[clang] Build UsingType for elaborated type specifiers.

Support building UsingType for elaborated type specifiers:

```
namespace ns { class Foo {}; }

using ns::Foo;

// The TypeLoc of `Foo` below should be a ElaboratedTypeLoc with an
// inner UsingTypeLoc rather than the underlying `CXXRecordTypeLoc`
class Foo foo;
```

Differential Revision: https://reviews.llvm.org/D141280

clang-tools-extra/include-cleaner/unittests/WalkASTTest.cpp

@@ -150,6 +150,9 @@ TEST(WalkAST, Using) {
     }
     using ns::$explicit^Y;)cpp",
            "^Y<int> x;");
+  testWalk(R"cpp(
+    namespace ns { class Foo {}; }
+  )cpp", "using ns::$explicit^Foo; class ^Foo foo;");
 }
 
 TEST(WalkAST, Namespaces) {

clang/include/clang/Sema/DeclSpec.h

@@ -506,8 +506,16 @@ class DeclSpec {
     assert(isTypeRep((TST) TypeSpecType) && "DeclSpec does not store a type");
     return TypeRep;
   }
+  // Returns the underlying decl, if any.
   Decl *getRepAsDecl() const {
-    assert(isDeclRep((TST) TypeSpecType) && "DeclSpec does not store a decl");
+    auto *D = getRepAsFoundDecl();
+    if (const auto *Using = dyn_cast_or_null<UsingShadowDecl>(D))
+      return Using->getTargetDecl();
+    return D;
+  }
+  // Returns the originally found decl, if any.
+  Decl *getRepAsFoundDecl() const {
+    assert(isDeclRep((TST)TypeSpecType) && "DeclSpec does not store a decl");
     return DeclRep;
   }
   Expr *getRepAsExpr() const {

clang/include/clang/Sema/Sema.h

@@ -3324,7 +3324,9 @@ class Sema final {
                       SourceLocation ScopedEnumKWLoc,
                       bool ScopedEnumUsesClassTag, TypeResult UnderlyingType,
                       bool IsTypeSpecifier, bool IsTemplateParamOrArg,
-                      OffsetOfKind OOK, SkipBodyInfo *SkipBody = nullptr);
+                      OffsetOfKind OOK,
+                      UsingShadowDecl*& FoundUsingShadow,
+                      SkipBodyInfo *SkipBody = nullptr);
 
   DeclResult ActOnTemplatedFriendTag(Scope *S, SourceLocation FriendLoc,
                                      unsigned TagSpec, SourceLocation TagLoc,

clang/lib/Parse/ParseDecl.cpp

@@ -4965,6 +4965,7 @@ void Parser::ParseEnumSpecifier(SourceLocation StartLoc, DeclSpec &DS,
   bool IsDependent = false;
   const char *PrevSpec = nullptr;
   unsigned DiagID;
+  UsingShadowDecl* FoundUsing = nullptr;
   Decl *TagDecl =
       Actions.ActOnTag(getCurScope(), DeclSpec::TST_enum, TUK, StartLoc, SS,
                     Name, NameLoc, attrs, AS, DS.getModulePrivateSpecLoc(),
@@ -4973,7 +4974,7 @@ void Parser::ParseEnumSpecifier(SourceLocation StartLoc, DeclSpec &DS,
                     BaseType, DSC == DeclSpecContext::DSC_type_specifier,
                     DSC == DeclSpecContext::DSC_template_param ||
                         DSC == DeclSpecContext::DSC_template_type_arg,
-                    OffsetOfState, &SkipBody).get();
+                    OffsetOfState, FoundUsing, &SkipBody).get();
 
   if (SkipBody.ShouldSkip) {
     assert(TUK == Sema::TUK_Definition && "can only skip a definition");
@@ -4983,8 +4984,8 @@ void Parser::ParseEnumSpecifier(SourceLocation StartLoc, DeclSpec &DS,
     T.skipToEnd();
 
