c++: Introduce USING_DECLs for non-function usings [PR114683]

With modules, a non-function using-declaration is not completely
interchangable with the declaration that it refers to; in particular,
such a using-declaration may be exported without revealing the name of
the entity it refers to.

This patch fixes this by building USING_DECLs for all using-declarations
that bind a non-function from a different scope.  These new decls can
than have purviewness and exportingness attached to them without
affecting the decl that they refer to.

We do this for all such usings, not just usings that may be revealed in
a module; this way we can verify the change in representation against
the (more comprehensive) non-modules testsuites, and in a future patch
we can use the locations of these using-decls to enhance relevant
diagnostics.

Another possible approach would be to reuse OVERLOADs for this, as is
already done within add_binding_entity for modules.  I didn't do this
because lots of code (as well as the names of the accessors) makes
assumptions that OVERLOADs refer to function overload sets, and so
splitting this up reduced semantic burden and made it easier to avoid
unintentional changes.  This did mean that we need to move out the
definitions of ovl_iterator::{purview,exporting}_p, because the
structures for module decls are declared later on in cp-tree.h.

Building USING_DECLs changed a couple of code paths when adjusting
bindings; in particular, pushdecl recognises global using-declarations
as usings now, and so checks fall through to update_binding.  To not
regress g++.dg/lookup/linkage2.C the checks for 'extern' declarations no
longer were sufficient (they don't handle 'extern "C"'); but
duplicate_decls performed all the relevant checks anyway.

Otherwise in general we strip using-decls from all lookup_* functions
where necessary.  Over time for diagnostics purposes it would probably
be good to slowly revert this (especially e.g. lookup_elaborated_type
causes some diagnostic quality regressions here) but this patch doesn't
do so to minimise churn.

This patch also tries not to build USING_DECLs when just redeclaring an
existing declaration, and instead reveals that declaration in-place.
This requires reworking some logic handling CONST_DECLs in module
streaming, since a non-using CONST_DECL may now be exported indepenently
of its containing enum.

'add_binding_entity' needs to explicitly write the names of unscoped
enumerators so that lazy loading will trigger when the name is found by
name lookup; it does this by pretending that the enum declarations are
always usings so that it doesn't double-write definitions.  By also
checking if the enumerator was marked purview/exported we can use that
to override a non-purview/non-exported TYPE_DECL and ensure it's made
visible regardless.

When reading we should get the exported flag on the enumeration
constant, and so should properly create a binding for it.  We don't need
to do anything to handle importedness as that checking is skipped for
EK_USINGs.

Some other places assume that module information for a CONST_DECL
inherits module information from its containing type.  This includes:

- get_originating_module_decl, for determining if the name was imported
  or has module attachment; I don't /think/ this change should affect
  that, so I'm leaving this untouched.

- binding_cmp, for sorting by exportedness; since now an enumerator
  could be exported without the containing decl being exported, we need
  to handle this here too.

	PR c++/114683

gcc/cp/ChangeLog:

	* cp-tree.h (class ovl_iterator): Move definitions of purview_p
	and exporting_p to name-lookup.cc.
	* module.cc (depset::hash::add_binding_entity): Strip
	using-decls.  Remove workarounds.  Handle CONST_DECLs with
	different purview/exported from their enum.
	(enum ct_bind_flags): Remove unnecessary cbf_wrapped flag.
	(module_state::write_cluster): Likewise.
	(module_state::read_cluster): Build USING_DECL for non-function
	usings.
	(binding_cmp): Handle CONST_DECLs with different purview and/or
	exported from their enum.
	(set_instantiating_module): Support CONST_DECLs.
	* name-lookup.cc (get_fixed_binding_slot): Strip USING_DECLs.
	(name_lookup::process_binding): Strip USING_DECLs.
	(name_lookup::process_module_binding): Remove workaround.
	(update_binding): Strip USING_DECLs, remove incorrect check for
	non-extern variables.
	(ovl_iterator::purview_p): Support USING_DECLs.
	(ovl_iterator::exporting_p): Support USING_DECLs.
	(walk_module_binding): Handle stat hack type.
	(do_nonmember_using_decl): Strip USING_DECLs when comparing;
	build USING_DECLs for non-function usings in different scope
	rather than binding directly.
	(get_namespace_binding): Strip USING_DECLs.
	(lookup_name): Strip USING_DECLs.
	(lookup_elaborated_type): Strip USING_DECLs.
	* decl.cc (poplevel): Still support -Wunused for using-decls.
	(lookup_and_check_tag): Remove unnecessary strip_using_decl.
	* parser.cc (cp_parser_template_name): Likewise.
	(cp_parser_nonclass_name): Likewise.
	(cp_parser_class_name): Likewise.

gcc/testsuite/ChangeLog:

	* g++.dg/lookup/using29.C: Update errors.
	* g++.dg/lookup/using53.C: Update errors, add XFAILs.
	* g++.dg/modules/using-22_b.C: Remove xfails.
	* g++.dg/warn/Wunused-var-18.C: Update error, add check.
	* g++.dg/lookup/using68.C: New test.
	* g++.dg/modules/using-24_a.C: New test.
	* g++.dg/modules/using-24_b.C: New test.
	* g++.dg/modules/using-25_a.C: New test.
	* g++.dg/modules/using-25_b.C: New test.
	* g++.dg/modules/using-enum-4_a.C: New test.
	* g++.dg/modules/using-enum-4_b.C: New test.
	* g++.dg/modules/using-enum-4_c.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
Reviewed-by: Jason Merrill <jason@redhat.com>

