Index: include/clang/Lex/Preprocessor.h
===================================================================
--- include/clang/Lex/Preprocessor.h
+++ include/clang/Lex/Preprocessor.h
@@ -887,7 +887,8 @@
     return appendDefMacroDirective(II, MI, MI->getDefinitionLoc());
   }
   /// \brief Set a MacroDirective that was loaded from a PCH file.
-  void setLoadedMacroDirective(IdentifierInfo *II, MacroDirective *MD);
+  void setLoadedMacroDirective(IdentifierInfo *II, MacroDirective *ED,
+                               MacroDirective *MD);
 
   /// \brief Register an exported macro for a module and identifier.
   ModuleMacro *addModuleMacro(Module *Mod, IdentifierInfo *II, MacroInfo *Macro,
Index: lib/Lex/PPMacroExpansion.cpp
===================================================================
--- lib/Lex/PPMacroExpansion.cpp
+++ lib/Lex/PPMacroExpansion.cpp
@@ -92,12 +92,33 @@
 }
 
 void Preprocessor::setLoadedMacroDirective(IdentifierInfo *II,
+                                           MacroDirective *ED,
                                            MacroDirective *MD) {
+  // Normally, when a macro is defined, it goes through appendMacroDirective()
+  // above, which chains a macro to previous defines, undefs, etc.
+  // However, in a pch, the whole macro history up to the end of the pch is
+  // stored, so ASTReader goes through this function instead.
+  // However, built-in macros are already registered in the Preprocessor
+  // ctor, and ASTWriter stops writing the macro chain at built-in macros,
+  // so in that case the chain from the pch needs to be spliced to the existing
+  // built-in.
+
   assert(II && MD);
   MacroState &StoredMD = CurSubmoduleState->Macros[II];
-  assert(!StoredMD.getLatest() &&
-         "the macro history was modified before initializing it from a pch");
-  StoredMD = MD;
+
+  if (auto *OldMD = StoredMD.getLatest()) {
+    // FIXME: shouldIgnoreMacro() in ASTWriter also stops at macros from the
+    // predefines buffer in module builds. Do we need to splice to those here
+    // too?
+    assert(OldMD->getMacroInfo()->isBuiltinMacro() &&
+           "only built-ins should have an entry here");
+    assert(!OldMD->getPrevious() && "builtin should only have a singe entry");
+    ED->setPrevious(OldMD);
+    StoredMD.setLatest(MD);
+  } else {
+    StoredMD = MD;
+  }
+
   // Setup the identifier as having associated macro history.
   II->setHasMacroDefinition(true);
   if (!MD->isDefined() && LeafModuleMacros.find(II) == LeafModuleMacros.end())
Index: lib/Serialization/ASTReader.cpp
===================================================================
--- lib/Serialization/ASTReader.cpp
+++ lib/Serialization/ASTReader.cpp
@@ -1953,7 +1953,7 @@
   }
 
   if (Latest)
-    PP.setLoadedMacroDirective(II, Latest);
+    PP.setLoadedMacroDirective(II, Earliest, Latest);
 }
 
 ASTReader::InputFileInfo
Index: test/PCH/builtin-macro.c
===================================================================
--- test/PCH/builtin-macro.c
+++ test/PCH/builtin-macro.c
@@ -0,0 +1,33 @@
+
+// Test this without pch.
+// RUN: %clang_cc1 -D__DATE__= -D__TIMESTAMP__= -include %s -Wno-builtin-macro-redefined -fsyntax-only -verify %s
+
+// Test with pch.
+// RUN: %clang_cc1 -D__DATE__= -D__TIMESTAMP__= -Wno-builtin-macro-redefined -emit-pch -o %t %s
+// RUN: %clang_cc1 -D__DATE__= -D__TIMESTAMP__= -Wno-builtin-macro-redefined -include-pch %t -fsyntax-only -verify %s 
+
+#if !defined(HEADER)
+#define HEADER
+
+#define __TIME__
+
+#undef __TIMESTAMP__
+#define __TIMESTAMP__
+
+// FIXME: undefs don't work well with pchs yet, see PR31311
+// Once that's fixed, add -U__COUNTER__ to all command lines and check that
+// an attempt to use __COUNTER__ at the bottom produces an error in both non-pch
+// and pch case (works fine in the former case already).
+// Same for #undef __FILE__ right here and a use of that at the bottom.
+//#undef __FILE__
+
+// Also spot-check a predefine
+#undef __STDC_HOSTED__
+
+#else
+
+const char s[] = __DATE__ " " __TIME__ " " __TIMESTAMP__;
+
+const int d = __STDC_HOSTED__; // expected-error{{use of undeclared identifier '__STDC_HOSTED__'}}
+
+#endif