[PATCH v2] c++: Check module attachment instead of just purview when necessary [PR112631]

[Nathaniel Shead <nathanieloshead@gmail.com>]

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.
	(start_preparsed_function): Ensure block-scope functions are
	emitted in module interfaces.
	* decl2.cc (determine_visibility): Likewise.

gcc/testsuite/ChangeLog:

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

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
---
 gcc/cp/cp-tree.h                              |   2 +
 gcc/cp/decl.cc                                |  22 ++-
 gcc/cp/decl2.cc                               |  23 +--
 gcc/testsuite/g++.dg/modules/block-decl-1_a.C |   9 ++
 gcc/testsuite/g++.dg/modules/block-decl-1_b.C |  10 ++
 gcc/testsuite/g++.dg/modules/block-decl-2_a.C | 143 ++++++++++++++++++
 gcc/testsuite/g++.dg/modules/block-decl-2_b.C |   8 +
 gcc/testsuite/g++.dg/modules/block-decl-2_c.C |  25 +++
 gcc/testsuite/g++.dg/modules/block-decl-3.C   |  21 +++
 9 files changed, 249 insertions(+), 14 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/modules/block-decl-1_a.C
 create mode 100644 gcc/testsuite/g++.dg/modules/block-decl-1_b.C
 create mode 100644 gcc/testsuite/g++.dg/modules/block-decl-2_a.C
 create mode 100644 gcc/testsuite/g++.dg/modules/block-decl-2_b.C
 create mode 100644 gcc/testsuite/g++.dg/modules/block-decl-2_c.C
 create mode 100644 gcc/testsuite/g++.dg/modules/block-decl-3.C

diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 14895bc6585..05913861e06 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -7381,6 +7381,8 @@ inline bool module_attach_p ()
 
 inline bool named_module_purview_p ()
 { return named_module_p () && module_purview_p (); }
+inline bool named_module_attach_p ()
+{ return named_module_p () && module_attach_p (); }
 
 /* We're currently exporting declarations.  */
 inline bool module_exporting_p ()
diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index dbc3df24e77..92475ecc28f 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -6092,10 +6092,10 @@ start_decl (const cp_declarator *declarator,
     {
       /* A function-scope decl of some namespace-scope decl.  */
       DECL_LOCAL_DECL_P (decl) = true;
-      if (named_module_purview_p ())
+      if (named_module_attach_p ())
 	error_at (declarator->id_loc,
-		  "block-scope extern declaration %q#D not permitted"
-		  " in module purview", decl);
+		  "block-scope extern declaration %q#D must not be"
+		  " attached to a named module", decl);
     }
 
   /* Enter this declaration into the symbol table.  Don't push the plain
@@ -18054,6 +18054,18 @@ start_preparsed_function (tree decl1, tree attrs, int flags)
 	/* This is a function in a local class in an extern inline
 	   or template function.  */
 	comdat_linkage (decl1);
+
+      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);
+	}
     }
   /* If this function belongs to an interface, it is public.
      If it belongs to someone else's interface, it is also external.
@@ -18907,10 +18919,10 @@ grokmethod (cp_decl_specifier_seq *declspecs,
   check_template_shadow (fndecl);
 
   /* p1779 ABI-Isolation makes inline not a default for in-class
-     definitions in named module purview.  If the user explicitly
+     definitions attached to a named module.  If the user explicitly
      made it inline, grokdeclarator will already have done the right
      things.  */
-  if ((!named_module_purview_p ()
+  if ((!named_module_attach_p ()
        || flag_module_implicit_inline
       /* Lambda's operator function remains inline.  */
        || LAMBDA_TYPE_P (DECL_CONTEXT (fndecl)))
diff --git a/gcc/cp/decl2.cc b/gcc/cp/decl2.cc
index 6c9fd415d40..94eaf98c609 100644
--- a/gcc/cp/decl2.cc
+++ b/gcc/cp/decl2.cc
@@ -3050,15 +3050,20 @@ determine_visibility (tree decl)
 	constrain_visibility (decl, tvis, false);
     }
   else if (no_linkage_check (TREE_TYPE (decl), /*relaxed_p=*/true))
-    /* 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.
-
-       Since non-extern "C" decls need to be defined in the same
-       translation unit, we can make the type internal.  */
-    constrain_visibility (decl, VISIBILITY_ANON, false);
+    {
+      /* 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.
+
+	 Since non-extern "C" decls need to be defined in the same
+	 translation unit, we can make the type internal, unless this
+	 type is part of a module CMI, in which case importers may need
+	 to access it.  */
+      if (!module_has_cmi_p ())
+	constrain_visibility (decl, VISIBILITY_ANON, false);
+    }
 
   /* If visibility changed and DECL already has DECL_RTL, ensure
      symbol flags are updated.  */
diff --git a/gcc/testsuite/g++.dg/modules/block-decl-1_a.C b/gcc/testsuite/g++.dg/modules/block-decl-1_a.C
new file mode 100644
index 00000000000..e7ffc629192
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/block-decl-1_a.C
@@ -0,0 +1,9 @@
+// { dg-additional-options "-fmodules-ts" }
+// { dg-module-cmi bla }
+
+export module bla;
+
+export extern "C++" inline void fun() {
+  void oops();  // { dg-bogus "block-scope extern declaration" }
+  oops();
+}
diff --git a/gcc/testsuite/g++.dg/modules/block-decl-1_b.C b/gcc/testsuite/g++.dg/modules/block-decl-1_b.C
new file mode 100644
index 00000000000..c0d724f25ac
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/block-decl-1_b.C
@@ -0,0 +1,10 @@
+// { dg-module-do link }
+// { dg-additional-options "-fmodules-ts" }
+
+import bla;
+
+void oops() {}
+
+int main() {
+  fun();
+}
diff --git a/gcc/testsuite/g++.dg/modules/block-decl-2_a.C b/gcc/testsuite/g++.dg/modules/block-decl-2_a.C
new file mode 100644
index 00000000000..00a4f229ae8
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/block-decl-2_a.C
@@ -0,0 +1,143 @@
+// { dg-additional-options "-fmodules-ts" }
+// { dg-module-cmi mod }
+
+export module mod;
+
+namespace {
+  void internal() {}
+}
+
+// singly-nested (i=inline, n=non-inline)
+
+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{};
+};
+
+
+// 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-2_b.C b/gcc/testsuite/g++.dg/modules/block-decl-2_b.C
new file mode 100644
index 00000000000..bc9b2a213f0
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/block-decl-2_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-2_c.C b/gcc/testsuite/g++.dg/modules/block-decl-2_c.C
new file mode 100644
index 00000000000..ef275d10f0e
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/block-decl-2_c.C
@@ -0,0 +1,25 @@
+// { 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();
+
+  test_from_impl_unit();
+}
diff --git a/gcc/testsuite/g++.dg/modules/block-decl-3.C b/gcc/testsuite/g++.dg/modules/block-decl-3.C
new file mode 100644
index 00000000000..8bbebd06bab
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/block-decl-3.C
@@ -0,0 +1,21 @@
+// { dg-additional-options "-fmodules-ts" }
+// { dg-module-cmi !mod }
+
+export module mod;
+
+namespace {
+  void internal();
+}
+
+export extern "C++" auto foo() {
+  struct X {
+    // `foo` is not attached to a named module, and as such
+    // `X::f` should be implicitly `inline` here
+    void f() {  // { dg-error "references internal linkage entity" }
+      internal();
+    }
+  };
+  return X{};
+}
+
+// { dg-prune-output "failed to write compiled module" }
-- 
2.43.2