17 files changed, 308 insertions, 135 deletions

diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index f270b0b..208947c 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -889,21 +889,12 @@ class ovl_iterator {
     return (TREE_CODE (ovl) == USING_DECL
 	    || (TREE_CODE (ovl) == OVERLOAD && OVL_USING_P (ovl)));
   }
-  /* Whether this using is in the module purview.  */
-  bool purview_p () const
-  {
-    return OVL_PURVIEW_P (get_using ());
-  }
-  /* Whether this using is being exported.  */
-  bool exporting_p () const
-  {
-    return OVL_EXPORT_P (get_using ());
-  }
-  
   bool hidden_p () const
   {
     return TREE_CODE (ovl) == OVERLOAD && OVL_HIDDEN_P (ovl);
   }
+  bool purview_p () const;
+  bool exporting_p () const;
 
  public:
   tree remove_node (tree head)
diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index edf4c15..4ffc9a6 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -702,12 +702,13 @@ poplevel (int keep, int reverse, int functionbody)
 	/* There are cases where D itself is a TREE_LIST.  See in
 	   push_local_binding where the list of decls returned by
 	   getdecls is built.  */
-	decl = TREE_CODE (d) == TREE_LIST ? TREE_VALUE (d) : d;
+	tree udecl = TREE_CODE (d) == TREE_LIST ? TREE_VALUE (d) : d;
+	decl = strip_using_decl (udecl);
 
 	tree type = TREE_TYPE (decl);
 	if (VAR_P (decl)
-	    && (! TREE_USED (decl) || !DECL_READ_P (decl))
-	    && ! DECL_IN_SYSTEM_HEADER (decl)
+	    && (!TREE_USED (decl) || !DECL_READ_P (decl))
+	    && !DECL_IN_SYSTEM_HEADER (udecl)
 	    /* For structured bindings, consider only real variables, not
 	       subobjects.  */
 	    && (DECL_DECOMPOSITION_P (decl) ? DECL_DECOMP_IS_BASE (decl)
@@ -720,9 +721,13 @@ poplevel (int keep, int reverse, int functionbody)
 		|| lookup_attribute ("warn_unused",
 				     TYPE_ATTRIBUTES (TREE_TYPE (decl)))))
 	  {
-	    if (! TREE_USED (decl))
+	    if (!TREE_USED (decl))
 	      {
-		if (!DECL_NAME (decl) && DECL_DECOMPOSITION_P (decl))
+		if (TREE_CODE (udecl) == USING_DECL)
+		  warning_at (DECL_SOURCE_LOCATION (udecl),
+			      OPT_Wunused_variable,
+			      "unused using-declaration %qD", udecl);
+		else if (!DECL_NAME (decl) && DECL_DECOMPOSITION_P (decl))
 		  warning_at (DECL_SOURCE_LOCATION (decl),
 			      OPT_Wunused_variable,
 			      "unused structured binding declaration");
@@ -16491,7 +16496,6 @@ lookup_and_check_tag (enum tag_types tag_code, tree name,
       /* First try ordinary name lookup, ignoring hidden class name
 	 injected via friend declaration.  */
       decl = lookup_name (name, LOOK_want::TYPE);
-      decl = strip_using_decl (decl);
       /* If that fails, the name will be placed in the smallest
 	 non-class, non-function-prototype scope according to 3.3.1/5.
 	 We may already have a hidden name declared as friend in this
diff --git a/gcc/cp/module.cc b/gcc/cp/module.cc
index da18024..d385b42 100644
--- a/gcc/cp/module.cc
+++ b/gcc/cp/module.cc
@@ -13122,13 +13122,17 @@ bool
 depset::hash::add_binding_entity (tree decl, WMB_Flags flags, void *data_)
 {
   auto data = static_cast <add_binding_data *> (data_);
+  decl = strip_using_decl (decl);
 
   if (!(TREE_CODE (decl) == NAMESPACE_DECL && !DECL_NAMESPACE_ALIAS (decl)))
     {
       tree inner = decl;
 
       if (TREE_CODE (inner) == CONST_DECL
-	  && TREE_CODE (DECL_CONTEXT (inner)) == ENUMERAL_TYPE)
+	  && TREE_CODE (DECL_CONTEXT (inner)) == ENUMERAL_TYPE
+	  /* A using-decl could make a CONST_DECL purview for a non-purview
+	     enumeration.  */
+	  && (!DECL_LANG_SPECIFIC (inner) || !DECL_MODULE_PURVIEW_P (inner)))
 	inner = TYPE_NAME (DECL_CONTEXT (inner));
       else if (TREE_CODE (inner) == TEMPLATE_DECL)
 	inner = DECL_TEMPLATE_RESULT (inner);
@@ -13156,18 +13160,17 @@ depset::hash::add_binding_entity (tree decl, WMB_Flags flags, void *data_)
 	/* Ignore NTTP objects.  */
 	return false;
 
