Why defaulted operator fails when returning int in C++20

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Summary

A developer attempted to leverage the C++20 defaulted three-way comparison operator (operator<=>) to automatically handle comparison logic for a wrapper struct. The intention was to rely on the language rule that allows a defaulted operator to cast the result of its members’ comparisons to the return type of the caller. However, the compiler rejected the code, citing that the defaulted definition is ill-formed because it cannot perform the necessary static_cast from std::strong_ordering to int.

Root Cause

The failure stems from a misunderstanding of how implicit conversion via static_cast works within the context of defaulted member functions.

  • The developer defined AA::operator<=> to return an int.
  • The developer defined BB::operator<=> = default.
  • When BB::operator<=> is defaulted, the compiler generates code that performs static_cast<int>(a.b <=> b.b).
  • The expression a.b <=> b.b returns an object of type std::strong_ordering.
  • std::strong_ordering is not implicitly convertible to int, and more importantly, it cannot be explicitly converted via static_cast<int>.

The documentation rule cited by the developer applies to the synthesized comparison result, but it does not bypass the fundamental requirement that the target type must be reachable via a valid static_cast from the comparison category.

Why This Happens in Real Systems

This issue typically arises during large-scale refactoring or when migrating legacy codebases to C++20.

  • Type Safety Rigidity: C++20 introduced strong types for comparisons (strong_ordering, weak_ordering, partial_ordering) to prevent accidental arithmetic mistakes.
  • Implicit vs. Explicit: Developers often assume that because std::strong_ordering “represents” an integer relationship, it will behave like one. However, to maintain type safety, the standard prevents direct casting to primitive integers.
  • Complexity of Composition: In complex systems where structs wrap other structs, the “chain of defaults” can hide the specific type mismatch until the compiler attempts to resolve the entire expression tree.

Real-World Impact

  • Compilation Failures: Broken builds during modernization efforts.
  • Increased Technical Debt: Developers might resort to manual, error-prone implementations of comparison operators instead of using = default, losing the benefits of compiler-generated logic.
  • Obscure Error Messages: The compiler error “default definition would be ill-formed” is notoriously difficult for non-experts to debug because it points to the line where = default is written, rather than the underlying type mismatch in the member struct.

Example or Code

#include 
#include 

struct AA {
    int a;
    // This returns int, which is non-standard but allowed by the user
    int operator(const AA& other) const {
        if (a  other.a) return 5;
        else return 0;
    }
};

struct BB {
    AA b;
    // This fails because static_cast(std::strong_ordering) is invalid
    auto operator(const BB& other) const = default;
};

int main() {
    BB b1{4}, b2{6};
    // This line will fail to compile
    std::cout << (b1  b2) << std::endl;
    return 0;
}

How Senior Engineers Fix It

A senior engineer recognizes that the mismatch is between a user-defined return type and the standard comparison categories. The fix is to adhere to the standard types or provide a proper conversion path.

  • Option 1: Use Standard Comparison Categories (Recommended): Change the return type of AA::operator<=> to std::strong_ordering. This is the most robust and “correct” way to use C++20.
  • Option 2: Explicit Implementation: If an int return is strictly required for legacy compatibility, do not use = default. Instead, implement the comparison manually to perform the conversion.
  • Option 3: Wrapper Helper: Use a helper function to map the std::strong_ordering to the required integer values.

Why Juniors Miss It

  • Over-reliance on Documentation: Juniors often read the “rules” of a feature (like the static_cast rule) in isolation without understanding the strict type requirements of the objects being cast.
  • Ignoring Type Semantics: They tend to view std::strong_ordering as a “fancy integer” rather than a distinct, opaque type designed to prevent logic errors.
  • Difficulty with Composition: They may understand how AA works and how BB works, but struggle to visualize the recursive nature of operator resolution in templated or defaulted code.

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