From 8e37e6f9e9d2cd2cd142165fe886c11a0bd1f3ab Mon Sep 17 00:00:00 2001
From: Antonio Abbatangelo <contact@antangelo.com>
Date: Thu, 5 Oct 2023 21:07:37 -0400
Subject: [PATCH 1/2] [clang][Sema] Use original template pattern when
 declaring implicit deduction guides for nested template classes

When a nested template is instantiated, the template pattern of the
inner class is not copied into the outer class ClassTemplateSpecializationDecl.
The specialization contains a ClassTemplateDecl with an empty record
that points to the original template pattern instead.

As a result, when looking up the constructors of the inner class, no
results are returned. This patch finds the original template
pattern and uses that for the lookup instead.

Based on CWG2471 we must also substitute the known outer template arguments
when creating deduction guides for the inner class.

Fixes #46200
Fixes #57812
---
 clang/lib/Sema/SemaTemplate.cpp               | 22 ++++++++++++++++++-
 .../nested-implicit-deduction-guides.cpp      | 12 ++++++++++
 2 files changed, 33 insertions(+), 1 deletion(-)
 create mode 100644 clang/test/SemaTemplate/nested-implicit-deduction-guides.cpp

diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index ff370dd1e080b..fba5b22139170 100644
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -2250,6 +2250,7 @@ struct ConvertConstructorToDeductionGuideTransform {
 
   Sema &SemaRef;
   ClassTemplateDecl *Template;
+  ClassTemplateDecl *NestedPattern = nullptr;
 
   DeclContext *DC = Template->getDeclContext();
   CXXRecordDecl *Primary = Template->getTemplatedDecl();
@@ -2327,6 +2328,8 @@ struct ConvertConstructorToDeductionGuideTransform {
     if (FTD) {
       Args.addOuterTemplateArguments(SubstArgs);
       Args.addOuterRetainedLevel();
+      if (NestedPattern)
+        Args.addOuterRetainedLevels(NestedPattern->getTemplateDepth());
     }
 
     FunctionProtoTypeLoc FPTL = CD->getTypeSourceInfo()->getTypeLoc()
@@ -2438,10 +2441,17 @@ struct ConvertConstructorToDeductionGuideTransform {
     SmallVector<QualType, 4> ParamTypes;
     const FunctionProtoType *T = TL.getTypePtr();
 
+    MultiLevelTemplateArgumentList OuterInstantiationArgs;
+    if (NestedPattern)
+      OuterInstantiationArgs = SemaRef.getTemplateInstantiationArgs(Template);
+
     //    -- The types of the function parameters are those of the constructor.
     for (auto *OldParam : TL.getParams()) {
       ParmVarDecl *NewParam =
           transformFunctionTypeParam(OldParam, Args, MaterializedTypedefs);
+      if (NestedPattern && NewParam)
+        NewParam = transformFunctionTypeParam(NewParam, OuterInstantiationArgs,
+                                              MaterializedTypedefs);
       if (!NewParam)
         return QualType();
       ParamTypes.push_back(NewParam->getType());
@@ -2647,13 +2657,23 @@ void Sema::DeclareImplicitDeductionGuides(TemplateDecl *Template,
   if (BuildingDeductionGuides.isInvalid())
     return;
 
+  // If the template is nested, then we need to use the original
+  // pattern to iterate over the constructors.
+  ClassTemplateDecl *Pattern = Transform.Template;
+  while (Pattern->getInstantiatedFromMemberTemplate()) {
+    if (Pattern->isMemberSpecialization())
+      break;
+    Pattern = Pattern->getInstantiatedFromMemberTemplate();
+    Transform.NestedPattern = Pattern;
+  }
+
   // Convert declared constructors into deduction guide templates.
   // FIXME: Skip constructors for which deduction must necessarily fail (those
   // for which some class template parameter without a default argument never
   // appears in a deduced context).
   llvm::SmallPtrSet<NamedDecl *, 8> ProcessedCtors;
   bool AddedAny = false;
-  for (NamedDecl *D : LookupConstructors(Transform.Primary)) {
+  for (NamedDecl *D : LookupConstructors(Pattern->getTemplatedDecl())) {
     D = D->getUnderlyingDecl();
     if (D->isInvalidDecl() || D->isImplicit())
       continue;
diff --git a/clang/test/SemaTemplate/nested-implicit-deduction-guides.cpp b/clang/test/SemaTemplate/nested-implicit-deduction-guides.cpp
new file mode 100644
index 0000000000000..4915c687cf4c4
--- /dev/null
+++ b/clang/test/SemaTemplate/nested-implicit-deduction-guides.cpp
@@ -0,0 +1,12 @@
+// RUN: %clang_cc1 -std=c++17 -verify %s
+// expected-no-diagnostics
+
+template<class T> struct S {
+    template<class U> struct N {
+        N(T) {}
+        N(T, U) {}
+        template<class V> N(V, U) {}
+    };
+};
+
+S<int>::N x{"a", 1};

From 67b8a02140cf1e8819822c3b95495531f97f6591 Mon Sep 17 00:00:00 2001
From: Antonio Abbatangelo <contact@antangelo.com>
Date: Fri, 6 Oct 2023 15:33:19 -0400
Subject: [PATCH 2/2] Add release note

---
 clang/docs/ReleaseNotes.rst | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 9782c123f4c93..11d0c443610ce 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -513,6 +513,11 @@ Bug Fixes to C++ Support
   rather than prefer the non-templated constructor as specified in
   [standard.group]p3.
 
+- Fix a bug where implicit deduction guides are not correctly generated for nested template
+  classes. Fixes:
+  (`#46200 <https://github.com/llvm/llvm-project/issues/46200>`_)
+  (`#57812 <https://github.com/llvm/llvm-project/issues/57812>`_)
+
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
 - Fixed an import failure of recursive friend class template.