diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 1234afb..ffd2239 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -16,6 +16,7 @@
 #include "clang/AST/ASTConsumer.h"
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/ASTLambda.h"
+#include "clang/AST/ASTMutationListener.h"
 #include "clang/AST/CXXInheritance.h"
 #include "clang/AST/CharUnits.h"
 #include "clang/AST/CommentDiagnostic.h"
@@ -11233,13 +11234,16 @@
   return FixItHint::CreateInsertion(NameLoc, Insertion);
 }
 
-/// ActOnTag - This is invoked when we see 'struct foo' or 'struct {'.  In the
+/// \brief This is invoked when we see 'struct foo' or 'struct {'.  In the
 /// former case, Name will be non-null.  In the later case, Name will be null.
 /// TagSpec indicates what kind of tag this is. TUK indicates whether this is a
 /// reference/declaration/definition of a tag.
 ///
-/// IsTypeSpecifier is true if this is a type-specifier (or
+/// \param IsTypeSpecifier \c true if this is a type-specifier (or
 /// trailing-type-specifier) other than one in an alias-declaration.
+///
+/// \param SkipBody If non-null, will be set to true if the caller should skip
+/// the definition of this tag, and treat it as if it were a declaration.
 Decl *Sema::ActOnTag(Scope *S, unsigned TagSpec, TagUseKind TUK,
                      SourceLocation KWLoc, CXXScopeSpec &SS,
                      IdentifierInfo *Name, SourceLocation NameLoc,
@@ -11250,7 +11254,7 @@
                      SourceLocation ScopedEnumKWLoc,
                      bool ScopedEnumUsesClassTag,
                      TypeResult UnderlyingType,
-                     bool IsTypeSpecifier) {
+                     bool IsTypeSpecifier, bool *SkipBody) {
   // If this is not a definition, it must have a name.
   IdentifierInfo *OrigName = Name;
   assert((Name != nullptr || TUK == TUK_Definition) &&
@@ -11676,7 +11680,7 @@
 
           // Diagnose attempts to redefine a tag.
           if (TUK == TUK_Definition) {
-            if (TagDecl *Def = PrevTagDecl->getDefinition()) {
+            if (NamedDecl *Def = PrevTagDecl->getDefinition()) {
               // If we're defining a specialization and the previous definition
               // is from an implicit instantiation, don't emit an error
               // here; we'll catch this in the general case below.
@@ -11692,7 +11696,19 @@
                     TSK_ExplicitSpecialization;
               }
 
-              if (!IsExplicitSpecializationAfterInstantiation) {
+              if (SkipBody && getLangOpts().CPlusPlus &&
+                  !hasVisibleDefinition(Def, &Def)) {
+                // 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 = true;
+                if (auto *Listener = getASTMutationListener())
+                  Listener->RedefinedHiddenDefinition(Def, KWLoc);
+                Def->setHidden(false);
+                return Def;
+              } else if (!IsExplicitSpecializationAfterInstantiation) {
                 // A redeclaration in function prototype scope in C isn't
                 // visible elsewhere, so merely issue a warning.
                 if (!getLangOpts().CPlusPlus && S->containedInPrototypeScope())