachment not purview.
> > > > 	(grokmethod): Likewise.
> > > 
> > > These changes are OK; the others I want to consider more.
> > 
> > Thanks, I can commit this as a separate commit if you prefer?
> 
> Please.
> 

Thanks, committed as r14-9501-gead3075406ece9.

> > > > +export auto n_n() {
> > > > +  internal();
> > > > +  struct X { void f() { internal(); } };
> > > > +  return X{};
> > > 
> > > Hmm, is this not a prohibited exposure?  Seems like X has no linkage because
> > > it's at block scope, and the deduced return type names it.
> > > 
> > > I know we try to support this "voldemort" pattern, but is that actually
> > > correct?
> > 
> > I had similar doubts, but this is not an especially uncommon pattern in
> > the wild either. I also asked some other people for their thoughts and
> > got told:
> > 
> >    "no linkage" doesn't mean it's ill-formed to name it in other scopes.
> >    It means a declaration in another scope cannot correspond to it
> > 
> > And that the wording in [basic.link] p2.4 is imprecise. (Apparently they
> > were going to raise a core issue about this too, I think?)
> > 
> > As for whether it's an exposure, looking at [basic.link] p15, the entity
> > 'X' doesn't actually appear to be TU-local: it doesn't have a name with
> > internal linkage (no linkage is different) and is not declared or
> > introduced within the definition of a TU-local entity (n_n is not
> > TU-local).
> 
> Hmm, I think you're right.  And this rule:
> 
> > -    /* DR 757: A type without linkage shall not be used as the type of a
> > -       variable or function with linkage, unless
> > -       o the variable or function has extern "C" linkage (7.5 [dcl.link]), or
> > -       o the variable or function is not used (3.2 [basic.def.odr]) or is
> > -       defined in the same translation unit.
> 
> is no longer part of the standard since C++20; the remnant of this rule is
> the example in
> 
> https://eel.is/c++draft/basic#def.odr-11
> 
> > auto f() {
> >   struct A {};
> >   return A{};
> > }
> > decltype(f()) g();
> > auto x = g();
> 
> > A program containing this translation unit is ill-formed because g is odr-used but not defined, and cannot be defined in any other translation unit because the local class A cannot be named outside this translation unit.
> 
> But g could be defined in another translation unit if f is inline or in a
> module interface unit.
> 
> So, I think no_linkage_check needs to consider module_has_cmi_p as well as
> vague_linkage_p for relaxed mode.  And in no_linkage_error if
> no_linkage_check returns null in relaxed mode, reduce the permerror to a
> pedwarn before C++20, and no diagnostic at all in C++20 and above.
> 

Thanks for the pointers, I've implemented this below.

> > +      if (ctx != NULL_TREE && TREE_PUBLIC (ctx) && module_has_cmi_p ())
> > +	{
> > +	  /* Ensure that functions in local classes within named modules
> > +	     have their definitions exported, in case they are (directly
> > +	     or indirectly) used by an importer.  */
> > +	  TREE_PUBLIC (decl1) = true;
> > +	  if (DECL_DECLARED_INLINE_P (decl1))
> > +	    comdat_linkage (decl1);
> > +	  else
> > +	    mark_needed (decl1);
> > +	}
> 
> Isn't the inline case handled by the comdat_linkage just above?
> 
> Jason
> 

It wasn't, because 'TREE_PUBLIC (decl1)' wasn't yet set. But this means
that it's ended up a lot cleaner to do this in grokfndecl instead then,
which (along with the no_linkage_check changes) means I didn't actually
need to change this code at all.

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

-- >8 --

When testing the changes for PR c++/112631 we discovered that currently
we don't emit definitions of block-scope function declarations if
they're not used in the module interface TU, which causes issues if they
are used by importers.

This patch fixes the handling of no-linkage declarations for C++20. In
particular, a type declared in a function with vague linkage or declared
in a module CMI could potentially be accessible outside its defining TU,
and as such we can't assume that function declarations using that type
can never be defined in another TU.

