[clang] [clang][Sema] Use original template pattern when declaring implicit deduction guides for nested template classes (PR #68379)

[via cfe-commits]

https://github.com/antangelo created https://github.com/llvm/llvm-project/pull/68379

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

>From e4f4a7bbaad90103e87ad6e6ba80f10fad1e0b58 Mon Sep 17 00:00:00 2001
From: Antonio Abbatangelo <contact at antangelo.com>
Date: Thu, 5 Oct 2023 21:07:37 -0400
Subject: [PATCH] [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 080f005e04402cb..33907bd58b44fc4 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 000000000000000..4915c687cf4c4ef
--- /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};