-      bool unscoped_enum_const_p = false;
       if (!(flags & WMB_Using) && CP_DECL_CONTEXT (decl) != data->ns)
 	{
-	  /* A using that lost its wrapper or an unscoped enum
-	     constant.  */
-	  /* FIXME: Ensure that unscoped enums are differentiated from
-	     'using enum' declarations when PR c++/114683 is fixed.  */
-	  unscoped_enum_const_p = (TREE_CODE (decl) == CONST_DECL);
+	  /* An unscoped enum constant implicitly brought into the containing
+	     namespace.  We treat this like a using-decl.  */
+	  gcc_checking_assert (TREE_CODE (decl) == CONST_DECL);
+
 	  flags = WMB_Flags (flags | WMB_Using);
-	  if (DECL_MODULE_EXPORT_P (TREE_CODE (decl) == CONST_DECL
-				    ? TYPE_NAME (TREE_TYPE (decl))
-				    : STRIP_TEMPLATE (decl)))
+	  if (DECL_MODULE_EXPORT_P (TYPE_NAME (TREE_TYPE (decl)))
+	      /* A using-decl can make an enum constant exported for a
+		 non-exported enumeration.  */
+	      || (DECL_LANG_SPECIFIC (decl) && DECL_MODULE_EXPORT_P (decl)))
 	    flags = WMB_Flags (flags | WMB_Export);
 	}
 
@@ -13225,8 +13228,7 @@ depset::hash::add_binding_entity (tree decl, WMB_Flags flags, void *data_)
       if (flags & WMB_Using)
 	{
 	  decl = ovl_make (decl, NULL_TREE);
-	  if (!unscoped_enum_const_p)
-	    OVL_USING_P (decl) = true;
+	  OVL_USING_P (decl) = true;
 	  OVL_PURVIEW_P (decl) = true;
 	  if (flags & WMB_Export)
 	    OVL_EXPORT_P (decl) = true;
@@ -13250,12 +13252,8 @@ depset::hash::add_binding_entity (tree decl, WMB_Flags flags, void *data_)
       data->met_namespace = true;
       if (data->hash->add_namespace_entities (decl, data->partitions))
 	{
-	  /* It contains an exported thing, so it is exported.
-
-	     FIXME we have to set DECL_MODULE_PURVIEW_P instead of asserting
-	     that it is already set because of the c++/114683 issue with
-	     exported using-declarations; see do_nonmember_using_decl.  */
-	  DECL_MODULE_PURVIEW_P (decl) = true;
+	  /* It contains an exported thing, so it is exported.  */
+	  gcc_checking_assert (DECL_MODULE_PURVIEW_P (decl));
 	  DECL_MODULE_EXPORT_P (decl) = true;
 	}
 
@@ -13744,6 +13742,10 @@ binding_cmp (const void *a_, const void *b_)
       a_export = OVL_EXPORT_P (a_ent);
       a_ent = OVL_FUNCTION (a_ent);
     }
+  else if (TREE_CODE (a_ent) == CONST_DECL
+	   && DECL_LANG_SPECIFIC (a_ent)
+	   && DECL_MODULE_EXPORT_P (a_ent))
+    a_export = true;
   else
     a_export = DECL_MODULE_EXPORT_P (TREE_CODE (a_ent) == CONST_DECL
 				     ? TYPE_NAME (TREE_TYPE (a_ent))
@@ -13756,6 +13758,10 @@ binding_cmp (const void *a_, const void *b_)
       b_export = OVL_EXPORT_P (b_ent);
       b_ent = OVL_FUNCTION (b_ent);
     }
+  else if (TREE_CODE (b_ent) == CONST_DECL
+	   && DECL_LANG_SPECIFIC (b_ent)
+	   && DECL_MODULE_EXPORT_P (b_ent))
+    b_export = true;
   else
     b_export = DECL_MODULE_EXPORT_P (TREE_CODE (b_ent) == CONST_DECL
 				     ? TYPE_NAME (TREE_TYPE (b_ent))
@@ -14964,7 +14970,6 @@ enum ct_bind_flags
   cbf_export = 0x1,	/* An exported decl.  */
   cbf_hidden = 0x2,	/* A hidden (friend) decl.  */
   cbf_using = 0x4,	/* A using decl.  */
-  cbf_wrapped = 0x8,  	/* ... that is wrapped.  */
 };
 