A complication with handling this is that we're only strictly interested
in declarations with a module CMI, but when parsing the global module
fragment we don't yet know whether or not this module will have a CMI
until we reach the "export module" line (or not). Since this case is
IFNDR anyway (by [basic.def.odr] p11) we just tentatively assume while
parsing the GMF that this module will have a CMI; once we see (or don't
see) an 'export module' declaration we can commit to that knowledge for
future declarations.

gcc/cp/ChangeLog:

	* cp-tree.h (module_maybe_has_cmi_p): New function.
	* decl.cc (grokfndecl): Mark block-scope functions as public if
	they could be visible in other TUs.
	* decl2.cc (no_linkage_error): Don't error for declarations that
	could be defined in other TUs since C++20. Suppress duplicate
	errors from 'check_global_declaration'.
	* tree.cc (no_linkage_check): In relaxed mode, don't consider
	types in a module CMI to have no linkage.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/linkage-1.C: New test.
	* g++.dg/modules/block-decl-3.h: New test.
	* g++.dg/modules/block-decl-3_a.C: New test.
	* g++.dg/modules/block-decl-3_b.C: New test.
	* g++.dg/modules/block-decl-3_c.C: New test.
	* g++.dg/modules/linkage-1_a.C: New test.
	* g++.dg/modules/linkage-1_b.C: New test.
	* g++.dg/modules/linkage-1_c.C: New test.
	* g++.dg/modules/linkage-2.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
---
 gcc/cp/cp-tree.h                              |   6 +
 gcc/cp/decl.cc                                |  10 +-
 gcc/cp/decl2.cc                               |  39 ++++-
 gcc/cp/tree.cc                                |  21 ++-
 gcc/testsuite/g++.dg/cpp2a/linkage-1.C        |  18 ++
 gcc/testsuite/g++.dg/modules/block-decl-3.h   |  39 +++++
 gcc/testsuite/g++.dg/modules/block-decl-3_a.C | 157 ++++++++++++++++++
 gcc/testsuite/g++.dg/modules/block-decl-3_b.C |   8 +
 gcc/testsuite/g++.dg/modules/block-decl-3_c.C |  30 ++++
 gcc/testsuite/g++.dg/modules/linkage-1_a.C    |  15 ++
 gcc/testsuite/g++.dg/modules/linkage-1_b.C    |   6 +
 gcc/testsuite/g++.dg/modules/linkage-1_c.C    |   9 +
 gcc/testsuite/g++.dg/modules/linkage-2.C      |  26 +++
 13 files changed, 372 insertions(+), 12 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/linkage-1.C
 create mode 100644 gcc/testsuite/g++.dg/modules/block-decl-3.h
 create mode 100644 gcc/testsuite/g++.dg/modules/block-decl-3_a.C
 create mode 100644 gcc/testsuite/g++.dg/modules/block-decl-3_b.C
 create mode 100644 gcc/testsuite/g++.dg/modules/block-decl-3_c.C
 create mode 100644 gcc/testsuite/g++.dg/modules/linkage-1_a.C
 create mode 100644 gcc/testsuite/g++.dg/modules/linkage-1_b.C
 create mode 100644 gcc/testsuite/g++.dg/modules/linkage-1_c.C
 create mode 100644 gcc/testsuite/g++.dg/modules/linkage-2.C

diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 05913861e06..52d53589e51 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -7384,6 +7384,12 @@ inline bool named_module_purview_p ()
 inline bool named_module_attach_p ()
 { return named_module_p () && module_attach_p (); }
 
