[clang] Linker failure for std::visit with a templated lambda using a forward declared templated function

[StefanPaulet]

#include <type_traits>
#include <variant>

using MyType = std::variant<double, int>;

template <typename T>
auto foo(T&& arg) -> std::remove_cvref_t<T>;

auto bar(MyType const& v) {
    auto callable = []<typename Arg>(Arg&& arg) -> MyType {
        return foo(std::forward<Arg>(arg));
    };
    return std::visit(callable, v);
}

template <typename T>
auto foo(T&& arg) -> std::remove_cvref_t<T> {
    return arg;
}

int main() {
    MyType b {5.3};
    bar(b);
}

https://godbolt.org/z/n54f19hn5

It seems that only the declarations get generated for foo, but no definitions. However, if foo is no longer forward declared, everything works: https://godbolt.org/z/c1WY647af.

Diagnostics:

/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/15.0.0/../../../../x86_64-linux-gnu/bin/ld: /tmp/example-c83671.o: in function `std::variant<double, int> bar(std::variant<double, int> const&)::$_0::operator()<double const&>(double const&) const':
<source>:11:(.text+0x315): undefined reference to `std::remove_cvref<double const&>::type foo<double const&>(double const&)'
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/15.0.0/../../../../x86_64-linux-gnu/bin/ld: /tmp/example-c83671.o: in function `std::variant<double, int> bar(std::variant<double, int> const&)::$_0::operator()<int const&>(int const&) const':
<source>:11:(.text+0x3f5): undefined reference to `std::remove_cvref<int const&>::type foo<int const&>(int const&)'

[StefanPaulet]

Might be related to #57561

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (StefanPaulet)

```cpp #include <type_traits> #include <variant>

using MyType = std::variant<double, int>;

template <typename T>
auto foo(T&& arg) -> std::remove_cvref_t<T>;

auto bar(MyType const& v) {
auto callable = []<typename Arg>(Arg&& arg) -> MyType {
return foo(std::forward<Arg>(arg));
};
return std::visit(callable, v);
}

template <typename T>
auto foo(T&& arg) -> std::remove_cvref_t<T> {
return arg;
}

int main() {
MyType b {5.3};
bar(b);
}

https://godbolt.org/z/n54f19hn5

It seems that only the declarations get generated for `foo`, but no definitions. However, if `foo` is no longer forward declared, everything works: https://godbolt.org/z/c1WY647af.
</details>

[shafik]

Adding diagnostics b/c it helps w/ searchability for folks looking for the same issue.

[StefanPaulet]

Managed to reduce the code and remove all includes:
https://godbolt.org/z/aMdr9P4eY

Also found that when the lambda has a deduced return type or when it is not locally declared inside the function everything works:
https://godbolt.org/z/dqGdW381M

The problem seems to be that a locally declared lambda without a deduced return type gets eagerly instantiated and that it recursively tries to instantiate foo before it has a definition.

[StefanPaulet]

I would like to try and tackle this issue. However, I'm having a hard time understanding what the expected behaviour should be.
I looked into Sema::PerformPendingInstantiations and the problem might be that if a function is not instantiated after InstantiateFunctionDefinition is called, no attempt to do it again at the end of the translation unit is made.
On the other hand, the problem might also be that instantiating operator() on the lambda should not try to recursively instantiate foo.
Any indications?

[zyn0217]

Also found that when the lambda has a deduced return type or when it is not locally declared inside the function everything works:
https://godbolt.org/z/dqGdW381M

The symptom then looks like a duplicate of #35052 to me. I tried to instantiate the local lambda eagerly so the constant evaluation would find a definition before it evaluates anything, but changing that behavior turns out to break DeduceReturnType for one case, so it gets reverted, unfortunately.

[StefanPaulet]

The symptom then looks like a duplicate of #35052 to me. I tried to instantiate the local lambda eagerly so the constant evaluation would find a definition before it evaluates anything, but changing that behavior turns out to break DeduceReturnType for one case, so it gets reverted, unfortunately.

Yep, they seem to be related. Although one is an issue of generic lambdas being too eagerly instantiated, and the other of a generic lambda not getting instantiated for constant evaluation (I think).
I've found out the issue actually goes much deeper, as instantiation for generic lambdas has some problems. I will have a look at it based on the discussion from #117167

[shafik]

Looks fixed in clang trunk but I have no idea why: https://godbolt.org/z/o4c8xfxYM

CC @erichkeane @cor3ntin

[zyn0217]

Looks fixed in clang trunk but I have no idea why: https://godbolt.org/z/o4c8xfxYM

CC @erichkeane @cor3ntin

#126723