[CXX] Templates specialization visibility can be wrong

[steven_wu]

Summary

Under some conditions, LinkageComputer can get the visibility for
ClassTemplateSpecializationDecl wrong because it failed to find the Decl
that has the explicit visibility.

This fixes:
llvm.org/bugs/pr36810
rdar://problem/38080953

[doug.gregor]

lib/AST/Decl.cpp

1097

Do we want to look at *all* redeclarations, or only those declarations that precede the declaration that we found? The latter seems more correct, but IIRC visibility has often been able to "look forward" to declarations that come later in the translation unit.

[steven_wu]

Address review feedback

[steven_wu]

lib/AST/Decl.cpp

1097

I agree the latter is more correct. Update the patch.

[doug.gregor]

LGTM, thanks

lib/AST/Decl.cpp

if (const auto *RD = dyn_cast<CXXRecordDecl>(ND)) {
     CXXRecordDecl *InstantiatedFrom = RD->getInstantiatedFromMemberClass();
     if (InstantiatedFrom)
       return getVisibilityOf(InstantiatedFrom, kind);
   }
 
   // If there wasn't explicit visibility there, and this is a
   // specialization of a class template, check for visibility
   // on the pattern.
-  if (const auto *spec = dyn_cast<ClassTemplateSpecializationDecl>(ND))
-    return getVisibilityOf(spec->getSpecializedTemplate()->getTemplatedDecl(),
-                           kind);
+  if (const auto *spec = dyn_cast<ClassTemplateSpecializationDecl>(ND)) {
+    // Walk all the template decl till this point to see if there are
+    // explicit visibility attributes.
+    const auto *TD = spec->getSpecializedTemplate()->getTemplatedDecl();
+    while (TD != nullptr) {
+      auto Vis = getVisibilityOf(TD, kind);
+      if (Vis != None)
+        return Vis;
+      TD = TD->getPreviousDecl();
+    }
+    return None;
+  }
 
   // Use the most recent declaration.
   if (!IsMostRecent && !isa<NamespaceDecl>(ND)) {
     const NamedDecl *MostRecent = ND->getMostRecentDecl();
     if (MostRecent != ND)
       return getExplicitVisibilityAux(MostRecent, kind, true);
   }
 

test/CodeGenCXX/visibility-pr36810.cpp

+// RUN: %clang_cc1 -triple x86_64-apple-macosx -std=c++11 -fvisibility hidden -emit-llvm -o - %s -O2 -disable-llvm-passes | FileCheck %s
+// RUN: %clang_cc1 -triple x86_64-apple-macosx -DUNDEF_G -std=c++11 -fvisibility hidden -emit-llvm -o - %s -O2 -disable-llvm-passes | FileCheck %s
+
+namespace std {
+template <class>
+class __attribute__((__type_visibility__("default"))) shared_ptr {
+  template <class> friend class shared_ptr;
+};
+}
+struct dict;
+#ifndef UNDEF_G
+std::shared_ptr<dict> g;
+#endif
+class __attribute__((visibility("default"))) Bar;
+template <class = std::shared_ptr<Bar>>
+class __attribute__((visibility("default"))) i {
+  std::shared_ptr<int> foo() const;
+};
+
+// CHECK: define void @_ZNK1iISt10shared_ptrI3BarEE3fooEv
+template <> std::shared_ptr<int> i<>::foo() const {
+  return std::shared_ptr<int>();
+}