[Clang] Instantiate Typedefs referenced by type alias deduction guides

[zyn0217]

TypedefNameDecl referenced by a synthesized CTAD guide for type aliases was not transformed previously, resulting in a substitution failure in BuildDeductionGuideForTypeAlias() when substituting into the right-hand-side deduction guide.

This patch fixes it in the way we have been doing since https://reviews.llvm.org/D80743. We transform all the function parameters, parenting referenced TypedefNameDecls with the CXXDeductionGuideDecl. Then we instantiate these declarations in FindInstantiatedDecl() as we build up the eventual deduction guide, using the mechanism introduced in D80743

Fixes #111508

[llvmbot]

@llvm/pr-subscribers-clang

Author: Younan Zhang (zyn0217)

Changes

TypedefNameDecl referenced by a synthesized CTAD guide for type aliases was not transformed previously, resulting in a substitution failure in BuildDeductionGuideForTypeAlias() when substituting into the right-hand-side deduction guide.

This patch fixes it in the way we have been doing since https://reviews.llvm.org/D80743. We transform all the function parameters, parenting referenced TypedefNameDecls with the CXXDeductionGuideDecl. Then we instantiate these declarations in FindInstantiatedDecl() as we build up the eventual deduction guide, using the mechanism introduced in D80743

Fixes #111508

---

Full diff: https://github.com/llvm/llvm-project/pull/111804.diff

2 Files Affected:

• (modified) clang/lib/Sema/SemaTemplateDeductionGuide.cpp (+18-3)
• (modified) clang/test/SemaCXX/cxx20-ctad-type-alias.cpp (+13)

diff --git a/clang/lib/Sema/SemaTemplateDeductionGuide.cpp b/clang/lib/Sema/SemaTemplateDeductionGuide.cpp
index ca93c840f03215..871c705ab65421 100644
--- a/clang/lib/Sema/SemaTemplateDeductionGuide.cpp
+++ b/clang/lib/Sema/SemaTemplateDeductionGuide.cpp
@@ -70,8 +70,8 @@ class ExtractTypeForDeductionGuide
   ExtractTypeForDeductionGuide(
       Sema &SemaRef,
       llvm::SmallVectorImpl<TypedefNameDecl *> &MaterializedTypedefs,
-      ClassTemplateDecl *NestedPattern,
-      const MultiLevelTemplateArgumentList *OuterInstantiationArgs)
+      ClassTemplateDecl *NestedPattern = nullptr,
+      const MultiLevelTemplateArgumentList *OuterInstantiationArgs = nullptr)
       : Base(SemaRef), MaterializedTypedefs(MaterializedTypedefs),
         NestedPattern(NestedPattern),
         OuterInstantiationArgs(OuterInstantiationArgs) {
@@ -1215,10 +1215,25 @@ FunctionTemplateDecl *DeclareAggregateDeductionGuideForTypeAlias(
       getRHSTemplateDeclAndArgs(SemaRef, AliasTemplate).first;
   if (!RHSTemplate)
     return nullptr;
+
+  llvm::SmallVector<TypedefNameDecl *, 4> TypedefDecls;
+  llvm::SmallVector<QualType> NewParamTypes;
+  ExtractTypeForDeductionGuide TypeAliasTransformer(SemaRef, TypedefDecls);
+  for (QualType P : ParamTypes) {
+    QualType Type = TypeAliasTransformer.TransformType(P);
+    if (Type.isNull())
+      return nullptr;
+    NewParamTypes.push_back(Type);
+  }
+
   auto *RHSDeductionGuide = SemaRef.DeclareAggregateDeductionGuideFromInitList(
-      RHSTemplate, ParamTypes, Loc);
+      RHSTemplate, NewParamTypes, Loc);
   if (!RHSDeductionGuide)
     return nullptr;
+
+  for (TypedefNameDecl *TD : TypedefDecls)
+    TD->setDeclContext(RHSDeductionGuide->getTemplatedDecl());
+
   return BuildDeductionGuideForTypeAlias(SemaRef, AliasTemplate,
                                          RHSDeductionGuide, Loc);
 }
diff --git a/clang/test/SemaCXX/cxx20-ctad-type-alias.cpp b/clang/test/SemaCXX/cxx20-ctad-type-alias.cpp
index 5392573fcdb9d5..675c32a81f1ae8 100644
--- a/clang/test/SemaCXX/cxx20-ctad-type-alias.cpp
+++ b/clang/test/SemaCXX/cxx20-ctad-type-alias.cpp
@@ -481,3 +481,16 @@ struct Out {
 Out<float>::B out(100); // deduced to Out<float>::A<float>;
 static_assert(__is_same(decltype(out), Out<float>::A<float>));
 }
+
+namespace GH111508 {
+
+template <typename V> struct S {
+  using T = V;
+  T Data;
+};
+
+template <typename V> using Alias = S<V>;
+
+Alias A(42);
+
+} // namespace GH111508

[zyn0217]

Shall we backport this one?

[zyn0217]

I'll go ahead and backport it - this bug was first introduced by sugar preservation on CTAD guides included in the 19.x cycle.

/cherry-pick 0bc02b9

[llvmbot]

/pull-request #112293