bruno updated this revision to Diff 94471.
bruno added a comment.

Update patch on top of Adrian comments


https://reviews.llvm.org/D31778

Files:
  include/clang/AST/ASTStructuralEquivalence.h
  include/clang/Basic/DiagnosticASTKinds.td
  include/clang/Parse/Parser.h
  include/clang/Sema/Sema.h
  lib/AST/ASTStructuralEquivalence.cpp
  lib/Parse/ParseDecl.cpp
  lib/Parse/ParseDeclCXX.cpp
  lib/Parse/ParseExpr.cpp
  lib/Sema/SemaDecl.cpp
  lib/Sema/SemaExpr.cpp
  lib/Sema/SemaType.cpp
  test/Modules/Inputs/F.framework/Headers/F.h
  test/Modules/Inputs/F.framework/Modules/module.modulemap
  test/Modules/Inputs/F.framework/Modules/module.private.modulemap
  test/Modules/Inputs/F.framework/PrivateHeaders/NS.h
  test/Modules/elaborated-type-specifier-from-hidden-module.m
  test/Modules/redefinition-c-tagtypes.m

Index: test/Modules/redefinition-c-tagtypes.m
===================================================================
--- /dev/null
+++ test/Modules/redefinition-c-tagtypes.m
@@ -0,0 +1,48 @@
+// RUN: rm -rf %t.cache
+// RUN: %clang_cc1 -fsyntax-only %s -fmodules -fmodules-cache-path=%t.cache \
+// RUN:   -fimplicit-module-maps -F%S/Inputs -verify
+// RUN: %clang_cc1 -fsyntax-only %s -fmodules -fmodules-cache-path=%t.cache \
+// RUN:   -fimplicit-module-maps -F%S/Inputs -DCHANGE_TAGS -verify
+#include "F/F.h"
+
+#ifndef CHANGE_TAGS
+// expected-no-diagnostics
+#endif
+
+struct NS {
+  int a;
+#ifndef CHANGE_TAGS
+  int b;
+#else
+  int c; // expected-note {{field has name 'c' here}}
+  // expected-error@redefinition-c-tagtypes.m:12 {{type 'struct NS' has incompatible definitions}}
+  // expected-note@Inputs/F.framework/PrivateHeaders/NS.h:3 {{field has name 'b' here}}
+#endif
+};
+
+enum NSE {
+  FST = 22,
+#ifndef CHANGE_TAGS
+  SND = 43,
+#else
+  SND = 44, // expected-note {{enumerator 'SND' with value 44 here}}
+  // expected-error@redefinition-c-tagtypes.m:23 {{type 'enum NSE' has incompatible definitions}}
+  // expected-note@Inputs/F.framework/PrivateHeaders/NS.h:8 {{enumerator 'SND' with value 43 here}}
+#endif
+  TRD = 55
+};
+
+#define NS_ENUM(_type, _name) \
+  enum _name : _type _name;   \
+  enum _name : _type
+
+typedef NS_ENUM(int, NSMyEnum) {
+  MinX = 11,
+#ifndef CHANGE_TAGS
+  MinXOther = MinX,
+#else
+  MinXOther = TRD, // expected-note {{enumerator 'MinXOther' with value 55 here}}
+  // expected-error@redefinition-c-tagtypes.m:39 {{type 'enum NSMyEnum' has incompatible definitions}}
+  // expected-note@Inputs/F.framework/PrivateHeaders/NS.h:18 {{enumerator 'MinXOther' with value 11 here}}
+#endif
+};
Index: test/Modules/elaborated-type-specifier-from-hidden-module.m
===================================================================
--- test/Modules/elaborated-type-specifier-from-hidden-module.m
+++ test/Modules/elaborated-type-specifier-from-hidden-module.m
@@ -4,12 +4,11 @@
 @import ElaboratedTypeStructs.Empty; // The structs are now hidden.
 struct S1 *x;
 struct S2 *y;
-// FIXME: compatible definition should not be an error.
-struct S2 { int x; }; // expected-error {{redefinition}}
+struct S2 { int x; };
 struct S3 *z;
 // Incompatible definition.
-struct S3 { float y; }; // expected-error {{redefinition}}
-// expected-note@elaborated-type-structs.h:* 2 {{previous definition is here}}
+struct S3 { float y; }; // expected-error {{has incompatible definitions}} // expected-note {{field has name}}
+// expected-note@Inputs/elaborated-type-structs.h:3 {{field has name}}
 
 @import ElaboratedTypeStructs.Structs;
 