 /* DEP belongs to a different cluster, seed it to prevent
@@ -15127,13 +15132,9 @@ module_state::write_cluster (elf_out *to, depset *scc[], unsigned size,
 		    if (!(TREE_CODE (bound) == CONST_DECL
 			  && UNSCOPED_ENUM_P (TREE_TYPE (bound))
 			  && decl == TYPE_NAME (TREE_TYPE (bound))))
-		      {
-			/* An unscope enumerator in its enumeration's
-			   scope is not a using.  */
-			flags |= cbf_using;
-			if (OVL_USING_P (ovl))
-			  flags |= cbf_wrapped;
-		      }
+		      /* An unscoped enumerator in its enumeration's
+			 scope is not a using.  */
+		      flags |= cbf_using;
 		    if (OVL_EXPORT_P (ovl))
 		      flags |= cbf_export;
 		  }
@@ -15300,13 +15301,54 @@ module_state::read_cluster (unsigned snum)
 		    /* Stat hack.  */
 		    if (type || !DECL_IMPLICIT_TYPEDEF_P (decl))
 		      sec.set_overrun ();
-		    type = decl;
+
+		    if (flags & cbf_using)
+		      {
+			type = build_lang_decl_loc (UNKNOWN_LOCATION,
+						    USING_DECL,
+						    DECL_NAME (decl),
+						    NULL_TREE);
+			USING_DECL_DECLS (type) = decl;
+			USING_DECL_SCOPE (type) = CP_DECL_CONTEXT (decl);
+			DECL_CONTEXT (type) = ns;
+
+			DECL_MODULE_PURVIEW_P (type) = true;
+			if (flags & cbf_export)
+			  DECL_MODULE_EXPORT_P (type) = true;
+		      }
+		    else
+		      type = decl;
 		  }
 		else
 		  {
-		    if (decls
-			|| (flags & (cbf_hidden | cbf_wrapped))
-			|| DECL_FUNCTION_TEMPLATE_P (decl))
+		    if ((flags & cbf_using) &&
+			!DECL_DECLARES_FUNCTION_P (decl))
+		      {
+			/* We should only see a single non-function using-decl
+			   for a binding; more than that would clash.  */
+			if (decls)
+			  sec.set_overrun ();
+
+			/* FIXME: Propagate the location of the using-decl
+			   for use in diagnostics.  */
+			decls = build_lang_decl_loc (UNKNOWN_LOCATION,
+						     USING_DECL,
+						     DECL_NAME (decl),
+						     NULL_TREE);
+			USING_DECL_DECLS (decls) = decl;
+			/* We don't currently record the actual scope of the
+			   using-declaration, but this approximation should
+			   generally be good enough.  */
+			USING_DECL_SCOPE (decls) = CP_DECL_CONTEXT (decl);
+			DECL_CONTEXT (decls) = ns;
+
+			DECL_MODULE_PURVIEW_P (decls) = true;
+			if (flags & cbf_export)
+			  DECL_MODULE_EXPORT_P (decls) = true;
+		      }
+		    else if (decls
+			     || (flags & (cbf_hidden | cbf_using))
+			     || DECL_FUNCTION_TEMPLATE_P (decl))
 		      {
 			decls = ovl_make (decl, decls);
 			if (flags & cbf_using)
@@ -19202,6 +19244,7 @@ set_instantiating_module (tree decl)
 	      || TREE_CODE (decl) == TYPE_DECL
 	      || TREE_CODE (decl) == CONCEPT_DECL
 	      || TREE_CODE (decl) == TEMPLATE_DECL
+	      || TREE_CODE (decl) == CONST_DECL
 	      || (TREE_CODE (decl) == NAMESPACE_DECL
 		  && DECL_NAMESPACE_ALIAS (decl)));
 
diff --git a/gcc/cp/name-lookup.cc b/gcc/cp/name-lookup.cc
index b578931..e5f8562 100644
--- a/gcc/cp/name-lookup.cc
+++ b/gcc/cp/name-lookup.cc
@@ -281,10 +281,11 @@ get_fixed_binding_slot (tree *slot, tree name, unsigned ix, int create)
 
 	  /* Propagate global & module entities to the global and
 	     partition slots.  */
-	  if (tree type = MAYBE_STAT_TYPE (orig))
+	  if (tree type = strip_using_decl (MAYBE_STAT_TYPE (orig)))
 	    init_global_partition (cluster, type);
 
-	  for (ovl_iterator iter (MAYBE_STAT_DECL (orig)); iter; ++iter)
+	  for (ovl_iterator iter (strip_using_decl (MAYBE_STAT_DECL (orig)));
+	       iter; ++iter)
 	    {
 	      tree decl = *iter;
 
@@ -767,6 +768,9 @@ name_lookup::process_binding (tree new_val, tree new_type)
       && (want & LOOK_want::TYPE_NAMESPACE) == LOOK_want::NAMESPACE)
     new_type = NULL_TREE;
 
+  new_val = strip_using_decl (new_val);
+  new_type = strip_using_decl (new_type);
+
   /* Do we really see a value? */
   if (new_val)
     switch (TREE_CODE (new_val))
@@ -827,17 +831,6 @@ name_lookup::process_module_binding (tree new_val, tree new_type,
       marker |= 2;
     }
 
-  /* add_binding_entity wraps decls brought in by 'using' in an OVERLOAD even
-     for non-functions; strip it now.
-     ??? Why isn't it represented with a USING_DECL?  Or do we want to use
-     OVERLOAD for using more widely to address 114683?  */
-  if (new_val && TREE_CODE (new_val) == OVERLOAD
-      && !DECL_DECLARES_FUNCTION_P (OVL_FUNCTION (new_val)))
-    {
-      gcc_checking_assert (OVL_USING_P (new_val) && !OVL_CHAIN (new_val));
-      new_val = OVL_FUNCTION (new_val);
-    }
-
   if (new_type || new_val)
     marker |= process_binding (new_val, new_type);
 
@@ -2998,6 +2991,7 @@ update_binding (cp_binding_level *level, cxx_binding *binding, tree *slot,
 
   if (old == error_mark_node)
     old = NULL_TREE;
+  old = strip_using_decl (old);
 
   if (DECL_IMPLICIT_TYPEDEF_P (decl))
     {
@@ -3102,11 +3096,7 @@ update_binding (cp_binding_level *level, cxx_binding *binding, tree *slot,
 	}
       else if (TREE_CODE (old) == VAR_DECL)
 	{
-	  /* There can be two block-scope declarations of the same
-	     variable, so long as they are `extern' declarations.  */
-	  if (!DECL_EXTERNAL (old) || !DECL_EXTERNAL (decl))
-	    goto conflict;
-	  else if (tree match = duplicate_decls (decl, old))
+	  if (tree match = duplicate_decls (decl, old))
 	    {
 	      gcc_checking_assert (!hide_value && !hiding);
 	      return match;
@@ -4205,6 +4195,28 @@ lookup_class_binding (tree klass, tree name)
   return found;
 }
 
