From 90010c2e1d60c6a9a4a0b30a113d4dae2b7214eb Mon Sep 17 00:00:00 2001
From: Erich Keane <erich.keane@intel.com>
Date: Thu, 11 Nov 2021 12:55:45 -0800
Subject: [PATCH] Don't consider 'LinkageSpec' when calculating DeclContext
 'Encloses'

We don't properly handle lookup through using directives when there is
a linkage spec in the common chain.  This is because `CppLookupName` and
`CppNamespaceLookup` end up skipping `LinkageSpec`'s (correctly, as they
are not lookup scopes), but the `UnqualUsingDirectiveSet` does not.

I discovered that when we are calculating the `CommonAncestor` for a
using-directive, we were coming up with the `LinkageSpec`, instead of
the `LinkageSpec`'s parent.  Then, when we use
`UnqualUsingDirectiveSet::getNamespacesFor` a scope, we don't end up
finding any that were in the `LinkageSpec` (again, since `CppLookupName`
skips linkage specs), so those don't end up participating in the lookup.

The function `UnqualUsingDirectiveSet::addUsingDirective` calculates
this common ancestor via a loop through the the `DeclSpec::Encloses`
function.

Changing this Encloses function to believe that a `LinkageSpec`
`Encloses` nothing ends up fixing the problem without breaking any other tests,
so I opted to do that.  A less aggressive patch could perhaps change only
the `addUsingDirective`, but my examination of all uses of `Encloses`
showed that it seems to be used exclusively in lookup, which makes me think
this is correct everywhere.

Differential Revision: https://reviews.llvm.org/D113709
---
 clang/lib/AST/DeclBase.cpp                    |  2 +-
 clang/test/SemaCXX/lookup-through-linkage.cpp | 33 +++++++++++++++++++++++++++
 2 files changed, 34 insertions(+), 1 deletion(-)
 create mode 100644 clang/test/SemaCXX/lookup-through-linkage.cpp

diff --git a/clang/lib/AST/DeclBase.cpp b/clang/lib/AST/DeclBase.cpp
index d8eaf70..064012b 100644
--- a/clang/lib/AST/DeclBase.cpp
+++ b/clang/lib/AST/DeclBase.cpp
@@ -1212,7 +1212,7 @@ bool DeclContext::Encloses(const DeclContext *DC) const {
     return getPrimaryContext()->Encloses(DC);
 
   for (; DC; DC = DC->getParent())
-    if (DC->getPrimaryContext() == this)
+    if (!isa<LinkageSpecDecl>(DC) && DC->getPrimaryContext() == this)
       return true;
   return false;
 }
diff --git a/clang/test/SemaCXX/lookup-through-linkage.cpp b/clang/test/SemaCXX/lookup-through-linkage.cpp
new file mode 100644
index 0000000..0a2d609
--- /dev/null
+++ b/clang/test/SemaCXX/lookup-through-linkage.cpp
@@ -0,0 +1,33 @@
+// RUN: %clang_cc1 %s -verify
+
+// expected-no-diagnostics
+
+extern "C++" {
+namespace A {
+namespace B {
+int bar;
+}
+} // namespace A
+namespace C {
+void foo() {
+  using namespace A;
+  (void)B::bar;
+}
+} // namespace C
+}
+
+extern "C" {
+extern "C++" {
+namespace D {
+namespace E {
+int bar;
+}
+} // namespace A
+namespace F {
+void foo() {
+  using namespace D;
+  (void)E::bar;
+}
+} // namespace C
+}
+}
-- 
2.7.4