[gcc r15-1296] c++/modules: multiple usings of the same decl [PR115194]

[Jason Merrill]

https://gcc.gnu.org/g:8878fecc251762bc32c24e659695557797e03fd9

commit r15-1296-g8878fecc251762bc32c24e659695557797e03fd9
Author: Jason Merrill <jason@redhat.com>
Date:   Wed Jun 12 21:44:10 2024 -0400

    c++/modules: multiple usings of the same decl [PR115194]
    
    add_binding_entity creates an OVERLOAD to represent a using-declaration in
    module purview of a declaration in the global module, even for
    non-functions, and we were failing to merge that with the original
    declaration in name lookup.
    
    It's not clear to me that building the OVERLOAD is what should be happening,
    but let's work around it for now pending an overhaul of using-decl handling
    for c++/114683.
    
            PR c++/115194
    
    gcc/cp/ChangeLog:
    
            * name-lookup.cc (name_lookup::process_module_binding): Strip an
            OVERLOAD from a non-function.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/modules/using-23_a.C: New test.
            * g++.dg/modules/using-23_b.C: New test.

Diff:
---
 gcc/cp/name-lookup.cc                     | 11 +++++++++++
 gcc/testsuite/g++.dg/modules/using-23_a.C | 19 +++++++++++++++++++
 gcc/testsuite/g++.dg/modules/using-23_b.C |  7 +++++++
 3 files changed, 37 insertions(+)

diff --git a/gcc/cp/name-lookup.cc b/gcc/cp/name-lookup.cc
index 3d3e20f48cbd..71482db7b766 100644
--- a/gcc/cp/name-lookup.cc
+++ b/gcc/cp/name-lookup.cc
@@ -827,6 +827,17 @@ 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);
 
diff --git a/gcc/testsuite/g++.dg/modules/using-23_a.C b/gcc/testsuite/g++.dg/modules/using-23_a.C
new file mode 100644
index 000000000000..e7e6fecbea65
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/using-23_a.C
@@ -0,0 +1,19 @@
+// PR c++/115194
+// { dg-additional-options "-fmodules-ts -Wno-global-module" }
+
+module;
+
+namespace NS1 {
+  namespace NS2 {
+    class Thing {};
+  } // NS2
+  using NS2::Thing;
+} // NS1
+
+export module modA;
+
+export
+namespace NS1 {
+  using ::NS1::Thing;
+  namespace NS2 { }
+}
diff --git a/gcc/testsuite/g++.dg/modules/using-23_b.C b/gcc/testsuite/g++.dg/modules/using-23_b.C
new file mode 100644
index 000000000000..6502c476b9b1
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/using-23_b.C
@@ -0,0 +1,7 @@
+// { dg-additional-options "-fmodules-ts" }
+
+import modA;
+
+using NS1::Thing;
+using namespace NS1::NS2;
+Thing thing;