1 files changed, 10 insertions, 3 deletions
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 55b044d1c59..42f8dea6f6b 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -9938,10 +9938,17 @@ void Sema::AddInitializerToDecl(Decl *RealDecl, Expr *Init,
       VDecl->setInvalidDecl();
     }
   } else if (VDecl->isFileVarDecl()) {
+    // In C, extern is typically used to avoid tentative definitions when
+    // declaring variables in headers, but adding an intializer makes it a
+    // defintion. This is somewhat confusing, so GCC and Clang both warn on it.
+    // In C++, extern is often used to give implictly static const variables
+    // external linkage, so don't warn in that case. If selectany is present,
+    // this might be header code intended for C and C++ inclusion, so apply the
+    // C++ rules.
     if (VDecl->getStorageClass() == SC_Extern &&
-        (!getLangOpts().CPlusPlus ||
-         !(Context.getBaseElementType(VDecl->getType()).isConstQualified() ||
-           VDecl->isExternC())) &&
+        ((!getLangOpts().CPlusPlus && !VDecl->hasAttr<SelectAnyAttr>()) ||
+         !Context.getBaseElementType(VDecl->getType()).isConstQualified()) &&
+        !(getLangOpts().CPlusPlus && VDecl->isExternC()) &&
         !isTemplateInstantiation(VDecl->getTemplateSpecializationKind()))
       Diag(VDecl->getLocation(), diag::warn_extern_init);