[CSDiagnostics] Add fix checks before could_not_infer_pack_element fallback

Author: li3zhen1

lib/Sema/CSDiagnostics.cpp

@@ -8202,20 +8202,17 @@ bool UnableToInferClosureReturnType::diagnoseAsError() {
 bool UnableToInferGenericPackElementType::diagnoseAsError() {
   auto *locator = getLocator();
   auto path = locator->getPath();
-
-  const auto applyArgToParamElt =
-      (path.end() - 2)->getAs<LocatorPathElt::ApplyArgToParam>();
-  const auto packElementElt =
-      (path.end() - 1)->getAs<LocatorPathElt::PackElement>();
-  if (!applyArgToParamElt || !packElementElt) {
-    return false;
-  }
+  const auto packElementElt = (path.end() - 1)->getAs<LocatorPathElt::PackElement>();
 
   if (isExpr<NilLiteralExpr>(getAnchor())) {
     // `nil` appears as an element of generic pack params, let's record a
     // specify contextual type for nil fix.
     emitDiagnostic(diag::unresolved_nil_literal);
   } else {
+    if (llvm::any_of(getSolution().Fixes, [&locator](const ConstraintFix *fix) {
+          return fix->getLocator() == locator;
+        }))
+      return false;
     // unable to infer the type of an element of generic pack params
     emitDiagnostic(diag::could_not_infer_pack_element,
                    packElementElt->getIndex());
@@ -8226,6 +8223,8 @@ bool UnableToInferGenericPackElementType::diagnoseAsError() {
     if (auto *calleeLocator = getSolution().getCalleeLocator(locator)) {
       if (const auto choice = getOverloadChoiceIfAvailable(calleeLocator)) {
         if (auto *decl = choice->choice.getDeclOrNull()) {
+          const auto applyArgToParamElt =
+              (path.end() - 2)->getAs<LocatorPathElt::ApplyArgToParam>();
           if (auto paramDecl =
                   getParameterAt(decl, applyArgToParamElt->getParamIdx())) {
             emitDiagnosticAt(