[Bronek]

Helper-based trailing return types made some invalid monadic callbacks disappear through substitution failure. This could select a different cv/ref overload, including one that lets a callback borrow from a temporary owning optional despite deleting its rvalue overload.

For example, this callback explicitly rejects borrowing from an rvalue Person. Before this change, the rvalue and_then overloads disappear through substitution failure, allowing the const& member overload to invoke the callback with an lvalue instead:

#include <pfn/optional.hpp>

struct Person {
  int age;
};

struct BorrowAge {
  pfn::optional<int const&> operator()(Person const& person) const {
    return person.age;
  }
  pfn::optional<int const&> operator()(Person const&&) const = delete;
};

int main() {
  auto age = pfn::optional<Person>{Person{42}}.and_then(BorrowAge{});
  // The temporary Person is gone; before this fix, age still refers to its member.
  return *age; // Undefined behavior: dangling reference.
}

This compiles before the fix on GCC and Clang, and AddressSanitizer reports a stack-use-after-scope at *age. With the fix, both compilers reject the and_then call. Calling the same callback through a named optional that remains alive is still valid.

Match the current C++ draft's public declarations for owning and reference pfn::optional and both pfn::expected specializations. Use body-deduced returns where invalid callbacks must produce hard errors, preserve reference optional's explicit transform return type, and retain the specified invocability and constructibility constraints. Expected's permitted constructibility-based copy fallback remains available.

Add compile-failure tests with successful positive controls and separate fixture checks, plus runtime and constexpr preservation tests. Update TYPE_ALGEBRA, CHANGELOG, and the contributor guidance for rejection tests.

Assisted-by: Codex:gpt-6

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Codecov Report

✅ All modified and coverable lines are covered by tests.

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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🤖 Augment PR Summary

Summary: Aligns pfn monadic declarations and constraints with the C++ draft to prevent unintended cv/ref fallback.

• Uses body-deduced returns for owning optional's `and_then` and `transform`.
• Makes invalid selected callbacks hard errors, preventing the demonstrated dangling-reference case.
• Preserves reference optional's explicit `transform` return type and substitution behavior.
• Declares both optional specializations' `or_else` with a fixed optional return type.
• Uses body-deduced returns for all monadic operations in both expected specializations.
• Exposes specified constructibility constraints while preserving expected's permitted copy fallback.
• Retains conditional `noexcept` behavior for valid calls.
• Adds compile-failure tests with successful positive controls and separate fixture checks.
• Adds runtime/constexpr preservation tests and updates TYPE_ALGEBRA, CHANGELOG, and contributor guidance.

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