Index: clang/docs/ReleaseNotes.rst
===================================================================
--- clang/docs/ReleaseNotes.rst
+++ clang/docs/ReleaseNotes.rst
@@ -111,9 +111,17 @@
 C++ Language Changes in Clang
 -----------------------------
 
+- The oldest supported GNU libstdc++ is now 4.8.3 (released
+  2014-05-22).  Clang workarounds for bugs in earlier versions have
+  been removed.
+
 - ...
 
-C++1z Feature Support
+C++20 Feature Support
+^^^^^^^^^^^^^^^^^^^^^
+...
+
+C++2b Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
 ...
 
Index: clang/lib/Sema/SemaDeclCXX.cpp
===================================================================
--- clang/lib/Sema/SemaDeclCXX.cpp
+++ clang/lib/Sema/SemaDeclCXX.cpp
@@ -10874,26 +10874,6 @@
                                             NamespaceDecl *PrevNS) {
   assert(*IsInline != PrevNS->isInline());
 
-  // HACK: Work around a bug in libstdc++4.6's <atomic>, where
-  // std::__atomic[0,1,2] are defined as non-inline namespaces, then reopened as
-  // inline namespaces, with the intention of bringing names into namespace std.
-  //
-  // We support this just well enough to get that case working; this is not
-  // sufficient to support reopening namespaces as inline in general.
-  if (*IsInline && II && II->getName().startswith("__atomic") &&
-      S.getSourceManager().isInSystemHeader(Loc)) {
-    // Mark all prior declarations of the namespace as inline.
-    for (NamespaceDecl *NS = PrevNS->getMostRecentDecl(); NS;
-         NS = NS->getPreviousDecl())
-      NS->setInline(*IsInline);
-    // Patch up the lookup table for the containing namespace. This isn't really
-    // correct, but it's good enough for this particular case.
-    for (auto *I : PrevNS->decls())
-      if (auto *ND = dyn_cast<NamedDecl>(I))
-        PrevNS->getParent()->makeDeclVisibleInContext(ND);
-    return;
-  }
-
   if (PrevNS->isInline())
     // The user probably just forgot the 'inline', so suggest that it
     // be added back.
Index: clang/test/SemaCXX/libstdcxx_atomic_ns_hack.cpp
===================================================================
--- clang/test/SemaCXX/libstdcxx_atomic_ns_hack.cpp
+++ clang/test/SemaCXX/libstdcxx_atomic_ns_hack.cpp
@@ -7,29 +7,28 @@
 // namespace to be converted from non-inline to inline in this one specific
 // case.
 
+// the last 4.6 release was 2013, so the hack is removed.  This checks __atomic
+// is not special.
 #ifdef BE_THE_HEADER
 
 #pragma clang system_header
 
 namespace std {
-  namespace __atomic0 {
-    typedef int foobar;
-  }
-  namespace __atomic1 {
-    typedef void foobar;
-  }
+namespace __atomic0 { // expected-note {{previous definition}}
+typedef int foobar;
+} // namespace __atomic0
+namespace __atomic1 {
+typedef void foobar;
+} // namespace __atomic1
 
-  inline namespace __atomic0 {}
-}
+inline namespace __atomic0 {} // expected-error {{cannot be reopened as inline}}
+} // namespace std
 
 #else
 
 #define BE_THE_HEADER
 #include "libstdcxx_atomic_ns_hack.cpp"
 
-std::foobar fb;
-
-using T = void; // expected-note {{here}}
-using T = std::foobar; // expected-error {{different types ('std::foobar' (aka 'int') vs 'void')}}
+std::foobar fb; // expected-error {{no type named 'foobar' in namespace}}
 
 #endif