[clang] Fix CTAD for aggregates for nested template classes (#78387)

Use the template pattern in determining whether to synthesize the
aggregate deduction guide, and update
DeclareImplicitDeductionGuideFromInitList to substitute outer template
arguments.

Fixes #77599

Author: antangelo

clang/docs/ReleaseNotes.rst

@@ -942,6 +942,9 @@ Bug Fixes to C++ Support
   (`#57410 <https://github.com/llvm/llvm-project/issues/57410>`_) and
   (`#76604 <https://github.com/llvm/llvm-project/issues/57410>`_)
 
+- Fixes CTAD for aggregates on nested template classes. Fixes:
+  (`#77599 <https://github.com/llvm/llvm-project/issues/77599>`_)
+
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
 - Fixed an import failure of recursive friend class template.

clang/lib/Sema/SemaInit.cpp

@@ -10718,7 +10718,14 @@ QualType Sema::DeduceTemplateSpecializationFromInitializer(
     bool HasAnyDeductionGuide = false;
 
     auto SynthesizeAggrGuide = [&](InitListExpr *ListInit) {
-      auto *RD = cast<CXXRecordDecl>(Template->getTemplatedDecl());
+      auto *Pattern = Template;
+      while (Pattern->getInstantiatedFromMemberTemplate()) {
+        if (Pattern->isMemberSpecialization())
+          break;
+        Pattern = Pattern->getInstantiatedFromMemberTemplate();
+      }
+
+      auto *RD = cast<CXXRecordDecl>(Pattern->getTemplatedDecl());
       if (!(RD->getDefinition() && RD->isAggregate()))
         return;
       QualType Ty = Context.getRecordType(RD);

clang/lib/Sema/SemaTemplate.cpp

@@ -2418,16 +2418,23 @@ struct ConvertConstructorToDeductionGuideTransform {
     QualType Result = SemaRef.BuildFunctionType(DeducedType, ParamTypes, Loc,
                                                 DeductionGuideName, EPI);
     TypeSourceInfo *TSI = SemaRef.Context.getTrivialTypeSourceInfo(Result, Loc);
+    if (NestedPattern)
+      TSI = SemaRef.SubstType(TSI, OuterInstantiationArgs, Loc,
+                              DeductionGuideName);
 
     FunctionProtoTypeLoc FPTL =
         TSI->getTypeLoc().castAs<FunctionProtoTypeLoc>();
 
     // Build the parameters, needed during deduction / substitution.
     SmallVector<ParmVarDecl*, 4> Params;
     for (auto T : ParamTypes) {
-      ParmVarDecl *NewParam = ParmVarDecl::Create(
-          SemaRef.Context, DC, Loc, Loc, nullptr, T,
-          SemaRef.Context.getTrivialTypeSourceInfo(T, Loc), SC_None, nullptr);
+      auto *TSI = SemaRef.Context.getTrivialTypeSourceInfo(T, Loc);
+      if (NestedPattern)
+        TSI = SemaRef.SubstType(TSI, OuterInstantiationArgs, Loc,
+                                DeclarationName());
+      ParmVarDecl *NewParam =
+          ParmVarDecl::Create(SemaRef.Context, DC, Loc, Loc, nullptr,
+                              TSI->getType(), TSI, SC_None, nullptr);
       NewParam->setScopeInfo(0, Params.size());
       FPTL.setParam(Params.size(), NewParam);
       Params.push_back(NewParam);
@@ -2670,8 +2677,14 @@ FunctionTemplateDecl *Sema::DeclareImplicitDeductionGuideFromInitList(
   if (BuildingDeductionGuides.isInvalid())
     return nullptr;
 
-  return cast<FunctionTemplateDecl>(
+  ClassTemplateDecl *Pattern =
+      Transform.NestedPattern ? Transform.NestedPattern : Transform.Template;
+  ContextRAII SavedContext(*this, Pattern->getTemplatedDecl());
+
+  auto *DG = cast<FunctionTemplateDecl>(
       Transform.buildSimpleDeductionGuide(ParamTypes));
+  SavedContext.pop();
+  return DG;
 }
 
 void Sema::DeclareImplicitDeductionGuides(TemplateDecl *Template,

clang/test/SemaTemplate/nested-implicit-deduction-guides.cpp

@@ -1,5 +1,4 @@
 // RUN: %clang_cc1 -std=c++20 -verify %s
-// expected-no-diagnostics
 
 template<class T> struct S {
     template<class U> struct N {
@@ -58,3 +57,21 @@ template<class X> struct requires_clause {
 requires_clause<int>::B req(1, 2);
 using RC = decltype(req);
 using RC = requires_clause<int>::B<int>;
+
+template<typename X> struct nested_init_list {
+    template<C<X> Y>
+    struct B { // #INIT_LIST_INNER
+        X x;
+        Y y;
+    };
+};
+
+nested_init_list<int>::B nil {1, 2};
+using NIL = decltype(nil);
+using NIL = nested_init_list<int>::B<int>;
+
+// expected-error@+1 {{no viable constructor or deduction guide for deduction of template arguments of 'B'}}
+nested_init_list<int>::B nil_invalid {1, ""};
+// expected-note@#INIT_LIST_INNER {{candidate template ignored: substitution failure [with Y = const char *]: constraints not satisfied for class template 'B' [with Y = const char *]}}
+// expected-note@#INIT_LIST_INNER {{candidate function template not viable: requires 1 argument, but 2 were provided}}
+// expected-note@#INIT_LIST_INNER {{candidate function template not viable: requires 0 arguments, but 2 were provided}}