clang incorrectly emits `-Wundefined-func-template` warning

[ahatanak]

$ cat test.cpp

class c {
protected:
  virtual ~c();
  virtual int *d() const;
};

template <typename> class e : c {
  int *d() const override;
};

int t = sizeof(e<int>);

$ clang++ -c -std=gnu++2b -Wundefined-func-template test.cpp

test.cpp:7:27: warning: instantiation of function 'e::d' required here, but no definition is available [-Wundefined-func-template]
7 | template class e : c {

I don’t think instantiation of e::d is needed to compute the size of the class.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Akira Hatanaka (ahatanak)

$ cat test.cpp ``` class c { protected: virtual ~c(); virtual int *d() const; };

template <typename> class e : c {
int *d() const override;
};

int t = sizeof(e<int>);

$ clang++ -c -std=gnu++2b -Wundefined-func-template test.cpp

test.cpp:7:27: warning: instantiation of function 'e<int>::d' required here, but no definition is available [-Wundefined-func-template]
    7 | template <typename> class e : c {

I don’t think instantiation of e<int>::d is needed to compute the size of the class.
</details>

[ahatanak]

It looks like clang started emitting the warning after 99500e8. clang doesn't warn if I revert the commit.

[shafik]

CC @Fznamznon

[MitalAshok]

This also happens in Clang 18 if we have https://godbolt.org/z/7GzabWKxq

class c {
protected:
  virtual ~c();
  virtual int d() const;
};

template <typename> class e : c {
  constexpr ~e() = default;
  int d() const override;
};

int t = sizeof(e<int>);

This appears to have just been exposed when the implicitly-declared destructor became constexpr via https://wg21.link/p2448r2#pnum_23

[zygoloid]

I think this is ill-formed NDR -- the class is required to be complete, so its virtual functions are ODR-used.

In practice, e<int> has a constexpr virtual destructor, so we instantiate it eagerly, and we instantiate the complete vtable with the destructor, so the behavior here makes some sense. But it would seem better if we could defer marking the vtable used until the destructor is actually used (not just instantiated because it might be used at compile time, which doesn't need the vtable).