c++: Standardise errors for module_may_redeclare

Currently different places calling 'module_may_redeclare' all emit very
similar but slightly different error messages, and handle different
kinds of declarations differently.  This patch makes the function
perform its own error messages so that they're all in one place, and
prepares it for use with temploid friends.

gcc/cp/ChangeLog:

	* cp-tree.h (module_may_redeclare): Add default parameter.
	* decl.cc (duplicate_decls): Don't emit errors for failed
	module_may_redeclare.
	(xref_tag): Likewise.
	(start_enum): Likewise.
	* semantics.cc (begin_class_definition): Likewise.
	* module.cc (module_may_redeclare): Clean up logic. Emit error
	messages on failure.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/enum-12.C: Update error message.
	* g++.dg/modules/friend-5_b.C: Likewise.
	* g++.dg/modules/shadow-1_b.C: Likewise.

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

4 files changed, 86 insertions, 70 deletions

diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index bafdf63..4ac8cf6 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -7401,7 +7401,7 @@ inline bool module_exporting_p ()
 
 extern module_state *get_module (tree name, module_state *parent = NULL,
 				 bool partition = false);
-extern bool module_may_redeclare (tree decl);
+extern bool module_may_redeclare (tree olddecl, tree newdecl = NULL);
 
 extern bool module_global_init_needed ();
 extern bool module_determine_import_inits ();
diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index 2af026d..91268ff 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -2279,18 +2279,8 @@ duplicate_decls (tree newdecl, tree olddecl, bool hiding, bool was_hidden)
       && TREE_CODE (olddecl) != NAMESPACE_DECL
       && !hiding)
     {
-      if (!module_may_redeclare (olddecl))
-	{
-	  if (DECL_ARTIFICIAL (olddecl))
-	    error ("declaration %qD conflicts with builtin", newdecl);
-	  else
-	    {
-	      error ("declaration %qD conflicts with import", newdecl);
-	      inform (olddecl_loc, "import declared %q#D here", olddecl);
-	    }
-
-	  return error_mark_node;
-	}
+      if (!module_may_redeclare (olddecl, newdecl))
+	return error_mark_node;
 
       tree not_tmpl = STRIP_TEMPLATE (olddecl);
       if (DECL_LANG_SPECIFIC (not_tmpl)
@@ -16626,12 +16616,7 @@ xref_tag (enum tag_types tag_code, tree name,
 	{
 	  tree decl = TYPE_NAME (t);
 	  if (!module_may_redeclare (decl))
-	    {
-	      auto_diagnostic_group d;
-	      error ("cannot declare %qD in a different module", decl);
-	      inform (DECL_SOURCE_LOCATION (decl), "previously declared here");
-	      return error_mark_node;
-	    }
+	    return error_mark_node;
 
 	  tree not_tmpl = STRIP_TEMPLATE (decl);
 	  if (DECL_LANG_SPECIFIC (not_tmpl)
@@ -16979,12 +16964,7 @@ start_enum (tree name, tree enumtype, tree underlying_type,
 	{
 	  tree decl = TYPE_NAME (enumtype);
 	  if (!module_may_redeclare (decl))
-	    {
-	      auto_diagnostic_group d;
-	      error ("cannot declare %qD in different module", decl);
-	      inform (DECL_SOURCE_LOCATION (decl), "previously declared here");
-	      enumtype = error_mark_node;
-	    }
+	    enumtype = error_mark_node;
 	  else
 	    set_instantiating_module (decl);
 	}
diff --git a/gcc/cp/module.cc b/gcc/cp/module.cc
index 3bf863e..c2f077d 100644
--- a/gcc/cp/module.cc
+++ b/gcc/cp/module.cc
@@ -19003,11 +19003,15 @@ get_importing_module (tree decl, bool flexible)
   return module->mod;
 }
 
-/* Is it permissible to redeclare DECL.  */
+/* Is it permissible to redeclare OLDDECL with NEWDECL.
+
+   If NEWDECL is NULL, assumes that OLDDECL will be redeclared using
+   the current scope's module and attachment.  */
 
 bool
-module_may_redeclare (tree decl)
+module_may_redeclare (tree olddecl, tree newdecl)
 {
+  tree decl = olddecl;
   for (;;)
     {
       tree ctx = CP_DECL_CONTEXT (decl);
@@ -19021,58 +19025,94 @@ module_may_redeclare (tree decl)
       decl = TYPE_NAME (ctx);
     }
 
-  tree not_tmpl = STRIP_TEMPLATE (decl);
-
   int use_tpl = 0;
-  if (node_template_info (not_tmpl, use_tpl) && use_tpl)
+  if (node_template_info (STRIP_TEMPLATE (decl), use_tpl) && use_tpl)
     // Specializations of any kind can be redeclared anywhere.
     // FIXME: Should we be checking this in more places on the scope chain?
     return true;
 
-  if (!DECL_LANG_SPECIFIC (not_tmpl) || !DECL_MODULE_ATTACH_P (not_tmpl))
-    // Decl is attached to global module.  Current scope needs to be too.
-    return !module_attach_p ();
+  module_state *old_mod = (*modules)[0];
+  module_state *new_mod = old_mod;
 
