[llvm] [clang] [clang-tools-extra] [clang] Implement CWG1878 "`operator auto` template" (PR #78103)

[Shafik Yaghmour via cfe-commits]

================
@@ -11321,9 +11321,20 @@ Decl *Sema::ActOnConversionDeclarator(CXXConversionDecl *Conversion) {
       << ClassType << ConvType;
   }
 
-  if (FunctionTemplateDecl *ConversionTemplate
-                                = Conversion->getDescribedFunctionTemplate())
+  if (FunctionTemplateDecl *ConversionTemplate =
+          Conversion->getDescribedFunctionTemplate()) {
+    if (const auto *ConvTypePtr = ConvType->getAs<PointerType>()) {
+      ConvType = ConvTypePtr->getPointeeType();
+    }
+    if (ConvType->isUndeducedAutoType()) {
+      Diag(Conversion->getTypeSpecStartLoc(), diag::err_auto_not_allowed)
+          << getReturnTypeLoc(Conversion).getSourceRange()
+          << llvm::to_underlying(ConvType->getAs<AutoType>()->getKeyword())
+          << /* in declaration of conversion function template= */ 24;
----------------
shafik wrote:

`warn_cconv_unsupported` uses `CallingConventionIgnoredReason` enum to specify which one.

Not sure if this is something we want to use more often or not.

https://github.com/llvm/llvm-project/pull/78103