+/* Whether this using is declared in the module purview.  */
+
+bool
+ovl_iterator::purview_p () const
+{
+  gcc_checking_assert (using_p ());
+  if (TREE_CODE (ovl) == USING_DECL)
+    return DECL_MODULE_PURVIEW_P (ovl);
+  return OVL_PURVIEW_P (ovl);
+}
+
+/* Whether this using is exported from this module.  */
+
+bool
+ovl_iterator::exporting_p () const
+{
+  gcc_checking_assert (using_p ());
+  if (TREE_CODE (ovl) == USING_DECL)
+    return DECL_MODULE_EXPORT_P (ovl);
+  return OVL_EXPORT_P (ovl);
+}
+
 /* Given a namespace-level binding BINDING, walk it, calling CALLBACK
    for all decls of the current module.  When partitions are involved,
    decls might be mentioned more than once.   Return the accumulation of
@@ -4215,8 +4227,6 @@ walk_module_binding (tree binding, bitmap partitions,
 		     bool (*callback) (tree decl, WMB_Flags, void *data),
 		     void *data)
 {
-  // FIXME: We don't quite deal with using decls naming stat hack
-  // type.
   tree current = binding;
   unsigned count = 0;
 
@@ -4229,6 +4239,14 @@ walk_module_binding (tree binding, bitmap partitions,
       WMB_Flags flags = WMB_None;
       if (STAT_TYPE_HIDDEN_P (current))
 	flags = WMB_Flags (flags | WMB_Hidden);
+      if (TREE_CODE (type) == USING_DECL)
+	{
+	  flags = WMB_Flags (flags | WMB_Using);
+	  if (DECL_MODULE_PURVIEW_P (type))
+	    flags = WMB_Flags (flags | WMB_Purview);
+	  if (DECL_MODULE_EXPORT_P (type))
+	    flags = WMB_Flags (flags | WMB_Export);
+	}
       count += callback (type, flags, data);
       decl_hidden = STAT_DECL_HIDDEN_P (current);
     }
@@ -4300,7 +4318,14 @@ walk_module_binding (tree binding, bitmap partitions,
 			  flags = WMB_Flags (flags | WMB_Dups);
 			if (STAT_TYPE_HIDDEN_P (bind))
 			  flags = WMB_Flags (flags | WMB_Hidden);
-
+			if (TREE_CODE (btype) == USING_DECL)
+			  {
+			    flags = WMB_Flags (flags | WMB_Using);
+			    if (DECL_MODULE_PURVIEW_P (btype))
+			      flags = WMB_Flags (flags | WMB_Purview);
+			    if (DECL_MODULE_EXPORT_P (btype))
+			      flags = WMB_Flags (flags | WMB_Export);
+			  }
 			count += callback (btype, flags, data);
 		      }
 		    bool part_hidden = STAT_DECL_HIDDEN_P (bind);
@@ -5201,7 +5226,7 @@ do_nonmember_using_decl (name_lookup &lookup, bool fn_scope_p,
 
