[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:

I thought so too, but I cannot get it to work. I tried
```diff
diff --git a/llvm/include/llvm/IR/DiagnosticInfo.h b/llvm/include/llvm/IR/DiagnosticInfo.h
index 1e26cfdd17b5..9f77e7de68e1 100644
--- a/llvm/include/llvm/IR/DiagnosticInfo.h
+++ b/llvm/include/llvm/IR/DiagnosticInfo.h
@@ -613,27 +613,14 @@ protected:
 /// common base class.  This allows returning the result of the insertion
 /// directly by value, e.g. return OptimizationRemarkAnalysis(...) << "blah".
 template <class RemarkT>
-RemarkT &
-operator<<(RemarkT &R,
-           std::enable_if_t<
-               std::is_base_of<DiagnosticInfoOptimizationBase, RemarkT>::value,
-               StringRef>
-               S) {
-  R.insert(S);
-  return R;
-}
-
-/// Also allow r-value for the remark to allow insertion into a
-/// temporarily-constructed remark.
-template <class RemarkT>
-RemarkT &&
+auto
 operator<<(RemarkT &&R,
            std::enable_if_t<
-               std::is_base_of<DiagnosticInfoOptimizationBase, RemarkT>::value,
+               std::is_base_of<DiagnosticInfoOptimizationBase, std::remove_reference_t<RemarkT>>::value,
                StringRef>
                S) {
   R.insert(S);
-  return std::move(R);
+  return std::forward<RemarkT>(R);
 }
 
 template <class RemarkT>
```
but Clang says:
```
/code/LLVM/patches/llvm/include/llvm/IR/DiagnosticInfo.h:623:10: error: allocating an object of abstract class type 'llvm::DiagnosticInfoIROptimization'                                                                                      
  623 |   return std::forward<RemarkT>(R);                                                                                                                                                                                                    
      |          ^                                                                                                                                                                                                                            
/code/LLVM/patches/llvm/include/llvm/IR/DiagnosticInfo.h:728:11: note: in instantiation of function template specialization 'llvm::operator<<<llvm::DiagnosticInfoIROptimization &>' requested here                                           
  728 |     *this << Prepend;                                                                                                                                                                                                                 
      |           ^                                                                                                                                                                                                                           
/code/LLVM/patches/llvm/include/llvm/IR/DiagnosticInfo.h:551:16: note: unimplemented pure virtual method 'isEnabled' in 'DiagnosticInfoIROptimization'                                                                                        
  551 |   virtual bool isEnabled() const = 0;                                                                                                                                                                                                 
      |                ^
```

Unless there is a very obvious mistake / solution, I think this also gets out of scope for this PR which was "just" trying to resolve a compilation issue. If you don't mind, I would prefer landing this in its current form and future improvements can be tackled by the respective maintainer / people with more C++ skill than me.

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