+/* We don't know if this TU will have a CMI while parsing the GMF,
+   so tentatively assume that it might, for the purpose of determining
+   whether no-linkage decls could be used by an importer.  */
+inline bool module_maybe_has_cmi_p ()
+{ return module_has_cmi_p () || (named_module_p () && !module_purview_p ()); }
+
 /* We're currently exporting declarations.  */
 inline bool module_exporting_p ()
 { return module_kind & MK_EXPORTING; }
diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index 7a97b867199..65ab64885ff 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -10756,9 +10756,15 @@ grokfndecl (tree ctype,
 
   /* Members of anonymous types and local classes have no linkage; make
      them internal.  If a typedef is made later, this will be changed.  */
-  if (ctype && (!TREE_PUBLIC (TYPE_MAIN_DECL (ctype))
-		|| decl_function_context (TYPE_MAIN_DECL (ctype))))
+  if (ctype && !TREE_PUBLIC (TYPE_MAIN_DECL (ctype)))
     publicp = 0;
+  else if (ctype && decl_function_context (TYPE_MAIN_DECL (ctype)))
+    /* But members of local classes in a module CMI should have their
+       definitions exported, in case they are (directly or indirectly)
+       used by an importer.  We don't just use module_has_cmi_p here
+       because for entities in the GMF we don't yet know whether this
+       module will have a CMI, so we'll conservatively assume it might.  */
+    publicp = module_maybe_has_cmi_p ();
 
   if (publicp && cxx_dialect == cxx98)
     {
diff --git a/gcc/cp/decl2.cc b/gcc/cp/decl2.cc
index 6c9fd415d40..2562d8aeff6 100644
--- a/gcc/cp/decl2.cc
+++ b/gcc/cp/decl2.cc
@@ -4696,8 +4696,19 @@ no_linkage_error (tree decl)
       bool d = false;
       auto_diagnostic_group grp;
       if (cxx_dialect >= cxx11)
-	d = permerror (DECL_SOURCE_LOCATION (decl), "%q#D, declared using "
-		       "unnamed type, is used but never defined", decl);
+	{
+	  /* If t is declared in a module CMI, then decl could actually
+	     be defined in a different TU, so don't warn since C++20.  */
+	  tree relaxed = no_linkage_check (t, /*relaxed_p=*/true);
+	  if (relaxed != NULL_TREE)
+	    d = permerror (DECL_SOURCE_LOCATION (decl),
+			   "%q#D, declared using an unnamed type, "
+			   "is used but never defined", decl);
+	  else if (cxx_dialect < cxx20)
+	    d = pedwarn (DECL_SOURCE_LOCATION (decl), OPT_Wc__20_extensions,
+			 "%q#D, declared using an unnamed type, "
+			 "is used but not defined", decl);
+	}
       else if (DECL_EXTERN_C_P (decl))
 	/* Allow this; it's pretty common in C.  */;
       else if (VAR_P (decl))
@@ -4716,13 +4727,31 @@ no_linkage_error (tree decl)
 	inform (DECL_SOURCE_LOCATION (TYPE_NAME (t)), "%q#D does not refer "
 		"to the unqualified type, so it is not used for linkage",
 		TYPE_NAME (t));
+      /* Suppress warning from check_global_declaration if needed.  */
+      if (d)
+	suppress_warning (decl, OPT_Wunused);
     }
   else if (cxx_dialect >= cxx11)
     {
       if (VAR_P (decl) || !DECL_PURE_VIRTUAL_P (decl))
-	permerror (DECL_SOURCE_LOCATION (decl),
-		   "%q#D, declared using local type "
-		   "%qT, is used but never defined", decl, t);
+	{
+	  /* Similarly for local types in a function with vague linkage or
+	     defined in a module CMI, then decl could actually be defined
+	     in a different TU, so don't warn since C++20.  */
+	  bool d = false;
+	  tree relaxed = no_linkage_check (t, /*relaxed_p=*/true);
+	  if (relaxed != NULL_TREE)
+	    d = permerror (DECL_SOURCE_LOCATION (decl),
+			   "%q#D, declared using local type "
+			   "%qT, is used but never defined", decl, t);
+	  else if (cxx_dialect < cxx20)
+	    d = pedwarn (DECL_SOURCE_LOCATION (decl), OPT_Wc__20_extensions,
+			 "%q#D, declared using local type "
+			 "%qT, is used but not defined here", decl, t);
+	  /* Suppress warning from check_global_declaration if needed.  */
+	  if (d)
+	    suppress_warning (decl, OPT_Wunused);
+	}
     }
   else if (VAR_P (decl))
     warning_at (DECL_SOURCE_LOCATION (decl), 0, "type %qT with no linkage "
diff --git a/gcc/cp/tree.cc b/gcc/cp/tree.cc
index e75be9a4e66..f1a23ffe817 100644
--- a/gcc/cp/tree.cc
+++ b/gcc/cp/tree.cc
@@ -2971,7 +2971,8 @@ verify_stmt_tree (tree t)
 
