[clang] [clang] Implement CTAD for type alias template. (PR #77890)

[via cfe-commits]

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@@ -1446,6 +1447,58 @@ namespace {
       return inherited::TransformFunctionProtoType(TLB, TL);
     }
 
+    QualType TransformInjectedClassNameType(TypeLocBuilder &TLB,
+                                            InjectedClassNameTypeLoc TL) {
+      auto Type = inherited::TransformInjectedClassNameType(TLB, TL);
+      // Special case for transforming a deduction guide, we return a
+      // transformed TemplateSpecializationType.
+      if (Type.isNull() &&
+          SemaRef.CodeSynthesisContexts.back().Kind ==
+              Sema::CodeSynthesisContext::BuildingDeductionGuides) {
+        // Return a TemplateSpecializationType for transforming a deduction
+        // guide.
+        if (auto *ICT = TL.getType()->getAs<InjectedClassNameType>()) {
+          auto Type = 
+              inherited::TransformType(ICT->getInjectedSpecializationType());
+          TLB.pushTrivial(SemaRef.Context, Type, TL.getNameLoc());
+          return Type;
+        }
+      }
+      return Type;
+    }
+    // Override the default version to handle a rewrite-template-arg-pack case
+    // for building a deduction guide.
+    bool TransformTemplateArgument(const TemplateArgumentLoc &Input,
+                                   TemplateArgumentLoc &Output,
+                                   bool Uneval = false) {
+      const TemplateArgument &Arg = Input.getArgument();
+      std::vector<TemplateArgument> TArgs;
+      switch (Arg.getKind()) {
+      case TemplateArgument::Pack:
+        // Iterially rewrite the template argument pack, instead of unpacking
+        // it.
+        assert(
+            SemaRef.CodeSynthesisContexts.back().Kind ==
+                Sema::CodeSynthesisContext::BuildingDeductionGuides &&
+            "Transforming a template argument pack is only allowed in building "
+            "deduction guide");
+        for (auto &pack : Arg.getPackAsArray()) {
+          TemplateArgumentLoc Input = SemaRef.getTrivialTemplateArgumentLoc(
+              pack, QualType(), SourceLocation{});
+          TemplateArgumentLoc Output;
+          if (!SemaRef.SubstTemplateArgument(Input, TemplateArgs, Output))
+            TArgs.push_back(Output.getArgument());
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cor3ntin wrote:

Shouldn't we fail if SubstTemplateArgument fails?

https://github.com/llvm/llvm-project/pull/77890