-  module_state *me = (*modules)[0];
-  module_state *them = me;
+  tree old_origin = get_originating_module_decl (decl);
+  tree old_inner = STRIP_TEMPLATE (old_origin);
+  bool olddecl_attached_p = (DECL_LANG_SPECIFIC (old_inner)
+			     && DECL_MODULE_ATTACH_P (old_inner));
+  if (DECL_LANG_SPECIFIC (old_inner) && DECL_MODULE_IMPORT_P (old_inner))
+    {
+      unsigned index = import_entity_index (old_origin);
+      old_mod = import_entity_module (index);
+    }
 
-  if (DECL_LANG_SPECIFIC (not_tmpl) && DECL_MODULE_IMPORT_P (not_tmpl))
+  bool newdecl_attached_p = module_attach_p ();
+  if (newdecl)
     {
-      /* We can be given the TEMPLATE_RESULT.  We want the
-	 TEMPLATE_DECL.  */
-      int use_tpl = -1;
-      if (tree ti = node_template_info (decl, use_tpl))
+      tree new_origin = get_originating_module_decl (newdecl);
+      tree new_inner = STRIP_TEMPLATE (new_origin);
+      newdecl_attached_p = (DECL_LANG_SPECIFIC (new_inner)
+			    && DECL_MODULE_ATTACH_P (new_inner));
+      if (DECL_LANG_SPECIFIC (new_inner) && DECL_MODULE_IMPORT_P (new_inner))
 	{
-	  tree tmpl = TI_TEMPLATE (ti);
-	  if (use_tpl == 2)
-	    {
-	      /* A partial specialization.  Find that specialization's
-		 template_decl.  */
-	      for (tree list = DECL_TEMPLATE_SPECIALIZATIONS (tmpl);
-		   list; list = TREE_CHAIN (list))
-		if (DECL_TEMPLATE_RESULT (TREE_VALUE (list)) == decl)
-		  {
-		    decl = TREE_VALUE (list);
-		    break;
-		}
-	    }
-	  else if (DECL_TEMPLATE_RESULT (tmpl) == decl)
-	    decl = tmpl;
+	  unsigned index = import_entity_index (new_origin);
+	  new_mod = import_entity_module (index);
 	}
-      unsigned index = import_entity_index (decl);
-      them = import_entity_module (index);
     }
 
-  // Decl is attached to named module.  Current scope needs to be
-  // attaching to the same module.
-  if (!module_attach_p ())
-    return false;
+  /* Module attachment needs to match.  */
+  if (olddecl_attached_p == newdecl_attached_p)
+    {
+      if (!olddecl_attached_p)
+	/* Both are GM entities, OK.  */
+	return true;
 
-  // Both attached to named module.
-  if (me == them)
-    return true;
+      if (new_mod == old_mod
+	  || (new_mod && get_primary (new_mod) == get_primary (old_mod)))
+	/* Both attached to same named module, OK.  */
+	return true;
+    }
+
+  /* Attached to different modules, error.  */
+  decl = newdecl ? newdecl : olddecl;
+  location_t loc = newdecl ? DECL_SOURCE_LOCATION (newdecl) : input_location;
+  if (DECL_IS_UNDECLARED_BUILTIN (olddecl))
+    error_at (loc, "declaration %qD conflicts with builtin", decl);
+  else if (DECL_LANG_SPECIFIC (old_inner) && DECL_MODULE_IMPORT_P (old_inner))
+    {
+      auto_diagnostic_group d;
+      if (newdecl_attached_p)
+	error_at (loc, "redeclaring %qD in module %qs conflicts with import",
+		  decl, new_mod->get_flatname ());
+      else
+	error_at (loc, "redeclaring %qD in global module conflicts with import",
+		  decl);
 
-  return me && get_primary (them) == get_primary (me);
+      if (olddecl_attached_p)
+	inform (DECL_SOURCE_LOCATION (olddecl),
+		"import declared attached to module %qs",
+		old_mod->get_flatname ());
+      else
+	inform (DECL_SOURCE_LOCATION (olddecl),
+		"import declared in global module");
+    }
+  else
+    {
+      auto_diagnostic_group d;
+      if (newdecl_attached_p)
+	error_at (loc, "conflicting declaration of %qD in module %qs",
+		  decl, new_mod->get_flatname ());
+      else
+	error_at (loc, "conflicting declaration of %qD in global module",
+		  decl);
+
+      if (olddecl_attached_p)
+	inform (DECL_SOURCE_LOCATION (olddecl),
+		"previously declared in module %qs",
+		old_mod->get_flatname ());
+      else
+	inform (DECL_SOURCE_LOCATION (olddecl),
+		"previously declared in global module");
+    }
+  return false;
 }
 
 /* DECL is being created by this TU.  Record it came from here.  We
diff --git a/gcc/cp/semantics.cc b/gcc/cp/semantics.cc
index 02c7c1b..2dde65a 100644
--- a/gcc/cp/semantics.cc
+++ b/gcc/cp/semantics.cc
@@ -3777,11 +3777,7 @@ begin_class_definition (tree t)
   if (modules_p ())
     {
       if (!module_may_redeclare (TYPE_NAME (t)))
-	{
-	  error ("cannot declare %qD in a different module", TYPE_NAME (t));
-	  inform (DECL_SOURCE_LOCATION (TYPE_NAME (t)), "declared here");
-	  return error_mark_node;
-	}
+	return error_mark_node;
       set_instantiating_module (TYPE_NAME (t));
       set_defining_module (TYPE_NAME (t));
     }