     if (DS.SetTypeSpecType(DeclSpec::TST_enum, StartLoc,
-                           NameLoc.isValid() ? NameLoc : StartLoc,
-                           PrevSpec, DiagID, TagDecl, Owned,
+                           NameLoc.isValid() ? NameLoc : StartLoc, PrevSpec,
+                           DiagID, FoundUsing ? FoundUsing : TagDecl, Owned,
                            Actions.getASTContext().getPrintingPolicy()))
       Diag(StartLoc, DiagID) << PrevSpec;
     return;
@@ -5038,8 +5039,8 @@ void Parser::ParseEnumSpecifier(SourceLocation StartLoc, DeclSpec &DS,
   }
 
   if (DS.SetTypeSpecType(DeclSpec::TST_enum, StartLoc,
-                         NameLoc.isValid() ? NameLoc : StartLoc,
-                         PrevSpec, DiagID, TagDecl, Owned,
+                         NameLoc.isValid() ? NameLoc : StartLoc, PrevSpec,
+                         DiagID, FoundUsing ? FoundUsing : TagDecl, Owned,
                          Actions.getASTContext().getPrintingPolicy()))
     Diag(StartLoc, DiagID) << PrevSpec;
 }

clang/lib/Parse/ParseDeclCXX.cpp

@@ -1934,6 +1934,7 @@ void Parser::ParseClassSpecifier(tok::TokenKind TagTokKind,
   // Create the tag portion of the class or class template.
   DeclResult TagOrTempResult = true; // invalid
   TypeResult TypeResult = true;      // invalid
+  UsingShadowDecl *FoundUsing = nullptr;
 
   bool Owned = false;
   Sema::SkipBodyInfo SkipBody;
@@ -2074,7 +2075,7 @@ void Parser::ParseClassSpecifier(tok::TokenKind TagTokKind,
         DSC == DeclSpecContext::DSC_type_specifier,
         DSC == DeclSpecContext::DSC_template_param ||
             DSC == DeclSpecContext::DSC_template_type_arg,
-        OffsetOfState, &SkipBody);
+        OffsetOfState, FoundUsing, &SkipBody);
 
     // If ActOnTag said the type was dependent, try again with the
     // less common call.
@@ -2133,7 +2134,7 @@ void Parser::ParseClassSpecifier(tok::TokenKind TagTokKind,
   } else if (!TagOrTempResult.isInvalid()) {
     Result = DS.SetTypeSpecType(
         TagType, StartLoc, NameLoc.isValid() ? NameLoc : StartLoc, PrevSpec,
-        DiagID, TagOrTempResult.get(), Owned, Policy);
+        DiagID, FoundUsing ? FoundUsing : TagOrTempResult.get(), Owned, Policy);
   } else {
     DS.SetTypeSpecError();
     return;

clang/lib/Sema/SemaDecl.cpp

@@ -16592,7 +16592,8 @@ Sema::ActOnTag(Scope *S, unsigned TagSpec, TagUseKind TUK, SourceLocation KWLoc,
                bool &IsDependent, SourceLocation ScopedEnumKWLoc,
                bool ScopedEnumUsesClassTag, TypeResult UnderlyingType,
                bool IsTypeSpecifier, bool IsTemplateParamOrArg,
-               OffsetOfKind OOK, SkipBodyInfo *SkipBody) {
+               OffsetOfKind OOK, UsingShadowDecl *&FoundUsingShadow,
+               SkipBodyInfo *SkipBody) {
   // If this is not a definition, it must have a name.
   IdentifierInfo *OrigName = Name;
   assert((Name != nullptr || TUK == TUK_Definition) &&
@@ -17027,6 +17028,7 @@ Sema::ActOnTag(Scope *S, unsigned TagSpec, TagUseKind TUK, SourceLocation KWLoc,
     // redefinition if either context is within the other.
     if (auto *Shadow = dyn_cast<UsingShadowDecl>(DirectPrevDecl)) {
       auto *OldTag = dyn_cast<TagDecl>(PrevDecl);
+      FoundUsingShadow = Shadow;
       if (SS.isEmpty() && TUK != TUK_Reference && TUK != TUK_Friend &&
           isDeclInScope(Shadow, SearchDC, S, isMemberSpecialization) &&
           !(OldTag && isAcceptableTagRedeclContext(

