[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

Author: antangelo

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;

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};