[COFF] Don't try to emit weak aliases on COFF

This comes up when a derived class destructor is equivalent to a base
class destructor defined in the same TU, and we try to alias them.

A COFF weak alias cannot satisfy a normal undefined symbol reference
from another TU. The other TU must also mark the referenced symbol as
weak, and we can't rely on that.

Clang already has a special case here for dllexport, but we failed to
realize that the problem also applies to other non-discardable symbols
such as those from explicit template instantiations.

Fixes PR25477.

llvm-svn: 252659

clang/lib/CodeGen/CGCXX.cpp

@@ -131,11 +131,6 @@ bool CodeGenModule::TryEmitDefinitionAsAlias(GlobalDecl AliasDecl,
   if (!llvm::GlobalAlias::isValidLinkage(Linkage))
     return true;
 
-  // Don't create a weak alias for a dllexport'd symbol.
-  if (AliasDecl.getDecl()->hasAttr<DLLExportAttr>() &&
-      llvm::GlobalValue::isWeakForLinker(Linkage))
-    return true;
-
   llvm::GlobalValue::LinkageTypes TargetLinkage =
       getFunctionLinkage(TargetDecl);
 
@@ -173,6 +168,16 @@ bool CodeGenModule::TryEmitDefinitionAsAlias(GlobalDecl AliasDecl,
     return false;
   }
 
+  // If we have a weak, non-discardable alias (weak, weak_odr), like an extern
+  // template instantiation or a dllexported class, avoid forming it on COFF.
+  // A COFF weak external alias cannot satisfy a normal undefined symbol
+  // reference from another TU. The other TU must also mark the referenced
+  // symbol as weak, which we cannot rely on.
+  if (llvm::GlobalValue::isWeakForLinker(Linkage) &&
+      getTriple().isOSBinFormatCOFF()) {
+    return true;
+  }
+
   if (!InEveryTU) {
     // If we don't have a definition for the destructor yet, don't
     // emit.  We can't emit aliases to declarations; that's just not