[gcc r15-1224] c++: repeated export using

[Jason Merrill]

https://gcc.gnu.org/g:074c1fc797435979c00b24aff2a4f895b8273bcf

commit r15-1224-g074c1fc797435979c00b24aff2a4f895b8273bcf
Author: Jason Merrill <jason@redhat.com>
Date:   Wed Jun 12 08:06:47 2024 -0400

    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.

Diff:
---
 gcc/cp/tree.cc                            |  8 ++++++--
 gcc/testsuite/g++.dg/modules/using-21_a.C | 11 +++++++++++
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/gcc/cp/tree.cc b/gcc/cp/tree.cc
index d2a8f79ffab4..28648c14c6da 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 000000000000..ce6e3f920f1e
--- /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;
+}