Downgrade error about adding 'dllimport' to used free function to warning (PR24215)

The code will still work as it can reference the function via its thunk.

llvm-svn: 242973

2 files changed, 10 insertions, 4 deletions

diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index d25b4dda885..0eea0e689dc 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -5368,10 +5368,9 @@ static void checkDLLAttributeRedeclaration(Sema &S, NamedDecl *OldDecl,
   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 @@ static void checkDLLAttributeRedeclaration(Sema &S, NamedDecl *OldDecl,
         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)
diff --git a/clang/test/Sema/dllimport.c b/clang/test/Sema/dllimport.c
index 3ca1baa299c..d81aecd604c 100644
--- a/clang/test/Sema/dllimport.c
+++ b/clang/test/Sema/dllimport.c
@@ -143,7 +143,7 @@ __declspec(dllimport) void redecl3(); // expected-note{{previous declaration is
 
                       void redecl4(); // expected-note{{previous declaration is here}}
 void useRedecl4() { redecl4(); }
-__declspec(dllimport) void redecl4(); // expected-error{{redeclaration of 'redecl4' cannot add 'dllimport' attribute}}
+__declspec(dllimport) void redecl4(); // expected-warning{{redeclaration of 'redecl4' should not add 'dllimport' attribute}}
 
 // Allow with a warning if the decl hasn't been used yet.
                       void redecl5(); // expected-note{{previous declaration is here}}