 /* Check if the type T depends on a type with no linkage and if so,
    return it.  If RELAXED_P then do not consider a class type declared
-   within a vague-linkage function to have no linkage.  Remember:
+   within a vague-linkage function or in a module CMI to have no linkage,
+   since it can still be accessed within a different TU.  Remember:
    no-linkage is not the same as internal-linkage.  */
 
 tree
@@ -3012,7 +3013,15 @@ no_linkage_check (tree t, bool relaxed_p)
       /* Only treat unnamed types as having no linkage if they're at
 	 namespace scope.  This is core issue 966.  */
       if (TYPE_UNNAMED_P (t) && TYPE_NAMESPACE_SCOPE_P (t))
-	return t;
+	{
+	  if (relaxed_p
+	      && TREE_PUBLIC (CP_TYPE_CONTEXT (t))
+	      && module_maybe_has_cmi_p ())
+	    /* This type could possibly be accessed outside this TU.  */
+	    return NULL_TREE;
+	  else
+	    return t;
+	}
 
       for (r = CP_TYPE_CONTEXT (t); ; )
 	{
@@ -3023,10 +3032,12 @@ no_linkage_check (tree t, bool relaxed_p)
 	    return no_linkage_check (TYPE_CONTEXT (t), relaxed_p);
 	  else if (TREE_CODE (r) == FUNCTION_DECL)
 	    {
-	      if (!relaxed_p || !vague_linkage_p (r))
-		return t;
-	      else
+	      if (relaxed_p
+		  && (vague_linkage_p (r)
+		      || (TREE_PUBLIC (r) && module_maybe_has_cmi_p ())))
 		r = CP_DECL_CONTEXT (r);
+	      else
+		return t;
 	    }
 	  else
 	    break;
diff --git a/gcc/testsuite/g++.dg/cpp2a/linkage-1.C b/gcc/testsuite/g++.dg/cpp2a/linkage-1.C
new file mode 100644
index 00000000000..888ed6fa5b5
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/linkage-1.C
@@ -0,0 +1,18 @@
+// { dg-do compile { target c++11 } }
+
+inline auto f() {
+  struct A {};
+  return A{};
+}
+decltype(f()) a();  // { dg-error "used but not defined" "" { target c++17_down } }
+
+auto g() {
+  struct A {};
+  return A{};
+}
+decltype(g()) b();  // { dg-error "used but never defined" }
+
+int main() {
+  a();
+  b();
+}
diff --git a/gcc/testsuite/g++.dg/modules/block-decl-3.h b/gcc/testsuite/g++.dg/modules/block-decl-3.h
new file mode 100644
index 00000000000..4b155eb0054
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/block-decl-3.h
@@ -0,0 +1,39 @@
+// GMF
+
+// Non-inline function definitions in headers are a recipe for ODR violations,
+// but we should probably support that anyway as its not inherently wrong
+// if only ever included into the GMF of a single module.
+
+auto gmf_n_i() {
+  struct X { void f() {} };
+  return X{};
+}
+
+inline auto gmf_i_i() {
+  struct X { void f() {} };
+  return X{};
+}
+
+auto gmf_n_i_i() {
+  struct X {
+    auto f() {
+      struct Y {
+	void g() {}
+      };
+      return Y{};
+    }
+  };
+  return X{};
+}
+
+inline auto gmf_i_i_i() {
+  struct X {
+    auto f() {
+      struct Y {
+	void g() {}
+      };
+      return Y{};
+    }
+  };
+  return X{};
+}
diff --git a/gcc/testsuite/g++.dg/modules/block-decl-3_a.C b/gcc/testsuite/g++.dg/modules/block-decl-3_a.C