clang/lib/Sema/SemaDeclCXX.cpp

@@ -16972,6 +16972,7 @@ DeclResult Sema::ActOnTemplatedFriendTag(
     if (SS.isEmpty()) {
       bool Owned = false;
       bool IsDependent = false;
+      UsingShadowDecl* FoundUsing = nullptr;
       return ActOnTag(S, TagSpec, TUK_Friend, TagLoc, SS, Name, NameLoc, Attr,
                       AS_public,
                       /*ModulePrivateLoc=*/SourceLocation(),
@@ -16980,7 +16981,7 @@ DeclResult Sema::ActOnTemplatedFriendTag(
                       /*ScopedEnumUsesClassTag=*/false,
                       /*UnderlyingType=*/TypeResult(),
                       /*IsTypeSpecifier=*/false,
-                      /*IsTemplateParamOrArg=*/false, /*OOK=*/OOK_Outside);
+                      /*IsTemplateParamOrArg=*/false, /*OOK=*/OOK_Outside, FoundUsing);
     }
 
     NestedNameSpecifierLoc QualifierLoc = SS.getWithLocInContext(Context);

clang/lib/Sema/SemaTemplate.cpp

@@ -10181,11 +10181,14 @@ Sema::ActOnExplicitInstantiation(Scope *S, SourceLocation ExternLoc,
 
   bool Owned = false;
   bool IsDependent = false;
-  Decl *TagD = ActOnTag(S, TagSpec, Sema::TUK_Reference, KWLoc, SS, Name,
-               NameLoc, Attr, AS_none, /*ModulePrivateLoc=*/SourceLocation(),
+  UsingShadowDecl* FoundUsing = nullptr;
+  Decl *TagD =
+      ActOnTag(S, TagSpec, Sema::TUK_Reference, KWLoc, SS, Name, NameLoc, Attr,
+               AS_none, /*ModulePrivateLoc=*/SourceLocation(),
                MultiTemplateParamsArg(), Owned, IsDependent, SourceLocation(),
                false, TypeResult(), /*IsTypeSpecifier*/ false,
-               /*IsTemplateParamOrArg*/ false, /*OOK=*/OOK_Outside).get();
+               /*IsTemplateParamOrArg*/ false, /*OOK=*/OOK_Outside, FoundUsing)
+          .get();
   assert(!IsDependent && "explicit instantiation of dependent name not yet handled");
 
   if (!TagD)

clang/lib/Sema/SemaType.cpp

@@ -1588,6 +1588,9 @@ static QualType ConvertDeclSpecToType(TypeProcessingState &state) {
 
     // TypeQuals handled by caller.
     Result = Context.getTypeDeclType(D);
+    if (const auto *Using =
+            dyn_cast_or_null<UsingShadowDecl>(DS.getRepAsFoundDecl()))
+      Result = Context.getUsingType(Using, Result);
 
     // In both C and C++, make an ElaboratedType.
     ElaboratedTypeKeyword Keyword

clang/test/CXX/drs/dr2xx.cpp

@@ -992,7 +992,7 @@ namespace dr284 { // dr284: no
   }
   struct B::V {}; // expected-error {{no struct named 'V'}}
   struct B::W {};
-  struct B::X {}; // FIXME: ill-formed
+  struct B::X {}; // expected-error {{forward declaration of struct cannot have}}
   enum B::Y e; // ok per dr417
   class B::Z z; // ok per dr417
 
@@ -1009,7 +1009,7 @@ namespace dr284 { // dr284: no
   };
   struct D::V {}; // expected-error {{no struct named 'V'}}
   struct D::W {};
-  struct D::X {}; // FIXME: ill-formed
+  struct D::X {}; // expected-error {{forward declaration of struct cannot have}}
   enum D::Y e2; // ok per dr417
   class D::Z z2; // ok per dr417
 }

clang/test/CXX/drs/dr4xx.cpp

@@ -301,9 +301,8 @@ namespace dr417 { // dr417: no
     struct F;
     struct H;
   }
-  // FIXME: This is ill-formed.
   using N::D;
-  struct dr417::D {}; // expected-warning {{extra qualification}}
+  struct dr417::D {}; // expected-error {{forward declaration of struct cannot}} expected-warning {{extra qualification}}
   using namespace N;
   struct dr417::E {}; // expected-warning {{extra qualification}} expected-error {{no struct named 'E'}}
   struct N::F {};