--- a/clang/lib/Parse/ParseTemplate.cpp	Tue Jun 15 19:13:43 2021 +0900
+++ b/clang/lib/Parse/ParseTemplate.cpp	Tue Jun 15 19:15:29 2021 +0900
@@ -22,6 +22,16 @@
 #include "llvm/Support/TimeProfiler.h"
 using namespace clang;
 
+/// Re-enter a possible template scope, creating as many template parameter
+/// scopes as necessary.
+/// \return The number of template parameter scopes entered.
+unsigned Parser::ReenterTemplateScopes(MultiParseScope &S, Decl *D) {
+  return Actions.ActOnReenterTemplateScope(D, [&] {
+    S.Enter(Scope::TemplateParamScope);
+    return Actions.getCurScope();
+  });
+}
+
 /// Parse a template declaration, explicit instantiation, or
 /// explicit specialization.
 Decl *Parser::ParseDeclarationStartingWithTemplate(
@@ -67,8 +77,7 @@
   assert(Tok.isOneOf(tok::kw_export, tok::kw_template) &&
          "Token does not start a template declaration.");
 
-  // Enter template-parameter scope.
-  ParseScope TemplateParmScope(this, Scope::TemplateParamScope);
+  MultiParseScope TemplateParamScopes(*this);
 
   // Tell the action that names should be checked in the context of
   // the declaration to come.
@@ -116,7 +125,8 @@
     // Parse the '<' template-parameter-list '>'
     SourceLocation LAngleLoc, RAngleLoc;
     SmallVector<NamedDecl*, 4> TemplateParams;
-    if (ParseTemplateParameters(CurTemplateDepthTracker.getDepth(),
+    if (ParseTemplateParameters(TemplateParamScopes,
+                                CurTemplateDepthTracker.getDepth(),
                                 TemplateParams, LAngleLoc, RAngleLoc)) {
       // Skip until the semi-colon or a '}'.
       SkipUntil(tok::r_brace, StopAtSemi | StopBeforeMatch);
@@ -131,9 +141,8 @@
 
       if (TryConsumeToken(tok::kw_requires)) {
         OptionalRequiresClauseConstraintER =
-            Actions.CorrectDelayedTyposInExpr(
-                ParseConstraintLogicalOrExpression(
-                    /*IsTrailingRequiresClause=*/false));
+            Actions.ActOnRequiresClause(ParseConstraintLogicalOrExpression(
+                /*IsTrailingRequiresClause=*/false));
         if (!OptionalRequiresClauseConstraintER.isUsable()) {
           // Skip until the semi-colon or a '}'.
           SkipUntil(tok::r_brace, StopAtSemi | StopBeforeMatch);
@@ -150,9 +159,6 @@
         TemplateParams, RAngleLoc, OptionalRequiresClauseConstraintER.get()));
   } while (Tok.isOneOf(tok::kw_export, tok::kw_template));
 
-  unsigned NewFlags = getCurScope()->getFlags() & ~Scope::TemplateParamScope;
-  ParseScopeFlags TemplateScopeFlags(this, NewFlags, isSpecialization);
-
   // Parse the actual template declaration.
   if (Tok.is(tok::kw_concept))
     return ParseConceptDefinition(
@@ -191,7 +197,7 @@
     return ParseStaticAssertDeclaration(DeclEnd);
   }
 