new file mode 100644
index 00000000000..8cb4dde74d1
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/block-decl-3_a.C
@@ -0,0 +1,157 @@
+// { dg-additional-options "-fmodules-ts" }
+// { dg-module-cmi mod }
+
+// Test that we can link various forms of local class functions.
+// Function names use i=inline, n=non-inline, for each nesting.
+
+module;
+#include "block-decl-3.h"
+
+export module mod;
+
+namespace {
+  void internal() {}
+}
+
+// singly-nested
+
+export auto n_n() {
+  internal();
+  struct X { void f() { internal(); } };
+  return X{};
+}
+
+export auto n_i() {
+  internal();
+  struct X { inline void f() {} };
+  return X{};
+}
+
+export inline auto i_n() {
+  // `f` is not inline here, so this is OK
+  struct X { void f() { internal(); } };
+  return X{};
+}
+
+export inline auto i_i() {
+  struct X { inline void f() {} };
+  return X{};
+}
+
+
+// doubly nested
+
+export auto n_n_n() {
+  struct X {
+    auto f() {
+      struct Y {
+	void g() { internal(); }
+      };
+      return Y{};
+    }
+  };
+  return X{};
+}
+
+export auto n_i_n() {
+  struct X {
+    inline auto f() {
+      struct Y {
+	void g() { internal(); }
+      };
+      return Y{};
+    }
+  };
+  return X{};
+}
+
+export inline auto i_n_i() {
+  struct X {
+    auto f() {
+      struct Y {
+	inline void g() {}
+      };
+      return Y {};
+    }
+  };
+  return X{};
+}
+
+export inline auto i_i_i() {
+  struct X {
+    inline auto f() {
+      struct Y {
+	inline void g() {}
+      };
+      return Y{};
+    }
+  };
+  return X{};
+}
+
+
+// multiple types
+
+export auto multi_n_n() {
+  struct X {
+    void f() { internal(); }
+  };
+  struct Y {
+    X x;
+  };
+  return Y {};
+}
+
+export auto multi_n_i() {
+  struct X {
+    inline void f() {}
+  };
+  struct Y {
+    X x;
+  };
+  return Y {};
+}
+
+export inline auto multi_i_i() {
+  struct X {
+    inline void f() {}
+  };
+  struct Y {
+    X x;
+  };
+  return Y {};
+};
+
+
+// extern "C++"
+
+export extern "C++" auto extern_n_i() {
+  struct X {
+    void f() {}  // implicitly inline
+  };
+  return X{};
+};
+
+export extern "C++" inline auto extern_i_i() {
+  struct X {
+    void f() {}
+  };
+  return X{};
+};
+
+
+// GMF
+
+export using ::gmf_n_i;
+export using ::gmf_i_i;
+export using ::gmf_n_i_i;
+export using ::gmf_i_i_i;
+
+
+// can access from implementation unit
+
+auto only_used_in_impl() {
+  struct X { void f() {} };
+  return X{};
+}
+export void test_from_impl_unit();
diff --git a/gcc/testsuite/g++.dg/modules/block-decl-3_b.C b/gcc/testsuite/g++.dg/modules/block-decl-3_b.C
new file mode 100644
index 00000000000..bc9b2a213f0
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/block-decl-3_b.C
@@ -0,0 +1,8 @@
+// { dg-additional-options "-fmodules-ts" }
+
+module mod;
+
+// Test that we can access (and link) to declarations from the interface
+void test_from_impl_unit() {
