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

[via cfe-commits]

================
@@ -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()
+                 .getSourceRange()
+          << llvm::to_underlying(Conversion->getConversionType()
+                                     ->getAs<AutoType>()
+                                     ->getKeyword())
----------------
cor3ntin wrote:

You do not have to change it here, but this is kind of nasty.
`<< Conversion->getConversionType()` should just work, so this is an artifact of a weirdly constructed diagnostic.
I wonder if we should adapt the `err_auto_not_allowed` diagnostic.

We do this weird dance in 3 places
I also will not properly diagnose constrained return types

ie ```cpp
template <typename T>
concept C = true;

struct S {
    operator C auto();
};```


Writing that iI think we should bite the bullet and change it

```cpp

def err_auto_not_allowed : Error<
  "%select{"use of "
  "%select{class template|function template|variable template|alias template|"
  "template template parameter|concept|template}3 %4 requires template "
  "arguments; argument deduction|%2}0 not allowed "
  "%select{in function prototype"
  "|in non-static struct member|in struct member"
  "|in non-static union member|in union member"
  "|in non-static class member|in interface member"
  "|in exception declaration|in template parameter until C++17|in block literal"
  "|in template argument|in typedef|in type alias|in function return type"
  "|in conversion function type|here|in lambda parameter"
  "|in type allocated by 'new'|in K&R-style function parameter"
  "|in template parameter|in friend declaration|in function prototype that is "
  "not a function declaration|in requires expression parameter"
  "|in array declaration"
  "|in declaration of conversion function template}1">;
```
Should work, we would have to change the 3 uses of err_auto_not_allowed

Can you add tests for constrained auto?



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