-  if (Context == DeclaratorContext::MemberContext) {
+  if (Context == DeclaratorContext::Member) {
     // We are parsing a member template.
     ParseCXXClassMemberDeclaration(AS, AccessAttrs, TemplateInfo,
                                    &DiagsFromTParams);
@@ -272,7 +278,7 @@
     // Function definitions are only allowed at file scope and in C++ classes.
     // The C++ inline method definition case is handled elsewhere, so we only
     // need to handle the file scope definition case.
-    if (Context != DeclaratorContext::FileContext) {
+    if (Context != DeclaratorContext::File) {
       Diag(Tok, diag::err_function_definition_not_allowed);
       SkipMalformedDecl();
       return nullptr;
@@ -430,8 +436,9 @@
 ///
 /// \returns true if an error occurred, false otherwise.
 bool Parser::ParseTemplateParameters(
-    unsigned Depth, SmallVectorImpl<NamedDecl *> &TemplateParams,
-    SourceLocation &LAngleLoc, SourceLocation &RAngleLoc) {
+    MultiParseScope &TemplateScopes, unsigned Depth,
+    SmallVectorImpl<NamedDecl *> &TemplateParams, SourceLocation &LAngleLoc,
+    SourceLocation &RAngleLoc) {
   // Get the template parameter list.
   if (!TryConsumeToken(tok::less, LAngleLoc)) {
     Diag(Tok.getLocation(), diag::err_expected_less_after) << "template";
@@ -440,8 +447,11 @@
 
   // Try to parse the template parameter list.
   bool Failed = false;
-  if (!Tok.is(tok::greater) && !Tok.is(tok::greatergreater))
+  // FIXME: Missing greatergreatergreater support.
+  if (!Tok.is(tok::greater) && !Tok.is(tok::greatergreater)) {
+    TemplateScopes.Enter(Scope::TemplateParamScope);
     Failed = ParseTemplateParameterList(Depth, TemplateParams);
+  }
 
   if (Tok.is(tok::greatergreater)) {
     // No diagnostic required here: a template-parameter-list can only be
@@ -635,7 +645,7 @@
     // probably meant to write the type of a NTTP.
     DeclSpec DS(getAttrFactory());
     DS.SetTypeSpecError();
-    Declarator D(DS, DeclaratorContext::TemplateParamContext);
+    Declarator D(DS, DeclaratorContext::TemplateParam);
     D.SetIdentifier(nullptr, Tok.getLocation());
     D.setInvalidType(true);
     NamedDecl *ErrorParam = Actions.ActOnNonTypeTemplateParameter(
@@ -810,9 +820,9 @@
   SourceLocation EqualLoc;
   ParsedType DefaultArg;
   if (TryConsumeToken(tok::equal, EqualLoc))
-    DefaultArg = ParseTypeName(/*Range=*/nullptr,
-                               DeclaratorContext::TemplateTypeArgContext)
-                     .get();
+    DefaultArg =
+        ParseTypeName(/*Range=*/nullptr, DeclaratorContext::TemplateTypeArg)
+            .get();
 
   NamedDecl *NewDecl = Actions.ActOnTypeParameter(getCurScope(),
                                                   TypenameKeyword, EllipsisLoc,
@@ -850,9 +860,9 @@
   SmallVector<NamedDecl*,8> TemplateParams;
   SourceLocation LAngleLoc, RAngleLoc;
   {
-    ParseScope TemplateParmScope(this, Scope::TemplateParamScope);
-    if (ParseTemplateParameters(Depth + 1, TemplateParams, LAngleLoc,
-                               RAngleLoc)) {
+    MultiParseScope TemplateParmScope(*this);
+    if (ParseTemplateParameters(TemplateParmScope, Depth + 1, TemplateParams,
+                                LAngleLoc, RAngleLoc)) {
       return nullptr;
     }
   }
@@ -956,7 +966,7 @@
                              DeclSpecContext::DSC_template_param);
 
   // Parse this as a typename.
-  Declarator ParamDecl(DS, DeclaratorContext::TemplateParamContext);
+  Declarator ParamDecl(DS, DeclaratorContext::TemplateParam);
   ParseDeclarator(ParamDecl);
   if (DS.getTypeSpecType() == DeclSpec::TST_unspecified) {
     Diag(Tok.getLocation(), diag::err_expected_template_parameter);
@@ -1516,7 +1526,7 @@
     /*ExprContext=*/Sema::ExpressionEvaluationContextRecord::EK_TemplateArgument);
   if (isCXXTypeId(TypeIdAsTemplateArgument)) {
     TypeResult TypeArg = ParseTypeName(
-        /*Range=*/nullptr, DeclaratorContext::TemplateArgContext);
+        /*Range=*/nullptr, DeclaratorContext::TemplateArg);
     return Actions.ActOnTemplateTypeArgument(TypeArg);
   }
 
