[Sema][NFC] Cleanups after 843cc47 (llvm#87996)

I forgot to tidy up these lines that should've been done in the previous
commit, specifically:

1. Merge two `CodeSynthesisContext`s into one in `CheckTemplateIdType`.
2. Remove some gratuitous `Sema::` specifiers.
3. Rename the parameter `Template` to `Entity` to avoid confusion.

Author: zyn0217

clang/include/clang/Sema/Sema.h

@@ -10077,7 +10077,7 @@ class Sema final : public SemaBase {
 
     /// Note that we are instantiating a type alias template declaration.
     InstantiatingTemplate(Sema &SemaRef, SourceLocation PointOfInstantiation,
-                          TypeAliasTemplateDecl *Template,
+                          TypeAliasTemplateDecl *Entity,
                           ArrayRef<TemplateArgument> TemplateArgs,
                           SourceRange InstantiationRange = SourceRange());
 

clang/lib/Frontend/FrontendActions.cpp

@@ -453,7 +453,7 @@ class DefaultTemplateInstCallback : public TemplateInstantiationCallback {
       return "BuildingBuiltinDumpStructCall";
     case CodeSynthesisContext::BuildingDeductionGuides:
       return "BuildingDeductionGuides";
-    case Sema::CodeSynthesisContext::TypeAliasTemplateInstantiation:
+    case CodeSynthesisContext::TypeAliasTemplateInstantiation:
       return "TypeAliasTemplateInstantiation";
     }
     return "";

clang/lib/Sema/SemaTemplate.cpp

@@ -4494,15 +4494,13 @@ QualType Sema::CheckTemplateIdType(TemplateName Name,
         AliasTemplate->getTemplateParameters()->getDepth());
 
     LocalInstantiationScope Scope(*this);
-    InstantiatingTemplate Inst(*this, TemplateLoc, Template);
+    InstantiatingTemplate Inst(
+        *this, /*PointOfInstantiation=*/TemplateLoc,
+        /*Entity=*/AliasTemplate,
+        /*TemplateArgs=*/TemplateArgLists.getInnermost());
     if (Inst.isInvalid())
       return QualType();
 
-    InstantiatingTemplate InstTemplate(
-        *this, /*PointOfInstantiation=*/AliasTemplate->getBeginLoc(),
-        /*Template=*/AliasTemplate,
-        /*TemplateArgs=*/TemplateArgLists.getInnermost());
-
     std::optional<ContextRAII> SavedContext;
     if (!AliasTemplate->getDeclContext()->isFileContext())
       SavedContext.emplace(*this, AliasTemplate->getDeclContext());

clang/lib/Sema/SemaTemplateInstantiate.cpp

@@ -737,11 +737,11 @@ Sema::InstantiatingTemplate::InstantiatingTemplate(
 
 Sema::InstantiatingTemplate::InstantiatingTemplate(
     Sema &SemaRef, SourceLocation PointOfInstantiation,
-    TypeAliasTemplateDecl *Template, ArrayRef<TemplateArgument> TemplateArgs,
+    TypeAliasTemplateDecl *Entity, ArrayRef<TemplateArgument> TemplateArgs,
     SourceRange InstantiationRange)
     : InstantiatingTemplate(
-          SemaRef, Sema::CodeSynthesisContext::TypeAliasTemplateInstantiation,
-          PointOfInstantiation, InstantiationRange, /*Entity=*/Template,
+          SemaRef, CodeSynthesisContext::TypeAliasTemplateInstantiation,
+          PointOfInstantiation, InstantiationRange, /*Entity=*/Entity,
           /*Template=*/nullptr, TemplateArgs) {}
 
 Sema::InstantiatingTemplate::InstantiatingTemplate(
@@ -983,11 +983,6 @@ void Sema::PrintInstantiationStack() {
         Diags.Report(Active->PointOfInstantiation,
                      diag::note_template_class_instantiation_here)
             << CTD << Active->InstantiationRange;
-      } else {
-        Diags.Report(Active->PointOfInstantiation,
-                     diag::note_template_type_alias_instantiation_here)
-          << cast<TypeAliasTemplateDecl>(D)
-          << Active->InstantiationRange;
       }
       break;
     }
@@ -1262,6 +1257,10 @@ void Sema::PrintInstantiationStack() {
                    diag::note_building_deduction_guide_here);
       break;
     case CodeSynthesisContext::TypeAliasTemplateInstantiation:
+      Diags.Report(Active->PointOfInstantiation,
+                   diag::note_template_type_alias_instantiation_here)
+          << cast<TypeAliasTemplateDecl>(Active->Entity)
+          << Active->InstantiationRange;
       break;
     }
   }
@@ -1278,12 +1277,13 @@ std::optional<TemplateDeductionInfo *> Sema::isSFINAEContext() const {
        ++Active)
   {
     switch (Active->Kind) {
-    case CodeSynthesisContext::TemplateInstantiation:
+    case CodeSynthesisContext::TypeAliasTemplateInstantiation:
       // An instantiation of an alias template may or may not be a SFINAE
       // context, depending on what else is on the stack.
       if (isa<TypeAliasTemplateDecl>(Active->Entity))
         break;
       [[fallthrough]];
+    case CodeSynthesisContext::TemplateInstantiation:
     case CodeSynthesisContext::DefaultFunctionArgumentInstantiation:
     case CodeSynthesisContext::ExceptionSpecInstantiation:
     case CodeSynthesisContext::ConstraintsCheck:
@@ -1340,7 +1340,6 @@ std::optional<TemplateDeductionInfo *> Sema::isSFINAEContext() const {
       break;
 
     case CodeSynthesisContext::Memoization:
-    case CodeSynthesisContext::TypeAliasTemplateInstantiation:
       break;
     }
 

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

@@ -1119,6 +1119,8 @@ TemplateDeclInstantiator::VisitTypeAliasTemplateDecl(TypeAliasTemplateDecl *D) {
           : (TemplateArgs.begin() + TemplateArgs.getNumLevels() - 1 -
              D->getTemplateDepth())
                 ->Args);
+  if (InstTemplate.isInvalid())
+    return nullptr;
 
   TypeAliasTemplateDecl *PrevAliasTemplate = nullptr;
   if (getPreviousDeclForInstantiation<TypedefNameDecl>(Pattern)) {