[clang] [clang] Implement CWG1878 "`operator auto` template" (PR #78103)

[via cfe-commits]

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@@ -11322,9 +11322,22 @@ Decl *Sema::ActOnConversionDeclarator(CXXConversionDecl *Conversion) {
       << ClassType << ConvType;
   }
 
-  if (FunctionTemplateDecl *ConversionTemplate
-                                = Conversion->getDescribedFunctionTemplate())
+  if (FunctionTemplateDecl *ConversionTemplate =
+          Conversion->getDescribedFunctionTemplate()) {
+    if (ConvType->isUndeducedAutoType()) {
+      Diag(Conversion->getTypeSpecStartLoc(), diag::err_auto_not_allowed)
+          << Conversion->getTypeSourceInfo()
+                 ->getTypeLoc()
+                 .getContainedAutoTypeLoc()
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cor3ntin wrote:

I think this is a slightly bigger hammer than it needs to be.
I think we should just use `->getTypeLoc().getSourceRange()` (yes, it would be the source range of the type + qualifiers, but i think that's as good, if not better

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