@@ -1630,40 +1640,22 @@
   Sema::ContextRAII GlobalSavedContext(
       Actions, Actions.Context.getTranslationUnitDecl());
 
-  SmallVector<ParseScope*, 4> TemplateParamScopeStack;
+  MultiParseScope Scopes(*this);
+
+  // Get the list of DeclContexts to reenter.
+  SmallVector<DeclContext*, 4> DeclContextsToReenter;
+  for (DeclContext *DC = FunD; DC && !DC->isTranslationUnit();
+       DC = DC->getLexicalParent())
+    DeclContextsToReenter.push_back(DC);
 
-  // Get the list of DeclContexts to reenter. For inline methods, we only want
-  // to push the DeclContext of the outermost class. This matches the way the
-  // parser normally parses bodies of inline methods when the outermost class is
-  // complete.
-  struct ContainingDC {
-    ContainingDC(DeclContext *DC, bool ShouldPush) : Pair(DC, ShouldPush) {}
-    llvm::PointerIntPair<DeclContext *, 1, bool> Pair;
-    DeclContext *getDC() { return Pair.getPointer(); }
-    bool shouldPushDC() { return Pair.getInt(); }
-  };
-  SmallVector<ContainingDC, 4> DeclContextsToReenter;
-  DeclContext *DD = FunD;
-  DeclContext *NextContaining = Actions.getContainingDC(DD);
-  while (DD && !DD->isTranslationUnit()) {
-    bool ShouldPush = DD == NextContaining;
-    DeclContextsToReenter.push_back({DD, ShouldPush});
-    if (ShouldPush)
-      NextContaining = Actions.getContainingDC(DD);
-    DD = DD->getLexicalParent();
-  }
-
-  // Reenter template scopes from outermost to innermost.
-  for (ContainingDC CDC : reverse(DeclContextsToReenter)) {
-    TemplateParamScopeStack.push_back(
-        new ParseScope(this, Scope::TemplateParamScope));
-    unsigned NumParamLists = Actions.ActOnReenterTemplateScope(
-        getCurScope(), cast<Decl>(CDC.getDC()));
-    CurTemplateDepthTracker.addDepth(NumParamLists);
-    if (CDC.shouldPushDC()) {
-      TemplateParamScopeStack.push_back(new ParseScope(this, Scope::DeclScope));
-      Actions.PushDeclContext(Actions.getCurScope(), CDC.getDC());
-    }
+  // Reenter scopes from outermost to innermost.
+  for (DeclContext *DC : reverse(DeclContextsToReenter)) {
+    CurTemplateDepthTracker.addDepth(
+        ReenterTemplateScopes(Scopes, cast<Decl>(DC)));
+    Scopes.Enter(Scope::DeclScope);
+    // We'll reenter the function context itself below.
+    if (DC != FunD)
+      Actions.PushDeclContext(Actions.getCurScope(), DC);
   }
 
   assert(!LPT.Toks.empty() && "Empty body!");
@@ -1684,8 +1676,7 @@
                                Scope::CompoundStmtScope);
 
   // Recreate the containing function DeclContext.
-  Sema::ContextRAII FunctionSavedContext(Actions,
-                                         Actions.getContainingDC(FunD));
+  Sema::ContextRAII FunctionSavedContext(Actions, FunD->getLexicalParent());
 
   Actions.ActOnStartOfFunctionDef(getCurScope(), FunD);
 
@@ -1709,13 +1700,6 @@
     } else
       Actions.ActOnFinishFunctionBody(LPT.D, nullptr);
   }
-
-  // Exit scopes.
-  FnScope.Exit();
-  SmallVectorImpl<ParseScope *>::reverse_iterator I =
-   TemplateParamScopeStack.rbegin();
-  for (; I != TemplateParamScopeStack.rend(); ++I)
-    delete *I;
 }
 
 /// Lex a delayed template function for late parsing.