Index: test/Modules/Inputs/F.framework/PrivateHeaders/NS.h
===================================================================
--- /dev/null
+++ test/Modules/Inputs/F.framework/PrivateHeaders/NS.h
@@ -0,0 +1,19 @@
+struct NS {
+  int a;
+  int b;
+};
+
+enum NSE {
+  FST = 22,
+  SND = 43,
+  TRD = 55
+};
+
+#define NS_ENUM(_type, _name) \
+  enum _name : _type _name;   \
+  enum _name : _type
+
+typedef NS_ENUM(int, NSMyEnum) {
+  MinX = 11,
+  MinXOther = MinX,
+};
Index: test/Modules/Inputs/F.framework/Modules/module.private.modulemap
===================================================================
--- /dev/null
+++ test/Modules/Inputs/F.framework/Modules/module.private.modulemap
@@ -0,0 +1,7 @@
+module F.Private [system] {
+  explicit module NS {
+      header "NS.h"
+      export *
+  }
+  export *
+}
Index: test/Modules/Inputs/F.framework/Modules/module.modulemap
===================================================================
--- /dev/null
+++ test/Modules/Inputs/F.framework/Modules/module.modulemap
@@ -0,0 +1,7 @@
+framework module F [extern_c] [system] {
+  umbrella header "F.h"
+  module * {
+      export *
+  }
+  export *
+}
Index: test/Modules/Inputs/F.framework/Headers/F.h
===================================================================
--- /dev/null
+++ test/Modules/Inputs/F.framework/Headers/F.h
@@ -0,0 +1 @@
+// F.h
Index: lib/Sema/SemaType.cpp
===================================================================
--- lib/Sema/SemaType.cpp
+++ lib/Sema/SemaType.cpp
@@ -15,6 +15,7 @@
 #include "clang/AST/ASTConsumer.h"
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/ASTMutationListener.h"
+#include "clang/AST/ASTStructuralEquivalence.h"
 #include "clang/AST/CXXInheritance.h"
 #include "clang/AST/DeclObjC.h"
 #include "clang/AST/DeclTemplate.h"
@@ -7081,6 +7082,20 @@
   return false;
 }
 
+bool Sema::hasStructuralCompatLayout(Decl *D, Decl *Suggested) {
+  llvm::DenseSet<std::pair<Decl *, Decl *>> NonEquivalentDecls;
+  if (!Suggested)
+    return false;
+
+  // FIXME: Add a specific mode for C11 6.2.7/1 in StructuralEquivalenceContext
+  // and isolate from other C++ specific checks.
+  StructuralEquivalenceContext Ctx(
+      D->getASTContext(), Suggested->getASTContext(), NonEquivalentDecls,
+      false /*StrictTypeSpelling*/, true /*Complain*/,
+      true /*ErrorOnTagTypeMismatch*/);
+  return Ctx.IsStructurallyEquivalent(D, Suggested);
+}
+
 /// \brief Determine whether there is any declaration of \p D that was ever a
 ///        definition (perhaps before module merging) and is currently visible.
 /// \param D The definition of the entity.
Index: lib/Sema/SemaExpr.cpp
===================================================================
--- lib/Sema/SemaExpr.cpp
+++ lib/Sema/SemaExpr.cpp
@@ -2109,7 +2109,8 @@
                         SourceLocation TemplateKWLoc, UnqualifiedId &Id,
                         bool HasTrailingLParen, bool IsAddressOfOperand,
                         std::unique_ptr<CorrectionCandidateCallback> CCC,
-                        bool IsInlineAsmIdentifier, Token *KeywordReplacement) {
+                        bool IsInlineAsmIdentifier, Token *KeywordReplacement,
+                        bool IgnorePrevDef) {
   assert(!(IsAddressOfOperand && HasTrailingLParen) &&
          "cannot be direct & operand and have a trailing lparen");
   if (SS.isInvalid())
@@ -2194,8 +2195,18 @@
     }
   }
 
-  if (R.isAmbiguous())
-    return ExprError();
+  if (R.isAmbiguous()) {
+    if (!IgnorePrevDef)
+      return ExprError();
+    // We already know that there's a hidden decl included in the lookup,
+    // ignore it and only use the first found (the local) one...
+    auto RF = R.makeFilter();
+    NamedDecl *ND = R.getRepresentativeDecl();
+    while (RF.hasNext())
+      if (ND != RF.next())
+        RF.erase();
+    RF.done();
+  }
 
   // This could be an implicitly declared function reference (legal in C90,
   // extension in C99, forbidden in C++).
Index: lib/Sema/SemaDecl.cpp
===================================================================
--- lib/Sema/SemaDecl.cpp
+++ lib/Sema/SemaDecl.cpp
@@ -12822,9 +12822,9 @@
                      MultiTemplateParamsArg TemplateParameterLists,
                      bool &OwnedDecl, bool &IsDependent,
                      SourceLocation ScopedEnumKWLoc,
-                     bool ScopedEnumUsesClassTag,
-                     TypeResult UnderlyingType,
-                     bool IsTypeSpecifier, SkipBodyInfo *SkipBody) {
+                     bool ScopedEnumUsesClassTag, TypeResult UnderlyingType,
+                     bool IsTypeSpecifier, SkipBodyInfo *SkipBody,
+                     CheckCompatTagInfo *CheckCompatTag) {
   // If this is not a definition, it must have a name.
   IdentifierInfo *OrigName = Name;
   assert((Name != nullptr || TUK == TUK_Definition) &&
@@ -12923,6 +12923,55 @@
   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 = [&](void) -> 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 act on tag decl). 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').
@@ -13330,15 +13379,24 @@
                     TSK_ExplicitSpecialization;
               }
 
