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.