c++: repeated export using

A sample implementation of module std was breaking because the exports
included 'using std::operator&' twice.  Since Nathaniel's r15-964 for
PR114867, the first using added an extra instance of each function that was
revealed/exported by that using, resulting in duplicates for
lookup_maybe_add to dedup.  But if the duplicate is the first thing in the
list, lookup_add doesn't make an OVERLOAD, so trying to set OVL_USING_P
crashes.  Fixed by using ovl_make in the case where we want to set the flag.

gcc/cp/ChangeLog:

	* tree.cc (lookup_maybe_add): Use ovl_make when setting OVL_USING_P.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/using-21_a.C: New test.

2 files changed, 17 insertions, 2 deletions

diff --git a/gcc/cp/tree.cc b/gcc/cp/tree.cc
index d2a8f79..28648c1 100644
--- a/gcc/cp/tree.cc
+++ b/gcc/cp/tree.cc
@@ -2526,11 +2526,15 @@ lookup_maybe_add (tree fns, tree lookup, bool deduping)
 	       predecessors onto the lookup.  */
 	    for (; fns != probe; fns = OVL_CHAIN (fns))
 	      {
-		lookup = lookup_add (OVL_FUNCTION (fns), lookup);
 		/* Propagate OVL_USING, but OVL_HIDDEN &
 		   OVL_DEDUP_P don't matter.  */
 		if (OVL_USING_P (fns))
-		  OVL_USING_P (lookup) = true;
+		  {
+		    lookup = ovl_make (OVL_FUNCTION (fns), lookup);
+		    OVL_USING_P (lookup) = true;
+		  }
+		else
+		  lookup = lookup_add (OVL_FUNCTION (fns), lookup);
 	      }
 
 	    /* And now skip this function.  */
diff --git a/gcc/testsuite/g++.dg/modules/using-21_a.C b/gcc/testsuite/g++.dg/modules/using-21_a.C
new file mode 100644
index 0000000..ce6e3f9
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/using-21_a.C
@@ -0,0 +1,11 @@
+// { dg-additional-options "-fmodules-ts -Wno-global-module" }
+
+module;
+namespace foo {
+  void baz();
+}
+export module foo;
+namespace foo {
+  export using foo::baz;
+  export using foo::baz;
+}