Language Service: Unable to resolve type when managing namespaces via header and preprocessor

Reproduce the behavior using main.cpp and namespace.hpp from the issue, then compare resolution for b::Foo with a:🅱️:Foo and Foo. Investigate the language service's handling of preprocessor-managed nested namespaces and verify that main.cpp has no error squiggles and that b::Foo resolves to its declaration.

Bug type: Language Service

Describe the bug

• OS and Version: Windows 10 Pro 21H1 19043.1288
• VS Code Version: 1.61.2 (user setup)
• C/C++ Extension Version: v1.7.1
• Other extensions you installed (and if the issue persists after disabling them):
  • Local
    • jeff-hykin.better-cpp-syntax (disabled)
    • usernamehw.errorlens (disabled)
    • ms-vscode-remote.remote-ssh
    • ms-vscode-remote.remote-ssh-edit (disabled)
    • ms-vscode-remote.remote-wsl (disabled)
  • SSH
    • ms-vscode.cpptools
    • twxs.cmake (disabled)
    • ms-vscode.cmake-tools (disabled)
    • mhutchie.git-graph (disabled)
    • eamodio.gitlens (disabled)
    • ms-python.vscode-pylance (disabled)
    • ms-python.python (disabled)
• If using SSH remote, specify OS of remote machine: Ubuntu 20.04.3 LTS
• When managing a nested namespace with the preprocessor via a header file, a type inside the nested namespace is not recognized from inside the namespace when specifying the inner namespace explicitly. See Steps to reproduce for clarification.

Steps to reproduce

1. Open a folder with the following two files:

   // main.cpp
   
   #include "namespace.hpp"
   
   class Foo {};
   
   b::Foo *foo;
   
   #include "namespace.hpp"
   
   int main() {}
   

   // namespace.hpp
   
   #ifndef NAMESPACE_A_B_OPEN
       #define NAMESPACE_A_B_OPEN
       namespace a {
           namespace b {
   #else
           }
       }
   #endif
   

2. See the following errors:

   • b:: has error squiggles with the following two problems:

   this declaration has no storage class or type specifier C/C++(77)
   

   expected a ';'C/C++(65)
   

   • b::Foo does not correctly link to its declaration above when pressing CTRL+CLICK and shows no information when hovering the mouse over it.

   • endif has error squiggles with the following two problems (this one might be unrelated to the other errors - I don't really care about this one):

   PCH warning: header stop not at file scope.  An IntelliSense PCH file was not generated. C/C++(2923)
   

   expected a '}' C/C++(67)
   

Expected behavior

No error squiggles in main.cpp and correctly resolving b::Foo.

Additional context

The following changes to the code circumvent the problem:

• Specify the outer namespace as well:

  -b::Foo *foo;
  +a::b::Foo *foo;
  

• Don't specify the inner namespace:

  -b::Foo *foo;
  +Foo *foo;
  

• Manually including namespace.hpp by copying the content of namespace.hpp to the two lines with #include "namespace.hpp".