diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index d25b4dd..0eea0e6 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -5368,10 +5368,9 @@
   bool AddsAttr = !(OldImportAttr || OldExportAttr) && HasNewAttr;
 
   if (AddsAttr && !IsSpecialization && !OldDecl->isImplicit()) {
-    // If the declaration hasn't been used yet, allow with a warning for
-    // free functions and global variables.
+    // Allow with a warning for free functions and global variables.
     bool JustWarn = false;
-    if (!OldDecl->isUsed() && !OldDecl->isCXXClassMember()) {
+    if (!OldDecl->isCXXClassMember()) {
       auto *VD = dyn_cast<VarDecl>(OldDecl);
       if (VD && !VD->getDescribedVarTemplate())
         JustWarn = true;
@@ -5380,6 +5379,13 @@
         JustWarn = true;
     }
 
+    // We cannot change a declaration that's been used because IR has already
+    // been emitted. Dllimported functions will still work though (modulo
+    // address equality) as they can use the thunk.
+    if (OldDecl->isUsed())
+      if (!isa<FunctionDecl>(OldDecl) || !NewImportAttr)
+        JustWarn = false;
+
     unsigned DiagID = JustWarn ? diag::warn_attribute_dll_redeclaration
                                : diag::err_attribute_dll_redeclaration;
     S.Diag(NewDecl->getLocation(), DiagID)