   /* Shift the old and new bindings around so we're comparing class and
      enumeration names to each other.  */
-  if (value && DECL_IMPLICIT_TYPEDEF_P (value))
+  if (value && DECL_IMPLICIT_TYPEDEF_P (strip_using_decl (value)))
     {
       type = value;
       value = NULL_TREE;
@@ -5306,61 +5331,68 @@ do_nonmember_using_decl (name_lookup &lookup, bool fn_scope_p,
   else if (value
 	   /* Ignore anticipated builtins.  */
 	   && !anticipated_builtin_p (value)
-	   && (fn_scope_p || !decls_match (lookup.value, value)))
+	   && (fn_scope_p
+	       || !decls_match (lookup.value, strip_using_decl (value))))
     {
       diagnose_name_conflict (lookup.value, value);
       failed = true;
     }
   else if (insert_p)
     {
-      /* FIXME: Handle exporting declarations from a different scope
-	 without also marking those declarations as exported.
-	 This will require not just binding directly to the underlying
-	 value; see c++/114683 and c++/114685.  We allow the extra exports
-	 for now as this doesn't (currently) cause ICEs
-	 later down the line, but this should be revisited.  */
-      if (revealing_p)
+      /* A using-decl does not necessarily have the same purview-ness or
+	 exporting as the declaration it reveals, so build a USING_DECL
+	 that we can attach this information to.  This also gives us a
+	 location for the using-decl that we can use in diagnostics.
+
+	 But this is unnecessary if we're just redeclaring the same decl;
+	 in that case we can just mark it purview or exported directly.  */
+      if (value != lookup.value)
 	{
-	  if (module_exporting_p ()
-	      && check_can_export_using_decl (lookup.value)
-	      && !DECL_MODULE_EXPORT_P (lookup.value))
-	    {
-	      /* We're redeclaring the same value, but this time as
-		 newly exported: make sure to mark it as such.  */
-	      if (TREE_CODE (lookup.value) == TEMPLATE_DECL)
-		DECL_MODULE_EXPORT_P (DECL_TEMPLATE_RESULT (lookup.value)) = true;
-	      DECL_MODULE_EXPORT_P (lookup.value) = true;
-	    }
-	  /* We're also revealing this declaration with a using-decl,
-	     so mark it as purview to ensure it's emitted.  */
-	  tree inner = STRIP_TEMPLATE (lookup.value);
-	  retrofit_lang_decl (inner);
-	  DECL_MODULE_PURVIEW_P (inner) = true;
+	  value = build_lang_decl (USING_DECL, lookup.name, NULL_TREE);
+	  USING_DECL_DECLS (value) = lookup.value;
+	  USING_DECL_SCOPE (value) = CP_DECL_CONTEXT (lookup.value);
+	  DECL_CONTEXT (value) = current_scope ();
+	  DECL_MODULE_PURVIEW_P (value) = module_purview_p ();
+	}
+      else
+	set_instantiating_module (value);
+
+      if (revealing_p
+	  && module_exporting_p ()
+	  && check_can_export_using_decl (lookup.value))
+	{
+	  if (TREE_CODE (value) == TEMPLATE_DECL)
+	    DECL_MODULE_EXPORT_P (DECL_TEMPLATE_RESULT (value)) = true;
+	  DECL_MODULE_EXPORT_P (value) = true;
 	}
-      value = lookup.value;
     }
-  
+
   /* Now the type binding.  */
   if (lookup.type)
     {
-      if (type && !decls_match (lookup.type, type))
+      if (type && !decls_match (lookup.type, strip_using_decl (type)))
 	{
 	  diagnose_name_conflict (lookup.type, type);
 	  failed = true;
 	}
       else if (insert_p)
 	{
-	  if (revealing_p)
+	  /* As with revealing value bindings.  */
+	  if (type != lookup.type)
 	    {
-	      /* As with revealing value bindings.  */
-	      if (module_exporting_p ()
-		  && check_can_export_using_decl (lookup.type)
-		  && lookup.type == type)
-		DECL_MODULE_EXPORT_P (lookup.type) = true;
-	      retrofit_lang_decl (lookup.type);
-	      DECL_MODULE_PURVIEW_P (lookup.type) = true;
+	      type = build_lang_decl (USING_DECL, lookup.name, NULL_TREE);
+	      USING_DECL_DECLS (type) = lookup.type;
+	      USING_DECL_SCOPE (type) = CP_DECL_CONTEXT (lookup.type);
+	      DECL_CONTEXT (type) = current_scope ();
+	      DECL_MODULE_PURVIEW_P (type) = module_purview_p ();
 	    }
-	  type = lookup.type;
+	  else
+	    set_instantiating_module (type);
+
+	  if (revealing_p
+	      && module_exporting_p ()
+	      && check_can_export_using_decl (lookup.type))
+	    DECL_MODULE_EXPORT_P (type) = true;
 	}
     }
 
@@ -6217,7 +6249,7 @@ get_namespace_binding (tree ns, tree name)
       if (TREE_CODE (ret) == BINDING_VECTOR)
 	ret = BINDING_VECTOR_CLUSTER (ret, 0).slots[0];
       if (ret)
-	ret = MAYBE_STAT_DECL (ret);
+	ret = strip_using_decl (MAYBE_STAT_DECL (ret));
     }
 
   return ret;
@@ -8000,7 +8032,7 @@ lookup_name (tree name, LOOK_where where, LOOK_want want)
 
 	    if (binding)
 	      {
-		val = binding;
+		val = strip_using_decl (binding);
 		break;
 	      }
 	  }
@@ -8073,7 +8105,7 @@ lookup_elaborated_type (tree name, TAG_how how)
 	     typedef struct C {} C;
 	   correctly.  */
 
-	if (tree type = iter->type)
+	if (tree type = strip_using_decl (iter->type))
 	  {
 	    if (qualify_lookup (type, LOOK_want::TYPE)
 		&& (how != TAG_how::CURRENT_ONLY
@@ -8089,7 +8121,8 @@ lookup_elaborated_type (tree name, TAG_how how)
 	  }
 	else
 	  {
-	    if (qualify_lookup (iter->value, LOOK_want::TYPE)
+	    tree value = strip_using_decl (iter->value);
+	    if (qualify_lookup (value, LOOK_want::TYPE)
 		&& (how != TAG_how::CURRENT_ONLY
 		    || !INHERITED_VALUE_BINDING_P (iter)))
 	      {
@@ -8097,7 +8130,7 @@ lookup_elaborated_type (tree name, TAG_how how)
 		  /* It is no longer a hidden binding.  */
 		  HIDDEN_TYPE_BINDING_P (iter) = false;
 
-		return iter->value;
+		return value;
 	      }
 	  }
       }
@@ -8121,7 +8154,7 @@ lookup_elaborated_type (tree name, TAG_how how)
       if (bind)
 	{
 	  /* If this is the kind of thing we're looking for, we're done.  */
-	  if (tree type = MAYBE_STAT_TYPE (bind))
+	  if (tree type = strip_using_decl (MAYBE_STAT_TYPE (bind)))
 	    {
 	      if (how != TAG_how::HIDDEN_FRIEND)
 		/* No longer hidden.  */
@@ -8129,7 +8162,7 @@ lookup_elaborated_type (tree name, TAG_how how)
 	      
 	      return type;
 	    }
-	  else if (tree decl = MAYBE_STAT_DECL (bind))
+	  else if (tree decl = strip_using_decl (MAYBE_STAT_DECL (bind)))
 	    {
 	      if (qualify_lookup (decl, LOOK_want::TYPE))
 		{
@@ -8210,10 +8243,10 @@ lookup_elaborated_type (tree name, TAG_how how)
 		  }
 
 		if (type && qualify_lookup (type, LOOK_want::TYPE))
-		  return type;
+		  return strip_using_decl (type);
 
 		if (bind && qualify_lookup (bind, LOOK_want::TYPE))
-		  return bind;
+		  return strip_using_decl (bind);
 	      }
 
 	  if (!module_purview_p ())
diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index 6bf3f52..1dd0efa 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -19442,8 +19442,6 @@ cp_parser_template_name (cp_parser* parser,
 				/*ambiguous_decls=*/NULL,
 				token->location);
 
