diff --git a/clang/include/clang/AST/Decl.h b/clang/include/clang/AST/Decl.h
--- a/clang/include/clang/AST/Decl.h
+++ b/clang/include/clang/AST/Decl.h
@@ -3486,6 +3486,24 @@
   /// parameters.
   bool isDependentType() const { return isDependentContext(); }
 
+  /// Whether this declaration was a definition in some module but was forced
+  /// to be a declaration.
+  ///
+  /// Useful for clients checking if a module has a definition of a specific
+  /// symbol and not interested in the final AST with deduplicated definitions.
+  bool isThisDeclarationADemotedDefinition() const {
+    return TagDeclBits.IsThisDeclarationADemotedDefinition;
+  }
+
+  /// Mark a definition as a declaration and maintain information it _was_
+  /// a definition.
+  void demoteThisDefinitionToDeclaration() {
+    assert(isCompleteDefinition() &&
+           "Should demote definitions only, not forward declarations");
+    setCompleteDefinition(false);
+    TagDeclBits.IsThisDeclarationADemotedDefinition = true;
+  }
+
   /// Starts the definition of this tag declaration.
   ///
   /// This method should be invoked at the beginning of the definition
diff --git a/clang/include/clang/AST/DeclBase.h b/clang/include/clang/AST/DeclBase.h
--- a/clang/include/clang/AST/DeclBase.h
+++ b/clang/include/clang/AST/DeclBase.h
@@ -1443,10 +1443,14 @@
     /// Has the full definition of this type been required by a use somewhere in
     /// the TU.
     uint64_t IsCompleteDefinitionRequired : 1;
+
+    /// Whether this tag is a definition which was demoted due to
+    /// a module merge.
+    uint64_t IsThisDeclarationADemotedDefinition : 1;
   };
 
   /// Number of non-inherited bits in TagDeclBitfields.
-  enum { NumTagDeclBits = 9 };
+  enum { NumTagDeclBits = 10 };
 
   /// Stores the bits used by EnumDecl.
   /// If modified NumEnumDeclBit and the accessor
diff --git a/clang/lib/AST/Decl.cpp b/clang/lib/AST/Decl.cpp
--- a/clang/lib/AST/Decl.cpp
+++ b/clang/lib/AST/Decl.cpp
@@ -4301,6 +4301,7 @@
   setEmbeddedInDeclarator(false);
   setFreeStanding(false);
   setCompleteDefinitionRequired(false);
+  TagDeclBits.IsThisDeclarationADemotedDefinition = false;
 }
 
 SourceLocation TagDecl::getOuterLocStart() const {
diff --git a/clang/lib/Serialization/ASTReaderDecl.cpp b/clang/lib/Serialization/ASTReaderDecl.cpp
--- a/clang/lib/Serialization/ASTReaderDecl.cpp
+++ b/clang/lib/Serialization/ASTReaderDecl.cpp
@@ -773,7 +773,7 @@
     }
     if (OldDef) {
       Reader.MergedDeclContexts.insert(std::make_pair(ED, OldDef));
-      ED->setCompleteDefinition(false);
+      ED->demoteThisDefinitionToDeclaration();
       Reader.mergeDefinitionVisibility(OldDef, ED);
       if (OldDef->getODRHash() != ED->getODRHash())
         Reader.PendingEnumOdrMergeFailures[OldDef].push_back(ED);
@@ -828,7 +828,7 @@
     }
     if (OldDef) {
       Reader.MergedDeclContexts.insert(std::make_pair(RD, OldDef));
-      RD->setCompleteDefinition(false);
+      RD->demoteThisDefinitionToDeclaration();
       Reader.mergeDefinitionVisibility(OldDef, RD);
     } else {
       OldDef = RD;