[Sema] Fix a crash in access checking for deduction guides

[ilya-biryukov]

Summary

See the added test for a repro.

[sammccall]

clang/lib/Sema/SemaDeclCXX.cpp

3178

this isn't quite trivial, you might want to short circuit with &&, checking AS != TD->getAccess() first?
I don't think readability is too bad if you keep the existing comment.

[ilya-biryukov]

• Address comments

clang/lib/Sema/SemaDeclCXX.cpp

if (isInstField) {
     if (NonTemplateMember != Member)
       NonTemplateMember->setAccess(AS);
 
     // C++ [temp.deduct.guide]p3:
     //   A deduction guide [...] for a member class template [shall be
     //   declared] with the same access [as the template].
     if (auto *DG = dyn_cast<CXXDeductionGuideDecl>(NonTemplateMember)) {
       auto *TD = DG->getDeducedTemplate();
-      if (AS != TD->getAccess()) {
+      bool InSameScope = TD->getDeclContext()->getRedeclContext()->Equals(
+          DG->getDeclContext()->getRedeclContext());
+      // Access specifiers are only meaningful if both the template and the
+      // deduction guide are from the same scope.
+      if (InSameScope && AS != TD->getAccess()) {
         Diag(DG->getBeginLoc(), diag::err_deduction_guide_wrong_access);
         Diag(TD->getBeginLoc(), diag::note_deduction_guide_template_access)
             << TD->getAccess();
         const AccessSpecDecl *LastAccessSpec = nullptr;
         for (const auto *D : cast<CXXRecordDecl>(CurContext)->decls()) {
           if (const auto *AccessSpec = dyn_cast<AccessSpecDecl>(D))
             LastAccessSpec = AccessSpec;
         }

clang/test/Sema/crash-deduction-guide-access.cpp

+// RUN: not %clang_cc1 -x c++ -std=c++17 -fsyntax-only %s
+template <typename U>
+class Imp {
+  template <typename F>
+  explicit Imp(F f);
+};
+
+template <typename T>
+class Cls {
+  explicit Imp() : f() {}
+};