From 15b8e2af42168a41636186c53d1e833302195601 Mon Sep 17 00:00:00 2001
From: Nathan Sidwell <nathan@acm.org>
Date: Tue, 20 Oct 2020 12:17:21 -0700
Subject: [PATCH] c++: block-scope extern decl with default args

In adding the DECL_LOCAL_DECL handling, I'd forgotten that the
parm-decls also need cloning -- and resetting of their DECL_CONTEXT.
Also, any default args need droping when adding an alias, as those are
not propagated.  The std's not totally clear on this latter point when
there's no exising namespace decl, but that seems like the right thing
and is what clang does.

	gcc/cp/
	* name-lookup.c (push_local_extern_decl_alias): Reconstextualize
	alias' parm decls.  Drop any default args.
	gcc/testsuite/
	* g++.dg/lookup/local-extern.C: New.
---
 gcc/cp/name-lookup.c                       | 46 ++++++++++++++++++++++
 gcc/testsuite/g++.dg/lookup/local-extern.C | 13 ++++++
 2 files changed, 59 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/lookup/local-extern.C

diff --git a/gcc/cp/name-lookup.c b/gcc/cp/name-lookup.c
index e951fb7885b..46374278068 100644
--- a/gcc/cp/name-lookup.c
+++ b/gcc/cp/name-lookup.c
@@ -2969,6 +2969,52 @@ push_local_extern_decl_alias (tree decl)
 	{
 	  /* No existing namespace-scope decl.  Make one.  */
 	  alias = copy_decl (decl);
+	  if (TREE_CODE (alias) == FUNCTION_DECL)
+	    {
+	      /* Recontextualize the parms.  */
+	      for (tree *chain = &DECL_ARGUMENTS (alias);
+		   *chain; chain = &DECL_CHAIN (*chain))
+		{
+		  *chain = copy_decl (*chain);
+		  DECL_CONTEXT (*chain) = alias;
+		}
+
+	      tree type = TREE_TYPE (alias);
+	      for (tree args = TYPE_ARG_TYPES (type);
+		   args; args = TREE_CHAIN (args))
+		if (TREE_PURPOSE (args))
+		  {
+		    /* There are default args.  Lose them.  */
+		    tree nargs = NULL_TREE;
+		    tree *chain = &nargs;
+		    for (args = TYPE_ARG_TYPES (type);
+			 args; args = TREE_CHAIN (args))
+		      if (args == void_list_node)
+			{
+			  *chain = args;
+			  break;
+			}
+		      else
+			{
+			  *chain
+			    = build_tree_list (NULL_TREE, TREE_VALUE (args));
+			  chain = &TREE_CHAIN (*chain);
+			}
+
+		    tree fn_type = build_function_type (TREE_TYPE (type), nargs);
+
+		    fn_type = apply_memfn_quals
+		      (fn_type, type_memfn_quals (type));
+
+		    fn_type = build_cp_fntype_variant
+		      (fn_type, type_memfn_rqual (type),
+		       TYPE_RAISES_EXCEPTIONS (type),
+		       TYPE_HAS_LATE_RETURN_TYPE (type));
+
+		    TREE_TYPE (alias) = fn_type;
+		    break;
+		  }
+	    }
 
 	  /* This is the real thing.  */
 	  DECL_LOCAL_DECL_P (alias) = false;
diff --git a/gcc/testsuite/g++.dg/lookup/local-extern.C b/gcc/testsuite/g++.dg/lookup/local-extern.C
new file mode 100644
index 00000000000..1d6d8617bde
--- /dev/null
+++ b/gcc/testsuite/g++.dg/lookup/local-extern.C
@@ -0,0 +1,13 @@
+int foo ()
+{
+  extern int baz (int i = 5);
+  return baz ();
+}
+
+int baz (int i = 0);
+
+int bar ()
+{
+  extern int baz (int i = 6);
+  return baz ();
+}
-- 
2.30.2