C++ Discarded Return Statements Handling Clarification

Hey guys! Today, let's dive deep into a fascinating, albeit somewhat tricky, aspect of C++: the handling of discarded return statements. This topic stems from a discussion within the C++ Standard Committee regarding the correct interpretation and behavior of return statements within if constexpr blocks. Specifically, we're going to break down an ambiguity in the C++ standard and explore a proposed solution. So, buckle up, and let’s get started!

Issue Description: The Case of the Discarded return

At the heart of our discussion is a seemingly simple code snippet that exposes a subtle but significant issue:

auto f() {
 if constexpr (false) return;
 return 3;
}

Now, what’s the deal here? The question revolves around the validity of this code. The first return statement, nestled within an if constexpr (false) block, is essentially discarded during compilation because the condition is always false. This means that this particular return statement doesn't actively contribute to the return type deduction of the function f. Most compilers correctly deduce the return type based on the second return statement, which returns an integer (3).

However, here's where things get murky. According to the current C++ standard, specifically section [stmt.return]/2, there's a rule that states:

[...] A return statement with no operand shall be used only in a function whose return type is cv void, a constructor ([class.ctor]), or a destructor ([class.dtor]).

This rule seems straightforward, right? A return statement without a value (like our first one) should only appear in functions that return void, constructors, or destructors. But, and this is a big but, the standard doesn't explicitly say whether this check should apply to discarded return statements. One compiler implementation, interpreting the standard strictly, actually rejects the code because of this rule, while others accept it. This discrepancy highlights the ambiguity in the standard and the need for clarification.

So, why is this a problem? Well, for starters, it leads to inconsistent compiler behavior, which is a major headache for developers. Code that compiles on one compiler might fail on another, leading to portability issues and wasted debugging time. More fundamentally, it exposes a gap in the standard's specification, leaving room for misinterpretations and potential future issues. The goal of the C++ standard is to provide clear and unambiguous rules so that everyone – compiler writers and developers alike – can be on the same page. This particular case demonstrates a need for refinement.

Diving Deeper into the Standard: Why the Confusion?

To truly understand the issue, we need to dissect the relevant parts of the C++ standard. The key section we’re looking at is [stmt.return], which governs the semantics of return statements. Paragraph 2 of this section lays out the rules for return statements with and without operands (the value being returned). As we've seen, it states that a return without an operand is only valid in void functions, constructors, or destructors.

The problem arises from the fact that the standard doesn't explicitly address the concept of discarded statements in the context of return statements. A discarded statement, in this case, is a statement within a branch of an if constexpr that is never executed at compile time. The if constexpr construct is a powerful feature in C++ that allows for compile-time conditional execution. If the condition inside constexpr evaluates to false, the code within that branch is effectively removed from the compilation process.

In our example, the return; statement is inside an if constexpr (false) block. Since the condition is always false, this return statement is discarded. The question then becomes: should the standard's restrictions on return statements without operands apply to these discarded statements? The current wording doesn't provide a clear answer, leading to the conflicting interpretations we've observed.

One could argue that a discarded statement is essentially not part of the program, so it shouldn't be subject to the same rules as active statements. This is the rationale behind the compilers that accept the code. They essentially ignore the discarded return statement when checking for semantic validity. On the other hand, one could argue for a stricter interpretation, stating that the standard makes no exceptions for discarded statements, and therefore the rule should apply regardless. This is the reasoning behind the compiler that rejects the code.

The lack of clarity here highlights the importance of precise language in a programming language standard. Even a seemingly minor ambiguity can lead to significant differences in implementation and behavior. This is why the C++ Standard Committee spends considerable time and effort scrutinizing every detail of the standard, addressing potential ambiguities and ensuring consistency.

The Suggested Resolution: A Clearer Path Forward

To address this ambiguity, a suggested resolution has been proposed to the C++ Standard Committee. This resolution aims to clarify the standard's intent and ensure consistent behavior across different compilers. The proposed change focuses on modifying the wording in [stmt.return]/2 to explicitly exclude discarded return statements from the restriction on return statements without operands.

