False positive of -Wunused-local-typedef in dependent code.

[hokein]

Given the following code:

#define TYPE_OR_NONTYPE  typename
template<TYPE_OR_NONTYPE T>
struct BaseT {
  static int ff();
  void Foo();
};

template<TYPE_OR_NONTYPE T>
struct DerivedT : BaseT<T> {
  void Inner();
};

template<TYPE_OR_NONTYPE T>
void DerivedT<T>::Inner() {
  using K = BaseT<T>; 
  this->K::Foo(); 
}

clang emits a unused typedef warning.

⇒  ./bin/clang -Xclang  -fsyntax-only -Wunused-local-typedef  --std=c++20 /tmp/t2.cpp
/tmp/t2.cpp:15:9: warning: unused type alias 'K' [-Wunused-local-typedef]
   15 |   using K = BaseT<T>;
      |  

This is shown in abseil/abseil-cpp#1711.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Haojian Wu (hokein)

Given the following code:

#define TYPE_OR_NONTYPE  typename
template<TYPE_OR_NONTYPE T>
struct BaseT {
  static int ff();
  void Foo();
};

template<TYPE_OR_NONTYPE T>
struct DerivedT : BaseT<T> {
  void Inner();
};

template<TYPE_OR_NONTYPE T>
void DerivedT<T>::Inner() {
  using K = BaseT<T>; 
  this->K::Foo(); 
}

clang emits a unused typedef warning.

⇒  ./bin/clang -Xclang  -fsyntax-only -Wunused-local-typedef  --std=c++20 /tmp/t2.cpp
/tmp/t2.cpp:15:9: warning: unused type alias 'K' [-Wunused-local-typedef]
   15 |   using K = BaseT<T>;
      |  

This is shown in abseil/abseil-cpp#1711.

[nico]

This doesn't repro for me, using a 3-week old clang:

thakis@Nicos-MacBook-Pro llvm-project % cat test.cc
#define TYPE_OR_NONTYPE  typename

template<TYPE_OR_NONTYPE T>
struct BaseT {
  static int ff();
  void Foo();
};

template<TYPE_OR_NONTYPE T>
struct DerivedT : BaseT<T> {
  void Inner();
};

template<TYPE_OR_NONTYPE T>
void DerivedT<T>::Inner() {
  using K = BaseT<T>; 
  this->K::Foo(); 
}
thakis@Nicos-MacBook-Pro llvm-project % out/gn/bin/clang  -Xclang  -fsyntax-only -Wunused-local-typedef  -std=c++20 test.cc
-macosx_version_min has been renamed to -macos_version_min
-macosx_version_min has been renamed to -macos_version_min
ld: library 'System' not found
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Am I holding it wrong? Does it require a very recent clang to repro?

[hokein]

It can be reproduced in trunk, https://godbolt.org/z/Go4751bd8, looks like a recent regression.

[glandium]

I was definitely getting the warning on clang 18 in the non-reduced case in abseil.

[glandium]

This can be reproduced with clang 18 with:

#include <utility>

#define TYPE_OR_NONTYPE typename
template<TYPE_OR_NONTYPE T>
struct BaseT {
  static int ff();
  void Foo();
};

template<TYPE_OR_NONTYPE T>
struct DerivedT : BaseT<T> {
  void Inner();
};

template<TYPE_OR_NONTYPE T>
void DerivedT<T>::Inner() {
  using K = BaseT<T>; 
  std::move(*this)->K::Foo(); 
}

[shafik]

This looks like what you mean I think: https://godbolt.org/z/7nrdeM535

#define TYPE_OR_NONTYPE  typename
template<TYPE_OR_NONTYPE T>
struct BaseT {
  static int ff();
  void Foo();
};

template<TYPE_OR_NONTYPE T>
struct DerivedT : BaseT<T> {
  void Inner();
};

template<TYPE_OR_NONTYPE T>
void DerivedT<T>::Inner() {
  using K = BaseT<T>; 
  auto s = *this;
  s.K::Foo(); 
}

Looks like it goes back a while.