Hello,

I’d like to propose a small but meaningful change to C++: making auto optional in trailing return types.

1. Motivation

Currently, auto is required in functions that use trailing return types:

auto func() -> int;  // Required

However, this requirement is redundant and misleading because:

• The return type is explicitly specified (int), so auto adds no value.
• auto implies type deduction, but no deduction is happening here.

A more natural syntax would allow:

func() -> int;  // More intuitive Right?

Not only does this improve readability and consistency with standard function declarations, but it also aligns well with Software Engineering Structural Diagrams, such as UML Class Diagrams and Object Diagrams.

In UML Class Diagrams, method return types are typically represented after the function name, similar to how trailing return types work in C++. Allowing func() -> int; without auto makes the C++ syntax closer to UML representations, enhancing clarity and traceability between a codebase and its corresponding UML models.

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2. Why This Won’t Break Existing Code

This proposal is backward-compatible because:

• auto is already optional in normal return types:

  int func(); 
  

• This change is purely syntactic—it does not affect semantics.

• Existing code using auto remains fully valid.
  auto func() -> int;  // Remains valid
  auto func() -> decltype(some_expression);  // Also valid

Thus, this proposal only removes an unnecessary restriction without introducing breaking changes.

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3. Proposed Change

Modify the C++ grammar so that functions using trailing return types no longer require auto.
This would allow:

Current (C++ Standard Today)

auto func() -> int;  // Required

Proposed (New Syntax Allowance)

func() -> int;  // Allowed

This makes the syntax more consistent with regular function declarations.

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4. Next Steps

Would this be a reasonable proposal for a future C++ standard update?
I’d love to hear your thoughts and suggestions on how this could be refined further.

Best regards,
Fady al Dhaim