The proposed change involves replacing the phrase "be used" with "appear as a non-discarded statement." This seemingly small change in wording has a significant impact on the interpretation of the rule. Let's look at the original and proposed wording side-by-side:

Original Wording:

A return statement with no operand shall be used only in a function whose return type is cv void, a constructor ([class.ctor]), or a destructor ([class.dtor]).

Proposed Wording:

A return statement with no operand shall appear as a non-discarded statement ([stmt.if]) only in a function whose return type is cv void, a constructor ([class.ctor]), or a destructor ([class.dtor]).

By adding the phrase "appear as a non-discarded statement," the rule now explicitly applies only to return statements that are actually part of the program's execution flow. Discarded return statements, by definition, do not appear in the program's execution flow, and therefore are not subject to this restriction. This simple change effectively resolves the ambiguity and ensures that compilers treat the code snippet we discussed earlier as valid.

But the proposed resolution doesn't stop there. It also addresses another subtle point related to the semantic constraints for copy-initialization of the return object. The original wording implied that these constraints should be checked even for discarded return statements. The proposed change clarifies that these checks should only be performed for non-discarded return statements. This ensures that the compiler doesn't waste time and effort checking conditions that will never be relevant during program execution.

Here's the full proposed change to [stmt.return]/2, with the deletions marked with <del> and the insertions marked with <ins>:

The expr-or-braced-init-list of a return statement is called its operand. A return statement with no operand shall be used appear as a non-discarded statement ([stmt.if]) only in a function whose return type is cv void, a constructor ([class.ctor]), or a destructor ([class.dtor]). A return statement with an operand of type void shall be used appear as a non-discarded statement only in a function that has a cv void return type. A return statement with any other operand shall be used appear as a non-discarded statement only in a function that has a return type other than cv void; the such a non-discarded return statement initializes the returned reference or prvalue result object of the (explicit or implicit) function call by copy-initialization from the operand.

This comprehensive change not only resolves the immediate ambiguity but also clarifies the overall intent of the standard regarding discarded return statements.

Impact and Implications: Why This Matters

So, why is this clarification important? What are the real-world implications of this seemingly small change to the C++ standard? Well, there are several key reasons why this resolution matters.

First and foremost, it promotes consistency across different C++ compilers. As we've seen, the ambiguity in the current standard has led to divergent interpretations and behaviors. By explicitly clarifying the rules for discarded return statements, the proposed change ensures that all conforming compilers will treat the same code in the same way. This is crucial for code portability and reduces the risk of unexpected compilation errors.

Secondly, it simplifies the mental model of C++ for developers. When the rules of a language are clear and unambiguous, it's easier for programmers to understand how the language works and to write correct code. By explicitly excluding discarded return statements from the restriction on return statements without operands, the proposed change makes the behavior of return statements more intuitive and predictable.

Thirdly, it avoids unnecessary restrictions on code. The original interpretation of the standard could have led to situations where perfectly valid code was rejected by the compiler. By clarifying that discarded return statements are not subject to the same rules as active return statements, the proposed change allows developers to use if constexpr more freely and expressively, without worrying about artificial limitations.

Finally, this resolution highlights the importance of precision in language standards. Even a seemingly minor ambiguity can have significant consequences for compiler implementations and developer workflows. The C++ Standard Committee's attention to detail and commitment to resolving ambiguities like this are essential for maintaining the quality and consistency of the C++ language.

In conclusion, the issue of unclear handling of discarded return statements in C++, while seemingly a niche topic, underscores the complexities and nuances involved in crafting a robust and unambiguous programming language standard. The proposed resolution exemplifies the careful and deliberate approach taken by the C++ Standard Committee to ensure clarity, consistency, and developer-friendliness. This detailed examination not only sheds light on a specific technical point but also provides a glimpse into the intricate world of language standardization and its impact on the programming landscape.

Repair Input Keyword

Clarify whether the rule about return statements without operands applies to discarded return statements in C++.

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C++ Discarded Return Statements Handling Clarification