1 files changed, 77 insertions, 6 deletions

diff --git a/lib/Sema/SemaDecl.cpp b/lib/Sema/SemaDecl.cpp
index fd172def2d..59bfa49066 100644
--- a/lib/Sema/SemaDecl.cpp
+++ b/lib/Sema/SemaDecl.cpp
@@ -13233,6 +13233,55 @@ Decl *Sema::ActOnTag(Scope *S, unsigned TagSpec, TagUseKind TUK,
   if (TUK == TUK_Friend || TUK == TUK_Reference)
     Redecl = NotForRedeclaration;
 
+  /// Create a new tag decl in C/ObjC. Since the ODR-like semantics for ObjC/C
+  /// implemented asks for structural equivalence checking, the returned decl
+  /// here is passed back to the parser, allowing the tag body to be parsed.
+  auto createTagFromNewDecl = [&]() -> TagDecl * {
+    assert(!getLangOpts().CPlusPlus && "not meant for C++ usage");
+    // If there is an identifier, use the location of the identifier as the
+    // location of the decl, otherwise use the location of the struct/union
+    // keyword.
+    SourceLocation Loc = NameLoc.isValid() ? NameLoc : KWLoc;
+    TagDecl *New = nullptr;
+
+    if (Kind == TTK_Enum) {
+      New = EnumDecl::Create(Context, SearchDC, KWLoc, Loc, Name, nullptr,
+                             ScopedEnum, ScopedEnumUsesClassTag,
+                             !EnumUnderlying.isNull());
+      // If this is an undefined enum, bail.
+      if (TUK != TUK_Definition && !Invalid)
+        return nullptr;
+      if (EnumUnderlying) {
+        EnumDecl *ED = cast<EnumDecl>(New);
+        if (TypeSourceInfo *TI = EnumUnderlying.dyn_cast<TypeSourceInfo *>())
+          ED->setIntegerTypeSourceInfo(TI);
+        else
+          ED->setIntegerType(QualType(EnumUnderlying.get<const Type *>(), 0));
+        ED->setPromotionType(ED->getIntegerType());
+      }
+    } else { // struct/union
+      New = RecordDecl::Create(Context, Kind, SearchDC, KWLoc, Loc, Name,
+                               nullptr);
+    }
+
+    if (RecordDecl *RD = dyn_cast<RecordDecl>(New)) {
+      // Add alignment attributes if necessary; these attributes are checked
+      // when the ASTContext lays out the structure.
+      //
+      // It is important for implementing the correct semantics that this
+      // happen here (in ActOnTag). The #pragma pack stack is
+      // maintained as a result of parser callbacks which can occur at
+      // many points during the parsing of a struct declaration (because
+      // the #pragma tokens are effectively skipped over during the
+      // parsing of the struct).
+      if (TUK == TUK_Definition) {
+        AddAlignmentAttributesForRecord(RD);
+        AddMsStructLayoutForRecord(RD);
+      }
+    }
+    return New;
+  };
+
   LookupResult Previous(*this, Name, NameLoc, LookupTagName, Redecl);
   if (Name && SS.isNotEmpty()) {
     // We have a nested-name tag ('struct foo::bar').
@@ -13638,16 +13687,28 @@ Decl *Sema::ActOnTag(Scope *S, unsigned TagSpec, TagUseKind TUK,
                     TSK_ExplicitSpecialization;
               }
 
+              // Note that clang allows ODR-like semantics for ObjC/C, i.e., do
+              // not keep more that one definition around (merge them). However,
+              // ensure the decl passes the structural compatibility check in
+              // C11 6.2.7/1 (or 6.1.2.6/1 in C89).
               NamedDecl *Hidden = nullptr;
-              if (SkipBody && getLangOpts().CPlusPlus &&
-                  !hasVisibleDefinition(Def, &Hidden)) {
+              if (SkipBody && !hasVisibleDefinition(Def, &Hidden)) {
                 // There is a definition of this tag, but it is not visible. We
                 // explicitly make use of C++'s one definition rule here, and
                 // assume that this definition is identical to the hidden one
                 // we already have. Make the existing definition visible and
                 // use it in place of this one.
-                SkipBody->ShouldSkip = true;
-                makeMergedDefinitionVisible(Hidden);
+                if (!getLangOpts().CPlusPlus) {
+                  // Postpone making the old definition visible until after we
+                  // complete parsing the new one and do the structural
+                  // comparison.
+                  SkipBody->CheckSameAsPrevious = true;
+                  SkipBody->New = createTagFromNewDecl();
+                  SkipBody->Previous = Hidden;
+                } else {
+                  SkipBody->ShouldSkip = true;
+                  makeMergedDefinitionVisible(Hidden);
+                }
                 return Def;
               } else if (!IsExplicitSpecializationAfterInstantiation) {
                 // A redeclaration in function prototype scope in C isn't
@@ -13875,7 +13936,7 @@ CreateNewDecl:
     // the ASTContext lays out the structure.
     //
     // It is important for implementing the correct semantics that this
-    // happen here (in act on tag decl). The #pragma pack stack is
+    // happen here (in ActOnTag). The #pragma pack stack is
     // maintained as a result of parser callbacks which can occur at
     // many points during the parsing of a struct declaration (because
     // the #pragma tokens are effectively skipped over during the
@@ -14011,6 +14072,16 @@ void Sema::ActOnTagStartDefinition(Scope *S, Decl *TagD) {
   AddPushedVisibilityAttribute(Tag);
 }
 
+bool Sema::ActOnDuplicateDefinition(DeclSpec &DS, Decl *Prev,
+                                    SkipBodyInfo &SkipBody) {
+  if (!hasStructuralCompatLayout(Prev, SkipBody.New))
+    return false;
+
+  // Make the previous decl visible.
+  makeMergedDefinitionVisible(SkipBody.Previous);
+  return true;
+}
+
 Decl *Sema::ActOnObjCContainerStartDefinition(Decl *IDecl) {
   assert(isa<ObjCContainerDecl>(IDecl) &&
          "ActOnObjCContainerStartDefinition - Not ObjCContainerDecl");
@@ -15432,7 +15503,7 @@ Decl *Sema::ActOnEnumConstant(Scope *S, Decl *theEnumDecl, Decl *lastEnumConst,
   // different from T:
   // - every enumerator of every member of class T that is an unscoped
   // enumerated type
-  if (!TheEnumDecl->isScoped())
+  if (getLangOpts().CPlusPlus && !TheEnumDecl->isScoped())
     DiagnoseClassNameShadow(TheEnumDecl->getDeclContext(),
                             DeclarationNameInfo(Id, IdLoc));