Delay lookup of simple default template arguments under -fms-compatibility

MSVC delays parsing of default arguments until instantiation.  If the
default argument is never used, it is never parsed.  We don't model
this.

Instead, if lookup of a type name fails in a template argument context,
we form a DependentNameType, which will be looked up at instantiation
time.

This fixes errors about 'CControlWinTraits' in atlwin.h.

Reviewers: rsmith

Differential Revision: http://reviews.llvm.org/D3995

llvm-svn: 210382

clang/include/clang/AST/Type.h

@@ -4057,11 +4057,14 @@ public:
 /// dependent.
 ///
 /// DependentNameType represents a class of dependent types that involve a
-/// dependent nested-name-specifier (e.g., "T::") followed by a (dependent)
+/// possibly dependent nested-name-specifier (e.g., "T::") followed by a
 /// name of a type. The DependentNameType may start with a "typename" (for a
 /// typename-specifier), "class", "struct", "union", or "enum" (for a
 /// dependent elaborated-type-specifier), or nothing (in contexts where we
 /// know that we must be referring to a type, e.g., in a base class specifier).
+/// Typically the nested-name-specifier is dependent, but in MSVC compatibility
+/// mode, this type is used with non-dependent names to delay name lookup until
+/// instantiation.
 class DependentNameType : public TypeWithKeyword, public llvm::FoldingSetNode {
 
   /// \brief The nested name specifier containing the qualifier.
@@ -4076,10 +4079,7 @@ class DependentNameType : public TypeWithKeyword, public llvm::FoldingSetNode {
                       /*InstantiationDependent=*/true,
                       /*VariablyModified=*/false,
                       NNS->containsUnexpandedParameterPack()),
-      NNS(NNS), Name(Name) {
-    assert(NNS->isDependent() &&
-           "DependentNameType requires a dependent nested-name-specifier");
-  }
+      NNS(NNS), Name(Name) {}
 
   friend class ASTContext;  // ASTContext creates these
 

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -3224,6 +3224,9 @@ def err_pointer_to_member_oper_value_classify: Error<
 def ext_ms_deref_template_argument: ExtWarn<
   "non-type template argument containing a dereference operation is a "
   "Microsoft extension">, InGroup<Microsoft>;
+def ext_ms_delayed_template_argument: ExtWarn<
+  "using the undeclared type %0 as a default template argument is a "
+  "Microsoft extension">, InGroup<Microsoft>;
 
 // C++ template specialization
 def err_template_spec_unknown_kind : Error<

clang/include/clang/Parse/Parser.h

@@ -1691,7 +1691,8 @@ private:
     DSC_type_specifier, // C++ type-specifier-seq or C specifier-qualifier-list
     DSC_trailing, // C++11 trailing-type-specifier in a trailing return type
     DSC_alias_declaration, // C++11 type-specifier-seq in an alias-declaration
-    DSC_top_level // top-level/namespace declaration context
+    DSC_top_level, // top-level/namespace declaration context
+    DSC_template_type_arg // template type argument context
   };
 
   /// Is this a context in which we are parsing just a type-specifier (or
@@ -1703,6 +1704,7 @@ private:
     case DSC_top_level:
       return false;
 
+    case DSC_template_type_arg:
     case DSC_type_specifier:
     case DSC_trailing:
     case DSC_alias_declaration:

clang/include/clang/Sema/Sema.h

@@ -1418,6 +1418,13 @@ public:
                                ParsedType &SuggestedType,
                                bool AllowClassTemplates = false);
 
+  /// \brief For compatibility with MSVC, we delay parsing of some default
+  /// template type arguments until instantiation time.  Emits a warning and
+  /// returns a synthesized DependentNameType that isn't really dependent on any
+  /// other template arguments.
+  ParsedType ActOnDelayedDefaultTemplateArg(const IdentifierInfo &II,
+                                            SourceLocation NameLoc);
+
   /// \brief Describes the result of the name lookup and resolution performed
   /// by \c ClassifyName().
   enum NameClassificationKind {

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;

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) {

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,

clang/test/SemaTemplate/ms-delayed-default-template-args.cpp

@@ -0,0 +1,96 @@
+// RUN: %clang_cc1 -fms-compatibility -std=c++11 %s -verify
+
+// MSVC should compile this file without errors.
+
+namespace test_basic {
+template <typename T = Baz> // expected-warning {{using the undeclared type 'Baz' as a default template argument is a Microsoft extension}}
+struct Foo { T x; };
+typedef int Baz;
+template struct Foo<>;
+}
+
+namespace test_namespace {
+namespace nested {
+template <typename T = Baz> // expected-warning {{using the undeclared type 'Baz' as a default template argument is a Microsoft extension}}
+struct Foo {
+  static_assert(sizeof(T) == 4, "should get int, not double");
+};
+typedef int Baz;
+}
+typedef double Baz;
+template struct nested::Foo<>;
+}
+
+namespace test_inner_class_template {
+struct Outer {
+  template <typename T = Baz> // expected-warning {{using the undeclared type 'Baz' as a default template argument is a Microsoft extension}}
+  struct Foo {
+    static_assert(sizeof(T) == 4, "should get int, not double");
+  };
+  typedef int Baz;
+};
+typedef double Baz;
+template struct Outer::Foo<>;
+}
+
+namespace test_nontype_param {
+template <typename T> struct Bar { T x; };
+typedef int Qux;
+template <Bar<Qux> *P>
+struct Foo {
+};
+Bar<int> g;
+template struct Foo<&g>;
+}
+
+// MSVC accepts this, but Clang doesn't.
+namespace test_template_instantiation_arg {
+template <typename T> struct Bar { T x; };
+template <typename T = Bar<Weber>>  // expected-error {{use of undeclared identifier 'Weber'}}
+struct Foo {
+  static_assert(sizeof(T) == 4, "Bar should have gotten int");
+  // FIXME: These diagnostics are bad.
+}; // expected-error {{expected ',' or '>' in template-parameter-list}}
+// expected-warning@-1 {{does not declare anything}}
+typedef int Weber;
+}
+
+#ifdef __clang__
+// These are negative test cases that MSVC doesn't compile either.  Try to use
+// unique undeclared identifiers so typo correction doesn't find types declared
+// above.
+
+namespace test_undeclared_nontype_parm_type {
+template <Zargon N> // expected-error {{unknown type name 'Zargon'}}
+struct Foo { int x[N]; };
+typedef int Zargon;
+template struct Foo<4>;
+}
+
+namespace test_undeclared_nontype_parm_type_no_name {
+template <typename T, Asdf> // expected-error {{unknown type name 'Asdf'}}
+struct Foo { T x; };
+template struct Foo<int, 0>;
+}
+
+namespace test_undeclared_type_arg {
+template <typename T>
+struct Foo { T x; };
+template struct Foo<Yodel>; // expected-error {{use of undeclared identifier 'Yodel'}}
+}
+
+namespace test_undeclared_nontype_parm_arg {
+// Bury an undeclared type as a template argument to the type of a non-type
+// template parameter.
+template <typename T> struct Bar { T x; };
+
+template <Bar<Xylophone> *P> // expected-error {{use of undeclared identifier 'Xylophone'}}
+// expected-note@-1 {{template parameter is declared here}}
+struct Foo { };
+
+typedef int Xylophone;
+Bar<Xylophone> g;
+template struct Foo<&g>; // expected-error {{cannot be converted}}
+}
+
+#endif