+              // Allow ODR for ObjC/C in the sense that we won't keep more that
+              // one definition around (merge them). However, ensure the decl
+              // pass the structural compatibility check in C11 6.2.7/1.
+              bool AllowODR = getLangOpts().CPlusPlus || getLangOpts().ObjC1 ||
+                              getLangOpts().C11;
               NamedDecl *Hidden = nullptr;
-              if (SkipBody && getLangOpts().CPlusPlus &&
-                  !hasVisibleDefinition(Def, &Hidden)) {
+              if (SkipBody && AllowODR && !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;
+                if (!getLangOpts().CPlusPlus) {
+                  CheckCompatTag->CheckStructuralEquivalence = true;
+                  CheckCompatTag->NewDef = createTagFromNewDecl();
+                } else {
+                  SkipBody->ShouldSkip = true;
+                }
                 makeMergedDefinitionVisible(Hidden, KWLoc);
                 return Def;
               } else if (!IsExplicitSpecializationAfterInstantiation) {
@@ -15093,8 +15151,8 @@
 
 Decl *Sema::ActOnEnumConstant(Scope *S, Decl *theEnumDecl, Decl *lastEnumConst,
                               SourceLocation IdLoc, IdentifierInfo *Id,
-                              AttributeList *Attr,
-                              SourceLocation EqualLoc, Expr *Val) {
+                              AttributeList *Attr, SourceLocation EqualLoc,
+                              Expr *Val, bool IgnorePrevDef) {
   EnumDecl *TheEnumDecl = cast<EnumDecl>(theEnumDecl);
   EnumConstantDecl *LastEnumConst =
     cast_or_null<EnumConstantDecl>(lastEnumConst);
@@ -15119,18 +15177,20 @@
   // 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));
 
   EnumConstantDecl *New =
     CheckEnumConstant(TheEnumDecl, LastEnumConst, IdLoc, Id, Val);
   if (!New)
     return nullptr;
 
-  if (PrevDecl) {
+  if (!IgnorePrevDef && PrevDecl) {
     // When in C++, we may get a TagDecl with the same name; in this case the
-    // enum constant will 'hide' the tag.
+    // enum constant will 'hide' the tag. In C/ObjC callers of this action
+    // might set IgnorePrevDef in order to speculatively generate a new def,
+    // which will never be actually added to the scope.
     assert((getLangOpts().CPlusPlus || !isa<TagDecl>(PrevDecl)) &&
            "Received TagDecl when not in C++!");
     if (!isa<TagDecl>(PrevDecl) && isDeclInScope(PrevDecl, CurContext, S) &&
Index: lib/Parse/ParseExpr.cpp
===================================================================
--- lib/Parse/ParseExpr.cpp
+++ lib/Parse/ParseExpr.cpp
@@ -192,16 +192,16 @@
   return ParseRHSOfBinaryExpression(R, prec::Assignment);
 }
 
-
-ExprResult Parser::ParseConstantExpression(TypeCastState isTypeCast) {
+ExprResult Parser::ParseConstantExpression(TypeCastState isTypeCast,
+                                           bool IgnorePrevDef) {
   // C++03 [basic.def.odr]p2:
   //   An expression is potentially evaluated unless it appears where an
   //   integral constant expression is required (see 5.19) [...].
   // C++98 and C++11 have no such rule, but this is only a defect in C++98.
   EnterExpressionEvaluationContext ConstantEvaluated(Actions,
                                                Sema::ConstantEvaluated);
 