-  decl = strip_using_decl (decl);
-
   /* 13.3 [temp.names] A < is interpreted as the delimiter of a
     template-argument-list if it follows a name that is not a
     conversion-function-id and
@@ -21061,8 +21059,6 @@ cp_parser_nonclass_name (cp_parser* parser)
   /* Look up the type-name.  */
   type_decl = cp_parser_lookup_name_simple (parser, identifier, token->location);
 
-  type_decl = strip_using_decl (type_decl);
-
   if (TREE_CODE (type_decl) != TYPE_DECL
       && (objc_is_id (identifier) || objc_is_class_name (identifier)))
     {
@@ -26834,8 +26830,6 @@ cp_parser_class_name (cp_parser *parser,
 	decl = TYPE_NAME (decl);
     }
 
-  decl = strip_using_decl (decl);
-
   /* Check to see that it is really the name of a class.  */
   if (TREE_CODE (decl) == TEMPLATE_ID_EXPR
       && identifier_p (TREE_OPERAND (decl, 0))
diff --git a/gcc/testsuite/g++.dg/lookup/using29.C b/gcc/testsuite/g++.dg/lookup/using29.C
index 428021c..697a654 100644
--- a/gcc/testsuite/g++.dg/lookup/using29.C
+++ b/gcc/testsuite/g++.dg/lookup/using29.C
@@ -51,7 +51,7 @@ struct I2 : H
 
 struct J
 {
-  struct type {};
+  struct type {}; // { dg-message "previous" }
 };
 
 struct K : J
@@ -62,8 +62,8 @@ struct K : J
 
 struct L : J
 {
-  using J::type; // { dg-message "previous" }
-  struct type {}; // { dg-error "conflicts" }
+  using J::type;
+  struct type {}; // { dg-error "redefinition" }
 };
 
 struct M
diff --git a/gcc/testsuite/g++.dg/lookup/using53.C b/gcc/testsuite/g++.dg/lookup/using53.C
index f9e59e6..e91829e 100644
--- a/gcc/testsuite/g++.dg/lookup/using53.C
+++ b/gcc/testsuite/g++.dg/lookup/using53.C
@@ -1,21 +1,23 @@
 // PR c++/20420
+// FIXME: These error messages aren't great, rather than multiple definition
+// we should talk about conflicting declarations with the using-decl.
 
 class B
 {
 protected:
-  enum E { E1, E2, E3 };
-  struct S { int i; E e; };
+  enum E { E1, E2, E3 };     // { dg-message "previous" }
+  struct S { int i; E e; };  // { dg-message "previous" }
 };
 
 class D : private B
 {
 public:
-  using B::E;       // { dg-message "previous" }
-  using B::S;       // { dg-message "previous" }
+  using B::E;
+  using B::S;
 
 private:
-  enum E {};        // { dg-error "conflicts" }
-  struct S {};      // { dg-error "conflicts" }
+  enum E {};        // { dg-error "multiple definition" }
+  struct S {};      // { dg-error "redefinition" }
 };
 
 template<typename T>
@@ -30,11 +32,11 @@ template<typename T>
 class DT : private BT<T>
 {
 public:
-  using BT<T>::E;   // { dg-message "previous" }
+  using BT<T>::E;   // { dg-message "previous" "PR c++/115806" { xfail *-*-* } }
   using BT<T>::S;   // { dg-message "previous" }
 
 private:
-  enum E {};        // { dg-error "conflicts" }
+  enum E {};        // { dg-error "conflicts" "PR c++/115806" { xfail *-*-* } }
   struct S {};      // { dg-error "conflicts" }
 };
 
@@ -50,5 +52,5 @@ void
 f ()
 {
   using N::i;
-  using N::i;       // { dg-error "redeclaration" }
+  using N::i;       // { dg-bogus "conflicts" "See P1787 (CWG36)" { xfail *-*-* } }
 }
diff --git a/gcc/testsuite/g++.dg/lookup/using68.C b/gcc/testsuite/g++.dg/lookup/using68.C
new file mode 100644
index 0000000..6eb2be5
--- /dev/null
+++ b/gcc/testsuite/g++.dg/lookup/using68.C
@@ -0,0 +1,7 @@
+// { dg-do compile }
+
+struct S {}; // { dg-message "previous" }
+void foo() {
+  using ::S;
+  struct S {};  // { dg-error "redefinition" }
+}
diff --git a/gcc/testsuite/g++.dg/modules/using-22_b.C b/gcc/testsuite/g++.dg/modules/using-22_b.C
index 0b66f4a..be122aa 100644
--- a/gcc/testsuite/g++.dg/modules/using-22_b.C
+++ b/gcc/testsuite/g++.dg/modules/using-22_b.C
@@ -7,7 +7,9 @@ int main()
   std::basic_string<char> s;
 
