diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
index 02c31df..58d3d33 100644
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -1131,7 +1131,7 @@
   bool HasRevertedTokenIDToIdentifier = readBit(Bits);
   bool Poisoned = readBit(Bits);
   bool ExtensionToken = readBit(Bits);
-  bool HadMacroDefinition = readBit(Bits);
+  bool HasMacroDefinition = readBit(Bits);
 
   assert(Bits == 0 && "Extra bits in the identifier?");
   DataLen -= sizeof(uint16_t) * 2;
@@ -1151,14 +1151,17 @@
          "Incorrect C++ operator keyword flag");
   (void)CPlusPlusOperatorKeyword;
 
-  // If this identifier is a macro, deserialize the macro
-  // definition.
-  if (HadMacroDefinition) {
+  // If this identifier has a macro definition, deserialize it or notify the
+  // visitor the actual definition is in a different module.
+  if (HasMacroDefinition) {
     uint32_t MacroDirectivesOffset =
         endian::readNext<uint32_t, llvm::endianness::little>(d);
     DataLen -= 4;
 
-    Reader.addPendingMacro(II, &F, MacroDirectivesOffset);
+    if (MacroDirectivesOffset)
+      Reader.addPendingMacro(II, &F, MacroDirectivesOffset);
+    else
+      hasMacroDefinitionInDependencies = true;
   }
 
   Reader.SetIdentifierInfo(ID, II);
@@ -2419,6 +2422,10 @@
       // declarations it needs.
       ++NumIdentifierLookupHits;
       Found = *Pos;
+      if (Trait.hasMoreInformationInDependencies()) {
+        // Look for the identifier in extra modules as they contain more info.
+        return false;
+      }
       return true;
     }
 
diff --git a/clang/lib/Serialization/ASTReaderInternals.h b/clang/lib/Serialization/ASTReaderInternals.h
index 353e0a5..4a77948 100644
--- a/clang/lib/Serialization/ASTReaderInternals.h
+++ b/clang/lib/Serialization/ASTReaderInternals.h
@@ -286,6 +286,8 @@
   // identifier that was constructed before the AST file was read.
   IdentifierInfo *KnownII;
 
+  bool hasMacroDefinitionInDependencies = false;
+
 public:
   using data_type = IdentifierInfo *;
 
@@ -300,6 +302,10 @@
   IdentifierID ReadIdentifierID(const unsigned char *d);
 
   ASTReader &getReader() const { return Reader; }
+
+  bool hasMoreInformationInDependencies() const {
+    return hasMacroDefinitionInDependencies;
+  }
 };
 
 /// The on-disk hash table used to contain information about
diff --git a/clang/lib/Serialization/ASTWriter.cpp b/clang/lib/Serialization/ASTWriter.cpp
index 95b5718..8c261e1 100644
--- a/clang/lib/Serialization/ASTWriter.cpp
+++ b/clang/lib/Serialization/ASTWriter.cpp
@@ -3795,7 +3795,10 @@
       II->getNotableIdentifierID() != tok::NotableIdentifierKind::not_notable ||
       II->getBuiltinID() != Builtin::ID::NotBuiltin ||
       II->getObjCKeywordID() != tok::ObjCKeywordKind::objc_not_keyword;
-  if (MacroOffset || II->isPoisoned() || (!IsModule && IsInteresting) ||
+  if (MacroOffset ||
+      (II->hasMacroDefinition() &&
+       II->hasFETokenInfoChangedSinceDeserialization()) ||
+      II->isPoisoned() || (!IsModule && IsInteresting) ||
       II->hasRevertedTokenIDToIdentifier() ||
       (NeedDecls && II->getFETokenInfo()))
     return true;
@@ -3874,7 +3877,8 @@
     if (isInterestingIdentifier(II, MacroOffset)) {
       DataLen += 2; // 2 bytes for builtin ID
       DataLen += 2; // 2 bytes for flags
-      if (MacroOffset)
+      if (MacroOffset || (II->hasMacroDefinition() &&
+                          II->hasFETokenInfoChangedSinceDeserialization()))
         DataLen += 4; // MacroDirectives offset.
 
       if (NeedDecls && IdResolver)
@@ -3905,15 +3909,17 @@
     assert((Bits & 0xffff) == Bits && "ObjCOrBuiltinID too big for ASTReader.");
     LE.write<uint16_t>(Bits);
     Bits = 0;
-    bool HadMacroDefinition = MacroOffset != 0;
-    Bits = (Bits << 1) | unsigned(HadMacroDefinition);
+    bool HasMacroDefinition =
+        (MacroOffset != 0) || (II->hasMacroDefinition() &&
+                               II->hasFETokenInfoChangedSinceDeserialization());
+    Bits = (Bits << 1) | unsigned(HasMacroDefinition);
     Bits = (Bits << 1) | unsigned(II->isExtensionToken());
     Bits = (Bits << 1) | unsigned(II->isPoisoned());
     Bits = (Bits << 1) | unsigned(II->hasRevertedTokenIDToIdentifier());
     Bits = (Bits << 1) | unsigned(II->isCPlusPlusOperatorKeyword());
     LE.write<uint16_t>(Bits);
 
-    if (HadMacroDefinition)
+    if (HasMacroDefinition)
       LE.write<uint32_t>(MacroOffset);
 
     if (NeedDecls && IdResolver) {
diff --git a/clang/test/Modules/macro-identifier-hiding.c b/clang/test/Modules/macro-identifier-hiding.c
new file mode 100644
index 0000000..4cd7cf0
--- /dev/null
+++ b/clang/test/Modules/macro-identifier-hiding.c
@@ -0,0 +1,29 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+// RUN: %clang_cc1 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t/modules.cache \
+// RUN:            -fsyntax-only %t/test.c -verify
+// Test again with the populated module cache.
+// RUN: %clang_cc1 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t/modules.cache \
+// RUN:            -fsyntax-only %t/test.c -verify
+
+// Test that an identifier with the same name as a macro doesn't hide this
+// macro from the includers.
+
+//--- macro-definition.h
+#define __P(protos) ()
+#define __Q(protos) ()
+
+//--- macro-transitive.h
+#include "macro-definition.h"
+void test(int __P) {} // not "interesting" identifier
+struct __Q {};        // "interesting" identifier
+
+//--- module.modulemap
+module MacroDefinition { header "macro-definition.h" export * }
+module MacroTransitive { header "macro-transitive.h" export * }
+
+//--- test.c
+// expected-no-diagnostics
+#include "macro-transitive.h"
+void foo __P(());
+void bar __Q(());