-  ExprResult LHS(ParseCastExpression(false, false, isTypeCast));
+  ExprResult LHS(ParseCastExpression(false, false, isTypeCast, IgnorePrevDef));
   ExprResult Res(ParseRHSOfBinaryExpression(LHS, prec::Conditional));
   return Actions.ActOnConstantExpression(Res);
 }
@@ -473,12 +473,11 @@
 ///
 ExprResult Parser::ParseCastExpression(bool isUnaryExpression,
                                        bool isAddressOfOperand,
-                                       TypeCastState isTypeCast) {
+                                       TypeCastState isTypeCast,
+                                       bool IgnorePrevDef) {
   bool NotCastExpr;
-  ExprResult Res = ParseCastExpression(isUnaryExpression,
-                                       isAddressOfOperand,
-                                       NotCastExpr,
-                                       isTypeCast);
+  ExprResult Res = ParseCastExpression(isUnaryExpression, isAddressOfOperand,
+                                       NotCastExpr, isTypeCast, IgnorePrevDef);
   if (NotCastExpr)
     Diag(Tok, diag::err_expected_expression);
   return Res;
@@ -694,7 +693,8 @@
 ExprResult Parser::ParseCastExpression(bool isUnaryExpression,
                                        bool isAddressOfOperand,
                                        bool &NotCastExpr,
-                                       TypeCastState isTypeCast) {
+                                       TypeCastState isTypeCast,
+                                       bool IgnorePrevDef) {
   ExprResult Res;
   tok::TokenKind SavedKind = Tok.getKind();
   NotCastExpr = false;
@@ -980,7 +980,7 @@
         getCurScope(), ScopeSpec, TemplateKWLoc, Name, Tok.is(tok::l_paren),
         isAddressOfOperand, std::move(Validator),
         /*IsInlineAsmIdentifier=*/false,
-        Tok.is(tok::r_paren) ? nullptr : &Replacement);
+        Tok.is(tok::r_paren) ? nullptr : &Replacement, IgnorePrevDef);
     if (!Res.isInvalid() && !Res.get()) {
       UnconsumeToken(Replacement);
       return ParseCastExpression(isUnaryExpression, isAddressOfOperand,
Index: lib/Parse/ParseDeclCXX.cpp
===================================================================
--- lib/Parse/ParseDeclCXX.cpp
+++ lib/Parse/ParseDeclCXX.cpp
@@ -1734,6 +1734,7 @@
 
   bool Owned = false;
   Sema::SkipBodyInfo SkipBody;
+  Sema::CheckCompatTagInfo CheckCompatTag;
   if (TemplateId) {
     // Explicit specialization, class template partial specialization,
     // or explicit instantiation.
@@ -1883,7 +1884,7 @@
                                        SourceLocation(), false,
                                        clang::TypeResult(),
                                        DSC == DSC_type_specifier,
-                                       &SkipBody);
+                                       &SkipBody, &CheckCompatTag);
 
     // If ActOnTag said the type was dependent, try again with the
     // less common call.
@@ -1905,8 +1906,23 @@
     else if (getLangOpts().CPlusPlus)
       ParseCXXMemberSpecification(StartLoc, AttrFixitLoc, attrs, TagType,
                                   TagOrTempResult.get());
-    else
-      ParseStructUnionBody(StartLoc, TagType, TagOrTempResult.get());
+    else {
+      Decl *D = CheckCompatTag.CheckStructuralEquivalence
+                    ? CheckCompatTag.NewDef
+                    : TagOrTempResult.get();
+      // Parse the definition body.
+      ParseStructUnionBody(StartLoc, TagType, D);
+      // Perform ODR-like check for C/ObjC when merging tag types from modules.
+      // Differently from C++, actually parse the body and reject / error out
+      // in case of a structural mismatch.
+      if (CheckCompatTag.CheckStructuralEquivalence &&
+          !Actions.hasStructuralCompatLayout(TagOrTempResult.get(),
+                                             CheckCompatTag.NewDef)) {
+        // Bail out.
+        DS.SetTypeSpecError();
+        return;
+      }
+    }
   }
 
   if (!TagOrTempResult.isInvalid())
Index: lib/Parse/ParseDecl.cpp
===================================================================
--- lib/Parse/ParseDecl.cpp
+++ lib/Parse/ParseDecl.cpp
@@ -4234,6 +4234,7 @@
   stripTypeAttributesOffDeclSpec(attrs, DS, TUK);
 
   Sema::SkipBodyInfo SkipBody;
+  Sema::CheckCompatTagInfo CheckCompatTag;
   if (!Name && TUK == Sema::TUK_Definition && Tok.is(tok::l_brace) &&
       NextToken().is(tok::identifier))
     SkipBody = Actions.shouldSkipAnonEnumBody(getCurScope(),
@@ -4244,12 +4245,11 @@
   bool IsDependent = false;
   const char *PrevSpec = nullptr;
   unsigned DiagID;
-  Decl *TagDecl = Actions.ActOnTag(getCurScope(), DeclSpec::TST_enum, TUK,
-                                   StartLoc, SS, Name, NameLoc, attrs.getList(),
-                                   AS, DS.getModulePrivateSpecLoc(), TParams,
-                                   Owned, IsDependent, ScopedEnumKWLoc,
-                                   IsScopedUsingClassTag, BaseType,
-                                   DSC == DSC_type_specifier, &SkipBody);
+  Decl *TagDecl = Actions.ActOnTag(
+      getCurScope(), DeclSpec::TST_enum, TUK, StartLoc, SS, Name, NameLoc,
+      attrs.getList(), AS, DS.getModulePrivateSpecLoc(), TParams, Owned,
+      IsDependent, ScopedEnumKWLoc, IsScopedUsingClassTag, BaseType,
+      DSC == DSC_type_specifier, &SkipBody, &CheckCompatTag);
 
   if (SkipBody.ShouldSkip) {
     assert(TUK == Sema::TUK_Definition && "can only skip a definition");
@@ -4303,8 +4303,21 @@
     return;
   }
 