+  only_used_in_impl().f();
+}
diff --git a/gcc/testsuite/g++.dg/modules/block-decl-3_c.C b/gcc/testsuite/g++.dg/modules/block-decl-3_c.C
new file mode 100644
index 00000000000..5b39e038327
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/block-decl-3_c.C
@@ -0,0 +1,30 @@
+// { dg-module-do link }
+// { dg-additional-options "-fmodules-ts" }
+
+import mod;
+
+int main() {
+  n_n().f();
+  n_i().f();
+  i_n().f();
+  i_i().f();
+
+  n_n_n().f().g();
+  n_i_n().f().g();
+  i_n_i().f().g();
+  i_i_i().f().g();
+
+  multi_n_n().x.f();
+  multi_n_i().x.f();
+  multi_i_i().x.f();
+
+  extern_n_i().f();
+  extern_i_i().f();
+
+  gmf_n_i().f();
+  gmf_i_i().f();
+  gmf_n_i_i().f().g();
+  gmf_i_i_i().f().g();
+
+  test_from_impl_unit();
+}
diff --git a/gcc/testsuite/g++.dg/modules/linkage-1_a.C b/gcc/testsuite/g++.dg/modules/linkage-1_a.C
new file mode 100644
index 00000000000..750e31ff347
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/linkage-1_a.C
@@ -0,0 +1,15 @@
+// { dg-additional-options "-fmodules-ts -Wno-error=c++20-extensions" }
+// { dg-module-cmi M }
+
+export module M;
+
+auto f() {
+  struct A {};
+  return A{};
+}
+decltype(f()) g();  // { dg-warning "used but not defined" "" { target c++17_down } }
+export auto x = g();
+
+struct {} s;
+decltype(s) h();  // { dg-warning "used but not defined" "" { target c++17_down } }
+export auto y = h();
diff --git a/gcc/testsuite/g++.dg/modules/linkage-1_b.C b/gcc/testsuite/g++.dg/modules/linkage-1_b.C
new file mode 100644
index 00000000000..f23962d76b7
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/linkage-1_b.C
@@ -0,0 +1,6 @@
+// { dg-additional-options "-fmodules-ts" }
+
+module M;
+
+decltype(f()) g() { return {}; }
+decltype(s) h() { return {}; }
diff --git a/gcc/testsuite/g++.dg/modules/linkage-1_c.C b/gcc/testsuite/g++.dg/modules/linkage-1_c.C
new file mode 100644
index 00000000000..f1406b99032
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/linkage-1_c.C
@@ -0,0 +1,9 @@
+// { dg-module-do link }
+// { dg-additional-options "-fmodules-ts" }
+
+import M;
+
+int main() {
+  auto a = x;
+  auto b = y;
+}
diff --git a/gcc/testsuite/g++.dg/modules/linkage-2.C b/gcc/testsuite/g++.dg/modules/linkage-2.C
new file mode 100644
index 00000000000..eb4d7b051af
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/linkage-2.C
@@ -0,0 +1,26 @@
+// { dg-additional-options "-fmodules-ts" }
+// { dg-module-cmi !M }
+
+export module M;
+
+// Same as a linkage-1 except within an anonymous namespace;
+// now these declarations cannot possibly be defined outside this TU,
+// so we should error.
+
+namespace {
+  auto f() {
+    struct A {};
+    return A{};
+  }
+  decltype(f()) g();  // { dg-error "used but never defined" }
+
+  struct {} s;
+  decltype(s) h();  // { dg-error "used but never defined" }
+}
+
+export void use() {
+  g();
+  h();
+}
+
+// { dg-prune-output "not writing module" }
-- 
2.43.2