   // The inline namespace should not be exported, only the 'using' in std.
-  std::__cxx11::basic_string<char> s2; // { dg-error "has not been declared" "" { xfail *-*-* } }
+  std::__cxx11::basic_string<char> s2; // { dg-error "has not been declared" }
   // The non-exported using should also not be visible.
-  foo::basic_string<char> s3; // { dg-error "has not been declared" "" { xfail *-*-* } }
+  foo::basic_string<char> s3; // { dg-error "has not been declared" }
 }
+
+// { dg-prune-output "expected primary-expression" }
diff --git a/gcc/testsuite/g++.dg/modules/using-24_a.C b/gcc/testsuite/g++.dg/modules/using-24_a.C
new file mode 100644
index 0000000..872f4fc
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/using-24_a.C
@@ -0,0 +1,15 @@
+// { dg-additional-options "-fmodules-ts -Wno-global-module" }
+// { dg-module-cmi M }
+
+module;
+
+namespace foo {
+  struct S {} S;
+}
+
+export module M;
+
+namespace bar {
+  export using foo::S;
+  export using X = struct foo::S;
+}
diff --git a/gcc/testsuite/g++.dg/modules/using-24_b.C b/gcc/testsuite/g++.dg/modules/using-24_b.C
new file mode 100644
index 0000000..ea80b4c
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/using-24_b.C
@@ -0,0 +1,12 @@
+// { dg-additional-options "-fmodules-ts" }
+
+import M;
+namespace foo {}
+
+int main() {
+  struct bar::S a = bar::S;
+  bar::X b = bar::S;
+
+  struct foo::S c;  // { dg-error "does not name a type" }
+  auto d = foo::S;  // { dg-error "not a member" }
+}
diff --git a/gcc/testsuite/g++.dg/modules/using-25_a.C b/gcc/testsuite/g++.dg/modules/using-25_a.C
new file mode 100644
index 0000000..bc7dc5b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/using-25_a.C
@@ -0,0 +1,15 @@
+// { dg-additional-options "-fmodules-ts -Wno-global-module" }
+// { dg-module-cmi M }
+
+module;
+namespace A {
+  enum E1 { Exposed_1, Hidden_1 };
+  enum E2 { Exposed_2, Hidden_2 };
+}
+export module M;
+namespace A {
+  export using E1::Exposed_1;
+}
+namespace B {
+  export using A::E2::Exposed_2;
+}
diff --git a/gcc/testsuite/g++.dg/modules/using-25_b.C b/gcc/testsuite/g++.dg/modules/using-25_b.C
new file mode 100644
index 0000000..41fdbd1
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/using-25_b.C
@@ -0,0 +1,17 @@
+// { dg-additional-options "-fmodules-ts" }
+
+import M;
+
+int main() {
+  // These have been exported by using-decls and should be OK
+  auto a = A::Exposed_1;
+  auto b = B::Exposed_2;
+
+  // But we shouldn't have exposed the type names
+  A::E1 c;  // { dg-error "not a member" }
+  B::E2 d;  // { dg-error "not a member" }
+
+  // We also shouldn't have exposed any other enumerators
+  auto e = A::Hidden_1;  // { dg-error "not a member" }
+  auto f = B::Hidden_2;  // { dg-error "not a member" }
+}
diff --git a/gcc/testsuite/g++.dg/modules/using-enum-4_a.C b/gcc/testsuite/g++.dg/modules/using-enum-4_a.C
new file mode 100644
index 0000000..071f5ff
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/using-enum-4_a.C
@@ -0,0 +1,13 @@
+// PR c++/114683
+// { dg-do compile { target c++20 } }
+// { dg-additional-options "-fmodules-ts -Wno-global-module" }
+// { dg-module-cmi M:a }
+
+module;
+namespace foo {
+  enum class a { x, y, z };
+}
+export module M:a;
+namespace bar {
+  export using enum foo::a;
+}
diff --git a/gcc/testsuite/g++.dg/modules/using-enum-4_b.C b/gcc/testsuite/g++.dg/modules/using-enum-4_b.C
new file mode 100644
index 0000000..38e6081
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/using-enum-4_b.C
@@ -0,0 +1,7 @@
+// PR c++/114683
+// { dg-do compile { target c++20 } }
+// { dg-additional-options "-fmodules-ts" }
+// { dg-module-cmi M }
+
+export module M;
+export import :a;
diff --git a/gcc/testsuite/g++.dg/modules/using-enum-4_c.C b/gcc/testsuite/g++.dg/modules/using-enum-4_c.C
new file mode 100644
index 0000000..4f5965c
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/using-enum-4_c.C
@@ -0,0 +1,11 @@
+// PR c++/114683
+// { dg-do compile { target c++20 } }
+// { dg-additional-options "-fmodules-ts" }
+
+import M;
+int main() {
+  auto y = bar::y;
+
+  // This should not be made visible.
+  auto z = foo::a::z;  // { dg-error "not been declared" }
+}
diff --git a/gcc/testsuite/g++.dg/warn/Wunused-var-18.C b/gcc/testsuite/g++.dg/warn/Wunused-var-18.C
index 0339663..71331ba 100644
--- a/gcc/testsuite/g++.dg/warn/Wunused-var-18.C
+++ b/gcc/testsuite/g++.dg/warn/Wunused-var-18.C
@@ -4,11 +4,18 @@
 
 namespace N
 {
-    int i; // { dg-warning "unused variable" }
+    int i;
 }
 
 void
 f ()
 {
-    using N::i;
+    using N::i; // { dg-warning "unused using" }
+}
+
+void
+g ()
+{
+    using N::i; // { dg-bogus "set but not used" }
+    i = 10;
 }