-  if (Tok.is(tok::l_brace) && TUK != Sema::TUK_Reference)
-    ParseEnumBody(StartLoc, TagDecl);
+  if (Tok.is(tok::l_brace) && TUK != Sema::TUK_Reference) {
+    Decl *D = CheckCompatTag.CheckStructuralEquivalence ? CheckCompatTag.NewDef
+                                                        : TagDecl;
+    ParseEnumBody(StartLoc, D,
+                  CheckCompatTag.CheckStructuralEquivalence /*IgnorePrevDef*/);
+    // Perform ODR-like check for C/ObjC when merging tag types from modules.
+    // Differently from C++, actually parse the body and reject / error out
+    // in case of a structural mismatch.
+    if (CheckCompatTag.CheckStructuralEquivalence &&
+        !Actions.hasStructuralCompatLayout(TagDecl, CheckCompatTag.NewDef)) {
+      // Bail out.
+      DS.SetTypeSpecError();
+      return;
+    }
+  }
 
   if (DS.SetTypeSpecType(DeclSpec::TST_enum, StartLoc,
                          NameLoc.isValid() ? NameLoc : StartLoc,
@@ -4323,7 +4336,8 @@
 ///       enumeration-constant:
 ///         identifier
 ///
-void Parser::ParseEnumBody(SourceLocation StartLoc, Decl *EnumDecl) {
+void Parser::ParseEnumBody(SourceLocation StartLoc, Decl *EnumDecl,
+                           bool IgnorePrevDef) {
   // Enter the scope of the enum body and start the definition.
   ParseScope EnumScope(this, Scope::DeclScope | Scope::EnumScope);
   Actions.ActOnTagStartDefinition(getCurScope(), EnumDecl);
@@ -4370,17 +4384,16 @@
     EnumAvailabilityDiags.emplace_back(*this);
 
     if (TryConsumeToken(tok::equal, EqualLoc)) {
-      AssignedVal = ParseConstantExpression();
+      AssignedVal =
+          ParseConstantExpression(NotTypeCast /*isTypeCast*/, IgnorePrevDef);
       if (AssignedVal.isInvalid())
         SkipUntil(tok::comma, tok::r_brace, StopBeforeMatch);
     }
 
     // Install the enumerator constant into EnumDecl.
-    Decl *EnumConstDecl = Actions.ActOnEnumConstant(getCurScope(), EnumDecl,
-                                                    LastEnumConstDecl,
-                                                    IdentLoc, Ident,
-                                                    attrs.getList(), EqualLoc,
-                                                    AssignedVal.get());
+    Decl *EnumConstDecl = Actions.ActOnEnumConstant(
+        getCurScope(), EnumDecl, LastEnumConstDecl, IdentLoc, Ident,
+        attrs.getList(), EqualLoc, AssignedVal.get(), IgnorePrevDef);
     EnumAvailabilityDiags.back().done();
 
     EnumConstantDecls.push_back(EnumConstDecl);
Index: lib/AST/ASTStructuralEquivalence.cpp
===================================================================
--- lib/AST/ASTStructuralEquivalence.cpp
+++ lib/AST/ASTStructuralEquivalence.cpp
@@ -734,13 +734,28 @@
   // Check for equivalent field names.
   IdentifierInfo *Name1 = Field1->getIdentifier();
   IdentifierInfo *Name2 = Field2->getIdentifier();
-  if (!::IsStructurallyEquivalent(Name1, Name2))
+  if (!::IsStructurallyEquivalent(Name1, Name2)) {
+    if (Context.Complain) {
+      Context.Diag2(Owner2->getLocation(),
+                    Context.ErrorOnTagTypeMismatch
+                        ? diag::err_odr_tag_type_inconsistent
+                        : diag::warn_odr_tag_type_inconsistent)
+          << Context.C2.getTypeDeclType(Owner2);
+      Context.Diag2(Field2->getLocation(), diag::note_odr_field_name)
+          << Field2->getDeclName();
+      Context.Diag1(Field1->getLocation(), diag::note_odr_field_name)
+          << Field1->getDeclName();
+    }
     return false;
+  }
 
   if (!IsStructurallyEquivalent(Context, Field1->getType(),
                                 Field2->getType())) {
     if (Context.Complain) {
-      Context.Diag2(Owner2->getLocation(), diag::warn_odr_tag_type_inconsistent)
+      Context.Diag2(Owner2->getLocation(),
+                    Context.ErrorOnTagTypeMismatch
+                        ? diag::err_odr_tag_type_inconsistent
+                        : diag::warn_odr_tag_type_inconsistent)
           << Context.C2.getTypeDeclType(Owner2);
       Context.Diag2(Field2->getLocation(), diag::note_odr_field)
           << Field2->getDeclName() << Field2->getType();
@@ -752,7 +767,10 @@
 
