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

[Younan Zhang via cfe-commits]

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@@ -1171,17 +1171,46 @@ 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)) {
-      assert(TemplateArgsForBuildingFPrime[Index].isNull() &&
-             "InstantiatedArgs must be null before setting");
-      TemplateArgsForBuildingFPrime[Index] = Output.getArgument();
+    TemplateArgumentListInfo Output;
+    if (SemaRef.SubstTemplateArguments(Input, Args, Output))
+      return nullptr;
+    assert(TemplateArgsForBuildingFPrime[Index].isNull() &&
+           "InstantiatedArgs must be null before setting");
+    // CheckTemplateArgument is necessary for NTTP initializations.
+    // FIXME: We may want to call CheckTemplateArguments instead, but we cannot
+    // match packs as usual, since packs can appear in the middle of the
+    // parameter list of a synthesized CTAD guide. See also the FIXME in
+    // test/SemaCXX/cxx20-ctad-type-alias.cpp:test25.
+    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;
+      }
+      // We will substitute the non-deduced template arguments with these
+      // transformed (unpacked at this point) arguments, where that substitution
+      // requires a pack for the corresponding parameter packs.
+      TemplateArgsForBuildingFPrime[Index] =
+          TemplateArgument::CreatePackCopy(Context, CTAI.SugaredConverted);
+    } else {
+      assert(Output.arguments().size() == 1);
+      TemplateArgumentLoc Transformed = Output.arguments()[0];
+      if (SemaRef.CheckTemplateArgument(
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zyn0217 wrote:

Thanks, I filed https://github.com/llvm/llvm-project/pull/161948

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