[llvm] [Diagnostics] Return rvalue reference from temporary argument (PR #127400)

[Jonas Hahnfeld via llvm-commits]

================
@@ -625,14 +626,14 @@ operator<<(RemarkT &R,
 /// Also allow r-value for the remark to allow insertion into a
 /// temporarily-constructed remark.
 template <class RemarkT>
-RemarkT &
+RemarkT &&
 operator<<(RemarkT &&R,
            std::enable_if_t<
                std::is_base_of<DiagnosticInfoOptimizationBase, RemarkT>::value,
                StringRef>
                S) {
   R.insert(S);
-  return R;
+  return std::move(R);
----------------
hahnjo wrote:

> Try using `decltype(auto)` as return value rather than `auto`. The latter drops references from the type deduction, meaning it tries to copy or move construct an object of type `RemarkT`.

Thanks, that works. And thanks for the explanation, I didn't know / remember that detail.

https://github.com/llvm/llvm-project/pull/127400