   if (Field1->isBitField() != Field2->isBitField()) {
     if (Context.Complain) {
-      Context.Diag2(Owner2->getLocation(), diag::warn_odr_tag_type_inconsistent)
+      Context.Diag2(Owner2->getLocation(),
+                    Context.ErrorOnTagTypeMismatch
+                        ? diag::err_odr_tag_type_inconsistent
+                        : diag::warn_odr_tag_type_inconsistent)
           << Context.C2.getTypeDeclType(Owner2);
       if (Field1->isBitField()) {
         Context.Diag1(Field1->getLocation(), diag::note_odr_bit_field)
@@ -779,7 +797,9 @@
     if (Bits1 != Bits2) {
       if (Context.Complain) {
         Context.Diag2(Owner2->getLocation(),
-                      diag::warn_odr_tag_type_inconsistent)
+                      Context.ErrorOnTagTypeMismatch
+                          ? diag::err_odr_tag_type_inconsistent
+                          : diag::warn_odr_tag_type_inconsistent)
             << Context.C2.getTypeDeclType(Owner2);
         Context.Diag2(Field2->getLocation(), diag::note_odr_bit_field)
             << Field2->getDeclName() << Field2->getType() << Bits2;
@@ -798,7 +818,10 @@
                                      RecordDecl *D1, RecordDecl *D2) {
   if (D1->isUnion() != D2->isUnion()) {
     if (Context.Complain) {
-      Context.Diag2(D2->getLocation(), diag::warn_odr_tag_type_inconsistent)
+      Context.Diag2(D2->getLocation(),
+                    Context.ErrorOnTagTypeMismatch
+                        ? diag::err_odr_tag_type_inconsistent
+                        : diag::warn_odr_tag_type_inconsistent)
           << Context.C2.getTypeDeclType(D2);
       Context.Diag1(D1->getLocation(), diag::note_odr_tag_kind_here)
           << D1->getDeclName() << (unsigned)D1->getTagKind();
@@ -921,7 +944,10 @@
        Field1 != Field1End; ++Field1, ++Field2) {
     if (Field2 == Field2End) {
       if (Context.Complain) {
-        Context.Diag2(D2->getLocation(), diag::warn_odr_tag_type_inconsistent)
+        Context.Diag2(D2->getLocation(),
+                      Context.ErrorOnTagTypeMismatch
+                          ? diag::err_odr_tag_type_inconsistent
+                          : diag::warn_odr_tag_type_inconsistent)
             << Context.C2.getTypeDeclType(D2);
         Context.Diag1(Field1->getLocation(), diag::note_odr_field)
             << Field1->getDeclName() << Field1->getType();
@@ -936,7 +962,10 @@
 
   if (Field2 != Field2End) {
     if (Context.Complain) {
-      Context.Diag2(D2->getLocation(), diag::warn_odr_tag_type_inconsistent)
+      Context.Diag2(D2->getLocation(),
+                    Context.ErrorOnTagTypeMismatch
+                        ? diag::err_odr_tag_type_inconsistent
+                        : diag::warn_odr_tag_type_inconsistent)
           << Context.C2.getTypeDeclType(D2);
       Context.Diag2(Field2->getLocation(), diag::note_odr_field)
           << Field2->getDeclName() << Field2->getType();
@@ -958,7 +987,10 @@
        EC1 != EC1End; ++EC1, ++EC2) {
     if (EC2 == EC2End) {
       if (Context.Complain) {
-        Context.Diag2(D2->getLocation(), diag::warn_odr_tag_type_inconsistent)
+        Context.Diag2(D2->getLocation(),
+                      Context.ErrorOnTagTypeMismatch
+                          ? diag::err_odr_tag_type_inconsistent
+                          : diag::warn_odr_tag_type_inconsistent)
             << Context.C2.getTypeDeclType(D2);
         Context.Diag1(EC1->getLocation(), diag::note_odr_enumerator)
             << EC1->getDeclName() << EC1->getInitVal().toString(10);
@@ -972,7 +1004,10 @@
     if (!llvm::APSInt::isSameValue(Val1, Val2) ||
         !IsStructurallyEquivalent(EC1->getIdentifier(), EC2->getIdentifier())) {
       if (Context.Complain) {
-        Context.Diag2(D2->getLocation(), diag::warn_odr_tag_type_inconsistent)
+        Context.Diag2(D2->getLocation(),
+                      Context.ErrorOnTagTypeMismatch
+                          ? diag::err_odr_tag_type_inconsistent
+                          : diag::warn_odr_tag_type_inconsistent)
             << Context.C2.getTypeDeclType(D2);
         Context.Diag2(EC2->getLocation(), diag::note_odr_enumerator)
             << EC2->getDeclName() << EC2->getInitVal().toString(10);
@@ -985,7 +1020,10 @@
 
