[clang] [llvm] [clang] Add scoped enum support to `StreamingDiagnostic` (PR #138089)

[via cfe-commits]

================
@@ -1429,6 +1429,22 @@ operator<<(const StreamingDiagnostic &DB, T *DC) {
   return DB;
 }
 
+// Convert scope enums to their underlying type, so that we don't have
+// clutter the emitting code with `llvm::to_underlying()`.
+// We also need to disable implicit conversion for the first argument,
+// because classes that derive from StreamingDiagnostic define their own
+// templated operator<< that accept a wide variety of types, leading
+// to ambiguity.
+template <typename T, typename U>
+inline std::enable_if_t<
+    std::is_same_v<std::remove_const_t<T>, StreamingDiagnostic> &&
+        llvm::is_scoped_enum_v<std::remove_reference_t<U>>,
+    const StreamingDiagnostic &>
+operator<<(const T &DB, U &&SE) {
----------------
cor3ntin wrote:


I'm not sure why `operator<<(const StreamingDiagnostic &DB, U &&SE)` would not work - if there are ambiguities in the derived classes, can we fix those instead? you have examples? 

https://github.com/llvm/llvm-project/pull/138089