[clang] Clang must diagnose the use of [[lifetimebound]] annotation in void-returning functions

[usx95]

https://godbolt.org/z/rcPnrWrr7

#include <string>
struct S {
    void setS(const std::string& s [[clang::lifetimebound]]) { this->s = &s; }
    const std::string* s;
};

void foo() {
    S s;
    s.setS(std::string());
}

Lifetime analysis is currently not defined for parameters annotated with [[lifetimebound]] in void-returning function.
Current analysis only enforces that the annotated parameter outlives the return value. With no return value, this analysis is silently disabled.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Utkarsh Saxena (usx95)

https://godbolt.org/z/rcPnrWrr7

#include <string>
struct S {
    void setS(const std::string& s [[clang::lifetimebound]]) { this->s = &s; }
    const std::string* s;
};

void foo() {
    S s;
    s.setS(std::string());
}

Lifetime analysis is currently not defined for parameters annotated with [[lifetimebound]] in void-returning function.
Current analysis only enforces that the annotated parameter outlives the return value. With no return value, this analysis is silently disabled.

[MitalAshok]

This is at least known about:

llvm-project/clang/test/SemaCXX/attr-lifetimebound.cpp

Lines 4 to 5 in 775f7f1

	// FIXME: Should we diagnose a void return type?
	void voidreturn(int &param [[clang::lifetimebound]]);

However, what about dependent void:

template<typename T>
std::add_lvalue_reference_t<const T> get_value(cached_function<T>& f [[clang::lifetimebound]]) {
  if (!f.has_value()) f.call_and_cache();
  return f.cached_value();
}

For T = void? The current behaviour of no extra analysis seems consistent

[changkhothuychung]

Can I pick this up? Can I have some more pointers what need to be done for this issue?