[clang] don't print redundant context notes when instantiating alias templates

The redundant notes were introduced with the workaround for finding
the template instantiationa args for lambdas inside template type aliases.

This removes the notes for the cases where we are simply instantiating an
outer template, and when diagnosing uses of the alias template.

Also adds comments calling the workaround explicitly.

Author: mizvekov

clang/lib/Sema/SemaTemplate.cpp

@@ -3822,14 +3822,19 @@ QualType Sema::CheckTemplateIdType(ElaboratedTypeKeyword Keyword,
         AliasTemplate->getTemplateParameters()->getDepth());
 
     LocalInstantiationScope Scope(*this);
-    InstantiatingTemplate Inst(
-        *this, /*PointOfInstantiation=*/TemplateLoc,
-        /*Entity=*/AliasTemplate,
-        /*TemplateArgs=*/TemplateArgLists.getInnermost());
 
     // Diagnose uses of this alias.
     (void)DiagnoseUseOfDecl(AliasTemplate, TemplateLoc);
 
+    // FIXME: The TemplateArgs passed here are not used for the context note,
+    // nor they should, because this note will be pointing to the specialization
+    // anyway. These arguments are needed for a hack for instantiating lambdas
+    // in the pattern of the alias. In getTemplateInstantiationArgs, these
+    // arguments will be used for collating the template arguments needed to
+    // instantiate the lambda.
+    InstantiatingTemplate Inst(*this, /*PointOfInstantiation=*/TemplateLoc,
+                               /*Entity=*/AliasTemplate,
+                               /*TemplateArgs=*/CTAI.SugaredConverted);
     if (Inst.isInvalid())
       return QualType();
 

clang/lib/Sema/SemaTemplateInstantiate.cpp

@@ -1271,6 +1271,12 @@ void Sema::PrintInstantiationStack(InstantiationContextDiagFuncRef DiagFunc) {
                PDiag(diag::note_building_deduction_guide_here));
       break;
     case CodeSynthesisContext::TypeAliasTemplateInstantiation:
+      // Workaround for a workaround: don't produce a note if we are merely
+      // instantiating some other template which contains this alias template.
+      // This would be redundant either with the error itself, or some other
+      // context note attached to it.
+      if (Active->NumTemplateArgs == 0)
+        break;
       DiagFunc(Active->PointOfInstantiation,
                PDiag(diag::note_template_type_alias_instantiation_here)
                    << cast<TypeAliasTemplateDecl>(Active->Entity)

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

@@ -1580,17 +1580,19 @@ Decl *TemplateDeclInstantiator::InstantiateTypeAliasTemplateDecl(
   if (!InstParams)
     return nullptr;
 
-  TypeAliasDecl *Pattern = D->getTemplatedDecl();
-  Sema::InstantiatingTemplate InstTemplate(
-      SemaRef, D->getBeginLoc(), D,
-      D->getTemplateDepth() >= TemplateArgs.getNumLevels()
-          ? ArrayRef<TemplateArgument>()
-          : (TemplateArgs.begin() + TemplateArgs.getNumLevels() - 1 -
-             D->getTemplateDepth())
-                ->Args);
+  // FIXME: This is a hack for instantiating lambdas in the pattern of the
+  // alias. We are not really instantiating the alias at its template level,
+  // that only happens in CheckTemplateId, this is only for outer templates
+  // which contain it. In getTemplateInstantiationArgs, the template arguments
+  // used here would be used for collating the template arguments needed to
+  // instantiate the lambda. Pass an empty argument list, so this workaround
+  // doesn't get confused if there is an outer alias being instantiated.
+  Sema::InstantiatingTemplate InstTemplate(SemaRef, D->getBeginLoc(), D,
+                                           ArrayRef<TemplateArgument>());
   if (InstTemplate.isInvalid())
     return nullptr;
 
+  TypeAliasDecl *Pattern = D->getTemplatedDecl();
   TypeAliasTemplateDecl *PrevAliasTemplate = nullptr;
   if (getPreviousDeclForInstantiation<TypedefNameDecl>(Pattern)) {
     DeclContext::lookup_result Found = Owner->lookup(Pattern->getDeclName());

clang/test/SemaTemplate/alias-template-deprecated.cpp

@@ -46,23 +46,19 @@ using UsingInstWithCPPAttr [[deprecated("Do not use this")]] = NoAttr<int>;
 void bar() {
   NoAttr<int> obj; // Okay
 
-  // expected-warning@+2 {{'UsingWithAttr' is deprecated}}
-  // expected-note@+1 {{in instantiation of template type alias 'UsingWithAttr' requested here}}
+  // expected-warning@+1 {{'UsingWithAttr' is deprecated}}
   UsingWithAttr<int> objUsingWA;
 
-  // expected-warning@+2 {{'UsingWithAttr' is deprecated}}
-  // expected-note@+1 {{in instantiation of template type alias 'UsingWithAttr' requested here}}
+  // expected-warning@+1 {{'UsingWithAttr' is deprecated}}
   NoAttr<UsingWithAttr<int>> s;
 
   // expected-note@+1 {{'DepInt' has been explicitly marked deprecated here}}
   using DepInt [[deprecated]] = int;
-  // expected-warning@+3 {{'UsingWithAttr' is deprecated}}
-  // expected-warning@+2 {{'DepInt' is deprecated}}
-  // expected-note@+1 {{in instantiation of template type alias 'UsingWithAttr' requested here}}
+  // expected-warning@+2 {{'UsingWithAttr' is deprecated}}
+  // expected-warning@+1 {{'DepInt' is deprecated}}
   using X = UsingWithAttr<DepInt>;
 
-  // expected-warning@+2 {{'UsingWithAttr' is deprecated}}
-  // expected-note@+1 {{in instantiation of template type alias 'UsingWithAttr' requested here}}
+  // expected-warning@+1 {{'UsingWithAttr' is deprecated}}
   UsingWithAttr<int>().foo();
 
   // expected-warning@+1 {{'UsingInstWithAttr' is deprecated}}
@@ -74,8 +70,7 @@ void bar() {
   // expected-warning@+1 {{'UsingTDWithAttr' is deprecated}}
   UsingTDWithAttr objUTDWA;
 
-  // expected-warning@+2 {{'UsingWithCPPAttr' is deprecated}}
-  // expected-note@+1 {{in instantiation of template type alias 'UsingWithCPPAttr' requested here}}
+  // expected-warning@+1 {{'UsingWithCPPAttr' is deprecated}}
   UsingWithCPPAttr<int> objUsingWCPPA;
 
   // expected-warning@+1 {{'UsingInstWithCPPAttr' is deprecated: Do not use this}}

clang/test/SemaTemplate/alias-templates.cpp

@@ -312,3 +312,11 @@ namespace resolved_nttp {
 
   using TC2 = decltype(C<bool, 2, 3>::p); // expected-note {{instantiation of}}
 }
+
+namespace OuterSubstFailure {
+  template <class T> struct A {
+      template <class> using B = T&;
+      // expected-error@-1 {{cannot form a reference to 'void'}}
+  };
+  template struct A<void>; // expected-note {{requested here}}
+} // namespace OuterSubstFailure