   if (EC2 != EC2End) {
     if (Context.Complain) {
-      Context.Diag2(D2->getLocation(), diag::warn_odr_tag_type_inconsistent)
+      Context.Diag2(D2->getLocation(),
+                    Context.ErrorOnTagTypeMismatch
+                        ? diag::err_odr_tag_type_inconsistent
+                        : diag::warn_odr_tag_type_inconsistent)
           << Context.C2.getTypeDeclType(D2);
       Context.Diag2(EC2->getLocation(), diag::note_odr_enumerator)
           << EC2->getDeclName() << EC2->getInitVal().toString(10);
Index: include/clang/Sema/Sema.h
===================================================================
--- include/clang/Sema/Sema.h
+++ include/clang/Sema/Sema.h
@@ -1461,6 +1461,10 @@
 
   bool hasVisibleMergedDefinition(NamedDecl *Def);
 
+  /// Determine if \p D abd \p Suggested have a structurally compatibale
+  /// layout as described in C11 6.2.7/1.
+  bool hasStructuralCompatLayout(Decl *D, Decl *Suggested);
+
   /// Determine if \p D has a visible definition. If not, suggest a declaration
   /// that should be made visible to expose the definition.
   bool hasVisibleDefinition(NamedDecl *D, NamedDecl **Suggested,
@@ -1547,6 +1551,12 @@
     NamedDecl *Previous;
   };
 
+  struct CheckCompatTagInfo {
+    CheckCompatTagInfo() : CheckStructuralEquivalence(false), NewDef(nullptr) {}
+    bool CheckStructuralEquivalence;
+    NamedDecl *NewDef;
+  };
+
   DeclGroupPtrTy ConvertDeclToDeclGroup(Decl *Ptr, Decl *OwnedType = nullptr);
 
   void DiagnoseUseOfUnimplementedSelectors();
@@ -2037,15 +2047,14 @@
   };
 
   Decl *ActOnTag(Scope *S, unsigned TagSpec, TagUseKind TUK,
-                 SourceLocation KWLoc, CXXScopeSpec &SS,
-                 IdentifierInfo *Name, SourceLocation NameLoc,
-                 AttributeList *Attr, AccessSpecifier AS,
-                 SourceLocation ModulePrivateLoc,
-                 MultiTemplateParamsArg TemplateParameterLists,
-                 bool &OwnedDecl, bool &IsDependent,
-                 SourceLocation ScopedEnumKWLoc,
+                 SourceLocation KWLoc, CXXScopeSpec &SS, IdentifierInfo *Name,
+                 SourceLocation NameLoc, AttributeList *Attr,
+                 AccessSpecifier AS, SourceLocation ModulePrivateLoc,
+                 MultiTemplateParamsArg TemplateParameterLists, bool &OwnedDecl,
+                 bool &IsDependent, SourceLocation ScopedEnumKWLoc,
                  bool ScopedEnumUsesClassTag, TypeResult UnderlyingType,
-                 bool IsTypeSpecifier, SkipBodyInfo *SkipBody = nullptr);
+                 bool IsTypeSpecifier, SkipBodyInfo *SkipBody = nullptr,
+                 CheckCompatTagInfo *CheckCompatTag = nullptr);
 
   Decl *ActOnTemplatedFriendTag(Scope *S, SourceLocation FriendLoc,
                                 unsigned TagSpec, SourceLocation TagLoc,
@@ -2163,8 +2172,8 @@
 
   Decl *ActOnEnumConstant(Scope *S, Decl *EnumDecl, Decl *LastEnumConstant,
                           SourceLocation IdLoc, IdentifierInfo *Id,
-                          AttributeList *Attrs,
-                          SourceLocation EqualLoc, Expr *Val);
+                          AttributeList *Attrs, SourceLocation EqualLoc,
+                          Expr *Val, bool IgnorePrevDef = false);
   void ActOnEnumBody(SourceLocation EnumLoc, SourceRange BraceRange,
                      Decl *EnumDecl,
                      ArrayRef<Decl *> Elements,
@@ -3908,7 +3917,8 @@
       Scope *S, CXXScopeSpec &SS, SourceLocation TemplateKWLoc,
       UnqualifiedId &Id, bool HasTrailingLParen, bool IsAddressOfOperand,
       std::unique_ptr<CorrectionCandidateCallback> CCC = nullptr,
-      bool IsInlineAsmIdentifier = false, Token *KeywordReplacement = nullptr);
+      bool IsInlineAsmIdentifier = false, Token *KeywordReplacement = nullptr,
+      bool IgnorePrevDef = false);
 
   void DecomposeUnqualifiedId(const UnqualifiedId &Id,
                               TemplateArgumentListInfo &Buffer,
Index: include/clang/Parse/Parser.h
===================================================================
--- include/clang/Parse/Parser.h
+++ include/clang/Parse/Parser.h
@@ -1429,7 +1429,8 @@
   };
 
