[Clang] Explicit function template instantiation leads to linker error (Regression)

[craffael]

Compiler Information

$ clang++-16 --version
Ubuntu clang version 16.0.1 (++20230328073357+42d1b276f779-1~exp1~20230328073502.65)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

Steps to reproduce

1. You need to create three files:

main.h

template <typename _Scalar>
struct Matrix {
  enum { IsVectorAtCompileTime = true };
};


template <typename Type>
using Vector = Matrix<Type>;

template <
    typename PlainObjectType,
    int StrideType = PlainObjectType::IsVectorAtCompileTime ? 0 : 1>
class Ref {};

template <class T>
class EigenSparseLU {
 public:
  template <class SCALAR_RHS>
  auto Apply(const Ref<Vector<SCALAR_RHS>>& rhs) const noexcept -> void;
};

main.cc

#include "main.h"

int main() {

  auto sparseLu = EigenSparseLU<double>();

  Matrix<double> q;
  sparseLu.Apply<double>({});

}

main2.cc

#include "main.h"

template <typename Scalar>
struct SparseLUImpl {
  typedef Ref<Matrix<Scalar>> BlockScalarVector;
};

template<class T>
template <class SCALAR_RHS>
auto EigenSparseLU<T>::Apply(
    const Ref<Vector<SCALAR_RHS>>& rhs) const noexcept
    -> void {
}

template auto EigenSparseLU<double>::Apply<double>(
    const Ref<Vector<double>>&) const noexcept -> void;

2. Compile and link the three files with clang++-16 main.cc main2.cc
3. Observe the error message:

/usr/bin/ld: /tmp/main-3e89ac.o: in function `main':
main.cc:(.text+0x1a): undefined reference to `void EigenSparseLU<double>::Apply<double>(Ref<Matrix<double>, Vector<double>::IsVectorAtCompileTime?(0) : (1)> const&) const'
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Note that this only fails with Clang 16 (I don't know about trunk). It works with Clang 14 and Clang 15. Also GCC compiles it fine.

[llvmbot]

@llvm/issue-subscribers-clang-codegen

[3y3p4tch]

The issue is still present in trunk - https://clang.godbolt.org/z/M1dsWrceY