`-Wundefined-func-template` warns that no definition is available for a pure virtual function

[ahatanak]

The warning seems incorrect as B::bar is pure virtual and its definition isn't needed.

$ cat test.cpp

template <typename T> class B {
public:
  constexpr void foo(const T &) { bar(1); }
  virtual constexpr void bar(unsigned int) = 0;
};

template <typename T> class D : public B<T> {
public:
  constexpr void bar(unsigned int) override {}
};

void test() {
  auto t = D<int>();
  t.foo(0);
}

$ clang++ -std=c++20 test.cpp -Wundefined-func-template -c
test.cpp:4:35: warning: instantiation of function 'B<int>::bar' required here, but no definition is available [-Wundefined-func-template]
  constexpr void foo(const T &) { bar(1); }
                                  ^
test.cpp:5:26: note: forward declaration of template entity is here
  virtual constexpr void bar(unsigned int) = 0;
                         ^
test.cpp:4:35: note: add an explicit instantiation declaration to suppress this warning if 'B<int>::bar' is explicitly instantiated in another translation unit
  constexpr void foo(const T &) { bar(1); }
                                  ^
1 warning generated.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Akira Hatanaka (ahatanak)

The warning seems incorrect as `B::bar` is pure virtual and its definition isn't needed.

$ cat test.cpp

template <typename T> class B {
public:
  constexpr void foo(const T &) { bar(1); }
  virtual constexpr void bar(unsigned int) = 0;
};

template <typename T> class D : public B<T> {
public:
  constexpr void bar(unsigned int) override {}
};

void test() {
  auto t = D<int>();
  t.foo(0);
}

$ clang++ -std=c++20 test.cpp -Wundefined-func-template -c
test.cpp:4:35: warning: instantiation of function 'B<int>::bar' required here, but no definition is available [-Wundefined-func-template]
constexpr void foo(const T &) { bar(1); }
^
test.cpp:5:26: note: forward declaration of template entity is here
virtual constexpr void bar(unsigned int) = 0;
^
test.cpp:4:35: note: add an explicit instantiation declaration to suppress this warning if 'B<int>::bar' is explicitly instantiated in another translation unit
constexpr void foo(const T &) { bar(1); }
^
1 warning generated.

[shafik]

Confirmed: https://godbolt.org/z/6a3vvveEG

[llvmbot]

Hi!

This issue may be a good introductory issue for people new to working on LLVM. If you would like to work on this issue, your first steps are:

1. In the comments of the issue, request for it to be assigned to you.
2. Fix the issue locally.
3. Run the test suite locally. Remember that the subdirectories under test/ create fine-grained testing targets, so you can e.g. use make check-clang-ast to only run Clang's AST tests.
4. Create a Git commit.
5. Run git clang-format HEAD~1 to format your changes.
6. Open a pull request to the upstream repository on GitHub. Detailed instructions can be found in GitHub's documentation.

If you have any further questions about this issue, don't hesitate to ask via a comment in the thread below.

[llvmbot]

@llvm/issue-subscribers-good-first-issue

Author: Akira Hatanaka (ahatanak)

The warning seems incorrect as `B::bar` is pure virtual and its definition isn't needed.

$ cat test.cpp

template <typename T> class B {
public:
  constexpr void foo(const T &) { bar(1); }
  virtual constexpr void bar(unsigned int) = 0;
};

template <typename T> class D : public B<T> {
public:
  constexpr void bar(unsigned int) override {}
};

void test() {
  auto t = D<int>();
  t.foo(0);
}

$ clang++ -std=c++20 test.cpp -Wundefined-func-template -c
test.cpp:4:35: warning: instantiation of function 'B<int>::bar' required here, but no definition is available [-Wundefined-func-template]
constexpr void foo(const T &) { bar(1); }
^
test.cpp:5:26: note: forward declaration of template entity is here
virtual constexpr void bar(unsigned int) = 0;
^
test.cpp:4:35: note: add an explicit instantiation declaration to suppress this warning if 'B<int>::bar' is explicitly instantiated in another translation unit
constexpr void foo(const T &) { bar(1); }
^
1 warning generated.

[charmitro]

I'd like to pick this up. I'm sharing my thoughts so we can discuss them.

Where does this happen?

The warning is coming from

llvm-project/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

Lines 4954 to 4958 in bc51a36

	if (AtEndOfTU && !getDiagnostics().hasErrorOccurred() &&
	!getSourceManager().isInSystemHeader(PatternDecl->getBeginLoc())) {
	Diag(PointOfInstantiation, diag::warn_func_template_missing)
	<< Function;
	Diag(PatternDecl->getLocation(), diag::note_forward_template_decl);

We do not check if the function is a pure virtual function via FunctionDecl::isVirtualAsWritten().

Issue fix

IMO we should just add another condition in the if statement checking for this, such as:

      if (AtEndOfTU && !getDiagnostics().hasErrorOccurred() &&
          !getSourceManager().isInSystemHeader(PatternDecl->getBeginLoc()) &&
          !Function->isVirtualAsWritten()) { ... }

Finally we should add a test-case for this under clang/test/SemaTemplate/ called instantiate-virtual-function.cpp (I'm not good at naming).

if all these sound good, I'll start implementing it.

[dwblaikie]

if all these sound good, I'll start implementing it.

Sounds about right/plausible to me, thanks!