   ExprResult ParseExpression(TypeCastState isTypeCast = NotTypeCast);
-  ExprResult ParseConstantExpression(TypeCastState isTypeCast = NotTypeCast);
+  ExprResult ParseConstantExpression(TypeCastState isTypeCast = NotTypeCast,
+                                     bool IgnorePrevDef = false);
   ExprResult ParseConstraintExpression();
   // Expr that doesn't include commas.
   ExprResult ParseAssignmentExpression(TypeCastState isTypeCast = NotTypeCast);
@@ -1447,12 +1448,13 @@
   ExprResult ParseRHSOfBinaryExpression(ExprResult LHS,
                                         prec::Level MinPrec);
   ExprResult ParseCastExpression(bool isUnaryExpression,
-                                 bool isAddressOfOperand,
-                                 bool &NotCastExpr,
-                                 TypeCastState isTypeCast);
+                                 bool isAddressOfOperand, bool &NotCastExpr,
+                                 TypeCastState isTypeCast,
+                                 bool IgnorePrevDef = false);
   ExprResult ParseCastExpression(bool isUnaryExpression,
                                  bool isAddressOfOperand = false,
-                                 TypeCastState isTypeCast = NotTypeCast);
+                                 TypeCastState isTypeCast = NotTypeCast,
+                                 bool IgnorePrevDef = false);
 
   /// Returns true if the next token cannot start an expression.
   bool isNotExpressionStart();
@@ -1918,7 +1920,8 @@
   void ParseEnumSpecifier(SourceLocation TagLoc, DeclSpec &DS,
                           const ParsedTemplateInfo &TemplateInfo,
                           AccessSpecifier AS, DeclSpecContext DSC);
-  void ParseEnumBody(SourceLocation StartLoc, Decl *TagDecl);
+  void ParseEnumBody(SourceLocation StartLoc, Decl *TagDecl,
+                     bool IgnorePrevDef = false);
   void ParseStructUnionBody(SourceLocation StartLoc, unsigned TagType,
                             Decl *TagDecl);
 
Index: include/clang/Basic/DiagnosticASTKinds.td
===================================================================
--- include/clang/Basic/DiagnosticASTKinds.td
+++ include/clang/Basic/DiagnosticASTKinds.td
@@ -200,12 +200,17 @@
 def err_odr_function_type_inconsistent : Error<
   "external function %0 declared with incompatible types in different "
   "translation units (%1 vs. %2)">;
-def warn_odr_tag_type_inconsistent : Warning<
-  "type %0 has incompatible definitions in different translation units">,
-  InGroup<DiagGroup<"odr">>;
+def warn_odr_tag_type_inconsistent
+    : Warning<"type %0 has incompatible definitions in different translation "
+              "units">,
+      InGroup<DiagGroup<"odr">>;
+def err_odr_tag_type_inconsistent
+    : Error<"type %0 has incompatible definitions in different translation "
+            "units">;
 def note_odr_tag_kind_here: Note<
   "%0 is a %select{struct|interface|union|class|enum}1 here">;
 def note_odr_field : Note<"field %0 has type %1 here">;
+def note_odr_field_name : Note<"field has name %0 here">;
 def note_odr_missing_field : Note<"no corresponding field here">;
 def note_odr_bit_field : Note<"bit-field %0 with type %1 and length %2 here">;
 def note_odr_not_bit_field : Note<"field %0 is not a bit-field">;
Index: include/clang/AST/ASTStructuralEquivalence.h
===================================================================
--- include/clang/AST/ASTStructuralEquivalence.h
+++ include/clang/AST/ASTStructuralEquivalence.h
@@ -61,9 +61,11 @@
   StructuralEquivalenceContext(
       ASTContext &C1, ASTContext &C2,
       llvm::DenseSet<std::pair<Decl *, Decl *>> &NonEquivalentDecls,
-      bool StrictTypeSpelling = false, bool Complain = true)
+      bool StrictTypeSpelling = false, bool Complain = true,
+      bool ErrorOnTagTypeMismatch = false)
       : C1(C1), C2(C2), NonEquivalentDecls(NonEquivalentDecls),
-        StrictTypeSpelling(StrictTypeSpelling), Complain(Complain),
+        StrictTypeSpelling(StrictTypeSpelling),
+        ErrorOnTagTypeMismatch(ErrorOnTagTypeMismatch), Complain(Complain),
         LastDiagFromC2(false) {}
 
   DiagnosticBuilder Diag1(SourceLocation Loc, unsigned DiagID);
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