3 files changed, 56 insertions, 2 deletions
diff --git a/clang/lib/AST/ASTContext.cpp b/clang/lib/AST/ASTContext.cpp
index 3f48e798687..4dff88bbebe 100644
--- a/clang/lib/AST/ASTContext.cpp
+++ b/clang/lib/AST/ASTContext.cpp
@@ -3318,8 +3318,6 @@ QualType ASTContext::getDependentNameType(ElaboratedTypeKeyword Keyword,
                                           NestedNameSpecifier *NNS,
                                           const IdentifierInfo *Name,
                                           QualType Canon) const {
-  assert(NNS->isDependent() && "nested-name-specifier must be dependent");
-
   if (Canon.isNull()) {
     NestedNameSpecifier *CanonNNS = getCanonicalNestedNameSpecifier(NNS);
     ElaboratedTypeKeyword CanonKeyword = Keyword;
diff --git a/clang/lib/Parse/ParseDecl.cpp b/clang/lib/Parse/ParseDecl.cpp
index 65ec44c5775..92a13659ea3 100644
--- a/clang/lib/Parse/ParseDecl.cpp
+++ b/clang/lib/Parse/ParseDecl.cpp
@@ -2227,6 +2227,8 @@ Parser::getDeclSpecContextFromDeclaratorContext(unsigned Context) {
     return DSC_class;
   if (Context == Declarator::FileContext)
     return DSC_top_level;
+  if (Context == Declarator::TemplateTypeArgContext)
+    return DSC_template_type_arg;
   if (Context == Declarator::TrailingReturnContext)
     return DSC_trailing;
   if (Context == Declarator::AliasDeclContext ||
@@ -2753,6 +2755,16 @@ void Parser::ParseDeclarationSpecifiers(DeclSpec &DS,
         Actions.getTypeName(*Tok.getIdentifierInfo(),
                             Tok.getLocation(), getCurScope());
 
+      // MSVC: If we weren't able to parse a default template argument, and it's
+      // just a simple identifier, create a DependentNameType.  This will allow us
+      // to defer the name lookup to template instantiation time, as long we forge a
+      // NestedNameSpecifier for the current context.
+      if (!TypeRep && DSContext == DSC_template_type_arg &&
+          getLangOpts().MSVCCompat && getCurScope()->isTemplateParamScope()) {
+        TypeRep = Actions.ActOnDelayedDefaultTemplateArg(
+            *Tok.getIdentifierInfo(), Tok.getLocation());
+      }
+
       // If this is not a typedef name, don't parse it as part of the declspec,
       // it must be an implicit int or an error.
       if (!TypeRep) {
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index ed7431ae3e4..ec0c27b2d2e 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -343,6 +343,50 @@ ParsedType Sema::getTypeName(const IdentifierInfo &II, SourceLocation NameLoc,
   return ParsedType::make(T);
 }
 
+// Builds a fake NNS for the given decl context.
+static NestedNameSpecifier *
+synthesizeCurrentNestedNameSpecifier(ASTContext &Context, DeclContext *DC) {
+  for (;; DC = DC->getLookupParent()) {
+    DC = DC->getPrimaryContext();
+    auto *ND = dyn_cast<NamespaceDecl>(DC);
+    if (ND && !ND->isInline() && !ND->isAnonymousNamespace())
+      return NestedNameSpecifier::Create(Context, nullptr, ND);
+    else if (auto *RD = dyn_cast<CXXRecordDecl>(DC))
+      return NestedNameSpecifier::Create(Context, nullptr, RD->isTemplateDecl(),
+                                         RD->getTypeForDecl());
+    else if (isa<TranslationUnitDecl>(DC))
+      return NestedNameSpecifier::GlobalSpecifier(Context);
+  }
+  llvm_unreachable("something isn't in TU scope?");
+}
+
+ParsedType Sema::ActOnDelayedDefaultTemplateArg(const IdentifierInfo &II,
+                                                SourceLocation NameLoc) {
+  // Accepting an undeclared identifier as a default argument for a template
+  // type parameter is a Microsoft extension.
+  Diag(NameLoc, diag::ext_ms_delayed_template_argument) << &II;
+
+  // Build a fake DependentNameType that will perform lookup into CurContext at
+  // instantiation time.  The name specifier isn't dependent, so template
+  // instantiation won't transform it.  It will retry the lookup, however.
+  NestedNameSpecifier *NNS =
+      synthesizeCurrentNestedNameSpecifier(Context, CurContext);
+  QualType T = Context.getDependentNameType(ETK_None, NNS, &II);
+
+  // Build type location information.  We synthesized the qualifier, so we have
+  // to build a fake NestedNameSpecifierLoc.
+  NestedNameSpecifierLocBuilder NNSLocBuilder;
+  NNSLocBuilder.MakeTrivial(Context, NNS, SourceRange(NameLoc));
+  NestedNameSpecifierLoc QualifierLoc = NNSLocBuilder.getWithLocInContext(Context);
+
+  TypeLocBuilder Builder;
+  DependentNameTypeLoc DepTL = Builder.push<DependentNameTypeLoc>(T);
+  DepTL.setNameLoc(NameLoc);
+  DepTL.setElaboratedKeywordLoc(SourceLocation());
+  DepTL.setQualifierLoc(QualifierLoc);
+  return CreateParsedType(T, Builder.getTypeSourceInfo(Context, T));
+}
+
 /// isTagName() - This method is called *for error recovery purposes only*
 /// to determine if the specified name is a valid tag name ("struct foo").  If
 /// so, this returns the TST for the tag corresponding to it (TST_enum,