[clang] Reland "[clang][Sema] Use original template pattern when declaring implicit deduction guides for nested template classes" (PR #69676)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: None (antangelo)

<details>
<summary>Changes</summary>

Reland of dd0fba11690f9fef304d5f48cde646e5eca8d3c0

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.

Changes from the last iteration:
1. The outer retained levels from the outer template are always added to the `MultiLevelTemplateArgumentList` for rewriting `FunctionTemplateDecl` arguments, even if there is no FTD and the arguments are empty.
2. When building implicit deduction guides, the template pattern underlying decl is pushed as the current context. This resolves the issue where `FindInstantiatedDecl` is unable to find the inner template class.
3. Tests are updated to cover the failing case, and to assert that the type is correct after argument deduction in the implicit case. 

---
Full diff: https://github.com/llvm/llvm-project/pull/69676.diff


4 Files Affected:

- (modified) clang/docs/ReleaseNotes.rst (+5) 
- (modified) clang/lib/Sema/SemaTemplate.cpp (+25-1) 
- (modified) clang/test/SemaTemplate/nested-deduction-guides.cpp (+5) 
- (added) clang/test/SemaTemplate/nested-implicit-deduction-guides.cpp (+15) 


``````````diff
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index fc8caf9221b9d29..53a9507fc26613a 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -536,6 +536,11 @@ Bug Fixes to C++ Support
   declaration definition. Fixes:
   (`#61763 <https://github.com/llvm/llvm-project/issues/61763>`_)
 
+- 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.
diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index f0197f7c102a857..c2477ec0063e418 100644
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -2253,6 +2253,7 @@ struct ConvertConstructorToDeductionGuideTransform {
 
   Sema &SemaRef;
   ClassTemplateDecl *Template;
+  ClassTemplateDecl *NestedPattern = nullptr;
 
   DeclContext *DC = Template->getDeclContext();
   CXXRecordDecl *Primary = Template->getTemplatedDecl();
@@ -2332,6 +2333,9 @@ struct ConvertConstructorToDeductionGuideTransform {
       Args.addOuterRetainedLevel();
     }
 
+    if (NestedPattern)
+      Args.addOuterRetainedLevels(NestedPattern->getTemplateDepth());
+
     FunctionProtoTypeLoc FPTL = CD->getTypeSourceInfo()->getTypeLoc()
                                    .getAsAdjusted<FunctionProtoTypeLoc>();
     assert(FPTL && "no prototype for constructor declaration");
@@ -2441,10 +2445,17 @@ 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());
@@ -2650,13 +2661,24 @@ 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).
+  ContextRAII SavedContext(*this, Pattern->getTemplatedDecl());
   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;
@@ -2702,6 +2724,8 @@ void Sema::DeclareImplicitDeductionGuides(TemplateDecl *Template,
           Transform.buildSimpleDeductionGuide(Transform.DeducedType))
           ->getTemplatedDecl())
       ->setDeductionCandidateKind(DeductionCandidate::Copy);
+
+  SavedContext.pop();
 }
 
 /// Diagnose the presence of a default template argument on a
diff --git a/clang/test/SemaTemplate/nested-deduction-guides.cpp b/clang/test/SemaTemplate/nested-deduction-guides.cpp
index 2c5dda456a138a8..38410b93ead3b97 100644
--- a/clang/test/SemaTemplate/nested-deduction-guides.cpp
+++ b/clang/test/SemaTemplate/nested-deduction-guides.cpp
@@ -4,10 +4,15 @@
 template<typename T> struct A {
   template<typename U> struct B {
     B(...);
+    B(const B &) = default;
   };
   template<typename U> B(U) -> B<U>;
 };
 A<void>::B b = 123;
+A<void>::B copy = b;
 
 using T = decltype(b);
 using T = A<void>::B<int>;
+
+using Copy = decltype(copy);
+using Copy = A<void>::B<int>;
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..10b70f9c8c4f357
--- /dev/null
+++ b/clang/test/SemaTemplate/nested-implicit-deduction-guides.cpp
@@ -0,0 +1,15 @@
+// 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};
+
+using T = decltype(x);
+using T = S<int>::N<int>;

``````````

</details>


https://github.com/llvm/llvm-project/pull/69676