[clang] [Clang] Ensure initialized NTTP expressions when building CTAD for type aliases (PR #161035)

[Matheus Izvekov via cfe-commits]

================
@@ -1171,17 +1171,38 @@ BuildDeductionGuideForTypeAlias(Sema &SemaRef,
   Args.addOuterTemplateArguments(TransformedDeducedAliasArgs);
   for (unsigned Index = 0; Index < DeduceResults.size(); ++Index) {
     const auto &D = DeduceResults[Index];
+    auto *TP = F->getTemplateParameters()->getParam(Index);
     if (IsNonDeducedArgument(D)) {
       // 2): Non-deduced template parameters would be substituted later.
       continue;
     }
     TemplateArgumentLoc Input =
         SemaRef.getTrivialTemplateArgumentLoc(D, QualType(), SourceLocation{});
-    TemplateArgumentLoc Output;
-    if (!SemaRef.SubstTemplateArgument(Input, Args, Output)) {
+    TemplateArgumentListInfo Output;
+    if (!SemaRef.SubstTemplateArguments(Input, Args, Output)) {
       assert(TemplateArgsForBuildingFPrime[Index].isNull() &&
              "InstantiatedArgs must be null before setting");
-      TemplateArgsForBuildingFPrime[Index] = Output.getArgument();
+      Sema::CheckTemplateArgumentInfo CTAI;
+      if (Input.getArgument().getKind() == TemplateArgument::Pack) {
+        for (auto TA : Output.arguments()) {
+          if (SemaRef.CheckTemplateArgument(
+                  TP, TA, F, F->getLocation(), F->getLocation(),
+                  /*ArgumentPackIndex=*/-1, CTAI,
+                  Sema::CheckTemplateArgumentKind::CTAK_Specified))
+            return nullptr;
+        }
+        TemplateArgsForBuildingFPrime[Index] =
+            TemplateArgument::CreatePackCopy(Context, CTAI.SugaredConverted);
+      } else {
+        assert(Output.arguments().size() == 1);
+        TemplateArgumentLoc Transformed = Output.arguments()[0];
+        if (SemaRef.CheckTemplateArgument(
+                TP, Transformed, F, F->getLocation(), F->getLocation(),
+                /*ArgumentPackIndex=*/-1, CTAI,
+                Sema::CheckTemplateArgumentKind::CTAK_Specified))
+          return nullptr;
+        TemplateArgsForBuildingFPrime[Index] = CTAI.SugaredConverted[0];
+      }
----------------
mizvekov wrote:

How does this relate to the rest of the patch?

This looks like just adding some complicated error recovery, but there are no new tests which are producing any kinds of diagnostics, so I am confused.

https://github.com/llvm/llvm-project/pull/161035