[Clang] Linker error after immediate escalation

[LHLaurini]

Found this while investigating #118560. This (started from libc++ std::visit, now reduced beyond recognition)

struct S {
    consteval void operator()() {}
};

template <class Fn>
constexpr void dispatch(Fn fn) {
    fn();
}

template <class Visitor>
struct value_visitor {
    constexpr void operator()() { visitor(); }
    Visitor&& visitor;
};

template <class Visitor>
constexpr auto make_dispatch() {
    return dispatch<value_visitor<S>>;
}

template <class Visitor>
constexpr void visit(Visitor&&) {
    make_dispatch<Visitor>();
}

int main() { visit(S{}); }

results in a linker error:

/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/15.0.0/../../../../x86_64-linux-gnu/bin/ld: /tmp/example-02182a.o: in function `auto make_dispatch<S>()':
<source>:19:(.text._Z13make_dispatchI1SEDav[_Z13make_dispatchI1SEDav]+0x7): undefined reference to `void dispatch<value_visitor<S> >(value_visitor<S>)'

GCC accepts it without issue. Compiler Explorer link.

Issue seems related to immediate escalation, as making make_dispatch or visit consteval solves the issue.

[shafik]

@cor3ntin

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Luiz Henrique Laurini (LHLaurini)

Found this while investigating #118560. This (started from libc++ `std::visit`, now reduced beyond recognition) ```c++ struct S { consteval void operator()() {} };

template <class Fn>
constexpr void dispatch(Fn fn) {
fn();
}

template <class Visitor>
struct value_visitor {
constexpr void operator()() { visitor(); }
Visitor&& visitor;
};

template <class Visitor>
constexpr auto make_dispatch() {
return dispatch<value_visitor<S>>;
}

template <class Visitor>
constexpr void visit(Visitor&&) {
make_dispatch<Visitor>();
}

int main() { visit(S{}); }

results in a linker error:

/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/15.0.0/../../../../x86_64-linux-gnu/bin/ld: /tmp/example-02182a.o: in function auto make_dispatch<S>()': <source>:19:(.text._Z13make_dispatchI1SEDav[_Z13make_dispatchI1SEDav]+0x7): undefined reference to void dispatch<value_visitor<S> >(value_visitor<S>)'

GCC accepts it without issue. [Compiler Explorer link](https://godbolt.org/z/hMjj67j8b).

Issue seems related to immediate escalation, as making `make_dispatch` or `visit` `consteval` solves the issue.
</details>

[frederick-vs-ja]

Looks like that no link reference should be generated, at all because the outermost visit specialization and all functions directly or indirectly called within it are immediately-escalated.