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

We missed calling CheckTemplateArgument when building CTAD deduction
guides. That ensures some InitExprs are correctly initialized, as in the
test that crashed due to incorrect NTTP initialization.

I don't use CheckTemplateArguments because, in CTAD synthesis, template
parameter packs don't always appear at the end of the parameter list,
unlike user-written ones mandated by the standard. This makes it
difficult for CheckTemplateArguments to determine how many arguments a
pack in middle should match, leading to unnecessary complexity.

On the other hand, since we substitute non-deduced template parameters
with deduced ones, we need to fold the packs midway through
substitution, where CheckTemplateArgument is more convenient.

As a drive-by this also removes some dead code in SemaInit.

Fixes llvm#131408

Author: zyn0217

clang/docs/ReleaseNotes.rst

@@ -361,7 +361,7 @@ Bug Fixes in This Version
   first parameter. (#GH113323).
 - Fixed a crash with incompatible pointer to integer conversions in designated
   initializers involving string literals. (#GH154046)
-- Fix crash on CTAD for alias template. (#GH131342)
+- Fix crash on CTAD for alias template. (#GH131342), (#GH131408)
 - Clang now emits a frontend error when a function marked with the `flatten` attribute
   calls another function that requires target features not enabled in the caller. This
   prevents a fatal error in the backend.

clang/lib/Sema/SemaInit.cpp

@@ -8219,8 +8219,8 @@ ExprResult InitializationSequence::Perform(Sema &S,
       // InitializeTemporary entity for our target type.
       QualType Ty = Step->Type;
       bool IsTemporary = !S.Context.hasSameType(Entity.getType(), Ty);
-      InitializedEntity TempEntity = InitializedEntity::InitializeTemporary(Ty);
-      InitializedEntity InitEntity = IsTemporary ? TempEntity : Entity;
+      InitializedEntity InitEntity =
+          IsTemporary ? InitializedEntity::InitializeTemporary(Ty) : Entity;
       InitListChecker PerformInitList(S, InitEntity,
           InitList, Ty, /*VerifyOnly=*/false,
           /*TreatUnavailableAsInvalid=*/false);
@@ -8242,7 +8242,6 @@ ExprResult InitializationSequence::Perform(Sema &S,
 
       InitListExpr *StructuredInitList =
           PerformInitList.getFullyStructuredList();
-      CurInit.get();
       CurInit = shouldBindAsTemporary(InitEntity)
           ? S.MaybeBindToTemporary(StructuredInitList)
           : StructuredInitList;

clang/lib/Sema/SemaTemplateDeductionGuide.cpp

@@ -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(
+              TP, Transformed, F, F->getLocation(), F->getLocation(),
+              /*ArgumentPackIndex=*/-1, CTAI,
+              Sema::CheckTemplateArgumentKind::CTAK_Specified))
+        return nullptr;
+      TemplateArgsForBuildingFPrime[Index] = CTAI.SugaredConverted[0];
     }
   }
 

clang/test/SemaCXX/cxx20-ctad-type-alias.cpp

@@ -587,6 +587,23 @@ static_assert(__is_same(decltype(a), A<A<int>>));
 
 } // namespace GH133132
 
+namespace GH131408 {
+
+struct Node {};
+
+template <class T, Node>
+struct A {
+    A(T) {}
+};
+
+template <class T>
+using AA = A<T, {}>;
+
+AA a{0};
+
+static_assert(__is_same(decltype(a), A<int, Node{}>));
+}
+
 namespace GH130604 {
 template <typename T> struct A {
     A(T);