[clangd] Fix the crash in getQualification

[kbobyrev]

Summary

Happens when DestContext is LinkageSpecDecl and hense CurContext happens to be
both not TagDecl and NamespaceDecl.

Minimal reproducer: trigger define outline in

namespace ns {
extern "C" {
typedef int foo;
}
foo Fo^o(int id) { return id; }
}

[kbobyrev]

Fix formatting

[kadircet]

thanks for chasing this down! as mentioned offline a test case like:

namespace ns {
extern "C" {
typedef int foo;
}
foo Fo^o(int id) { return id; }
}

in define outline should be enough to ensure it is WAI.

clang-tools-extra/clangd/AST.cpp

122–130

rather than having !NNS in the check below it might be nicer to put:

else {
// Other types of contexts cannot be spelled in code, just skip over them.
continue;
}

I believe continue makes sense, that way we can spell in cases like:

namespace ns {
extern "C" {
typedef int foo;
}
foo Foo(int id) { return id; }
}

foo should still become ns::foo in other contexts.

[kbobyrev]

Make the controol flow less confusing.

[kbobyrev]

Add test, improve docs in code.

[kadircet]

thanks, lgtm!

clang-tools-extra/clangd/AST.cpp

for (const DeclContext *CurContext = SourceContext; CurContext;
 
     NestedNameSpecifier *NNS = nullptr;
     if (auto *TD = llvm::dyn_cast<TagDecl>(CurContext)) {
       // There can't be any more tag parents after hitting a namespace.
       assert(!ReachedNS);
       (void)ReachedNS;
       NNS = NestedNameSpecifier::Create(Context, nullptr, false,
                                         TD->getTypeForDecl());
-    } else {
+    } else
+      (auto *NSD = llvm::dyn_cast<NamespaceDecl>(CurContext)) {
         ReachedNS = true;
-      auto *NSD = llvm::cast<NamespaceDecl>(CurContext);
         NNS = NestedNameSpecifier::Create(Context, nullptr, NSD);
-      // Anonymous and inline namespace names are not spelled while qualifying a
-      // name, so skip those.
+        // Anonymous and inline namespace names are not spelled while qualifying
+        // a name, so skip those.
         if (NSD->isAnonymousNamespace() || NSD->isInlineNamespace())
           continue;
       }
     // Stop if this namespace is already visible at DestContext.
-    if (IsVisible(NNS))
+    if (!NNS || IsVisible(NNS))
       break;
 
     Parents.push_back(NNS);
   }
 
   // Go over name-specifiers in reverse order to create necessary qualification,
   // since we stored inner-most parent first.
   std::string Result;