Index: include/clang/Basic/LangOptions.h
===================================================================
--- include/clang/Basic/LangOptions.h
+++ include/clang/Basic/LangOptions.h
@@ -132,6 +132,10 @@
   /// host code generation.
   std::string OMPHostIRFile;
 
+  /// \brief Indicates whether the front-end is explicitly told that the
+  /// input is a header file (i.e. -x c-header).
+  bool IsHeaderFile;
+
   LangOptions();
 
   // Define accessors/mutators for language options of enumeration type.
Index: lib/Basic/LangOptions.cpp
===================================================================
--- lib/Basic/LangOptions.cpp
+++ lib/Basic/LangOptions.cpp
@@ -15,7 +15,8 @@
 
 using namespace clang;
 
-LangOptions::LangOptions() {
+LangOptions::LangOptions()
+  : IsHeaderFile(false) {
 #define LANGOPT(Name, Bits, Default, Description) Name = Default;
 #define ENUM_LANGOPT(Name, Type, Bits, Default, Description) set##Name(Default);
 #include "clang/Basic/LangOptions.def"
@@ -34,6 +35,7 @@
   SanitizerBlacklistFiles.clear();
 
   CurrentModule.clear();
+  IsHeaderFile = false;
 }
 
 bool LangOptions::isNoBuiltinFunc(StringRef FuncName) const {
Index: lib/Frontend/CompilerInvocation.cpp
===================================================================
--- lib/Frontend/CompilerInvocation.cpp
+++ lib/Frontend/CompilerInvocation.cpp
@@ -1111,7 +1111,8 @@
 }
 
 static InputKind ParseFrontendArgs(FrontendOptions &Opts, ArgList &Args,
-                                   DiagnosticsEngine &Diags) {
+                                   DiagnosticsEngine &Diags,
+                                   bool &IsHeaderFile) {
   using namespace options;
   Opts.ProgramAction = frontend::ParseSyntaxOnly;
   if (const Arg *A = Args.getLastArg(OPT_Action_Group)) {
@@ -1358,6 +1359,13 @@
     if (DashX == IK_None)
       Diags.Report(diag::err_drv_invalid_value)
         << A->getAsString(Args) << A->getValue();
+    IsHeaderFile = llvm::StringSwitch<bool>(A->getValue())
+      .Case("c-header", true)
+      .Case("cl-header", true)
+      .Case("objective-c-header", true)
+      .Case("c++-header", true)
+      .Case("objective-c++-header", true)
+      .Default(false);
   }
 
   // '-' is the default input if none is given.
@@ -2415,7 +2423,8 @@
   ParseCommentArgs(LangOpts.CommentOpts, Args);
   ParseFileSystemArgs(Res.getFileSystemOpts(), Args);
   // FIXME: We shouldn't have to pass the DashX option around here
-  InputKind DashX = ParseFrontendArgs(Res.getFrontendOpts(), Args, Diags);
+  InputKind DashX = ParseFrontendArgs(Res.getFrontendOpts(), Args, Diags,
+                                      LangOpts.IsHeaderFile);
   ParseTargetArgs(Res.getTargetOpts(), Args, Diags);
   Success &= ParseCodeGenArgs(Res.getCodeGenOpts(), Args, DashX, Diags,
                               Res.getTargetOpts());
Index: lib/Lex/PPDirectives.cpp
===================================================================
--- lib/Lex/PPDirectives.cpp
+++ lib/Lex/PPDirectives.cpp
@@ -2058,7 +2058,11 @@
   // diagnostic.
   const DirectoryLookup *Lookup = CurDirLookup;
   const FileEntry *LookupFromFile = nullptr;
-  if (isInPrimaryFile()) {
+  if (isInPrimaryFile() && LangOpts.IsHeaderFile) {
+    // If the main file is a header, then it's either for PCH/AST generation,
+    // or libclang opened it. Either way, handle it as a normal include below
+    // and do not complain about include_next.
+  } else if (isInPrimaryFile()) {
     Lookup = nullptr;
     Diag(IncludeNextTok, diag::pp_include_next_in_primary);
   } else if (CurSubmodule) {
Index: lib/Lex/PPMacroExpansion.cpp
===================================================================
--- lib/Lex/PPMacroExpansion.cpp
+++ lib/Lex/PPMacroExpansion.cpp
@@ -1421,7 +1421,11 @@
   // Preprocessor::HandleIncludeNextDirective.
   const DirectoryLookup *Lookup = PP.GetCurDirLookup();
   const FileEntry *LookupFromFile = nullptr;
-  if (PP.isInPrimaryFile()) {
+  if (PP.isInPrimaryFile() && PP.getLangOpts().IsHeaderFile) {
+    // If the main file is a header, then it's either for PCH/AST generation,
+    // or libclang opened it. Either way, handle it as a normal include below
+    // and do not complain about __has_include_next.
+  } else if (PP.isInPrimaryFile()) {
     Lookup = nullptr;
     PP.Diag(Tok, diag::pp_include_next_in_primary);
   } else if (PP.getCurrentSubmodule()) {
Index: lib/Lex/Pragma.cpp
===================================================================
--- lib/Lex/Pragma.cpp
+++ lib/Lex/Pragma.cpp
@@ -372,8 +372,10 @@
 ///
 void Preprocessor::HandlePragmaOnce(Token &OnceTok) {
   // Don't honor the 'once' when handling the primary source file, unless
-  // this is a prefix to a TU, which indicates we're generating a PCH file.
-  if (isInPrimaryFile() && TUKind != TU_Prefix) {
+  // this is a prefix to a TU, which indicates we're generating a PCH file, or
+  // when the main file is a header (e.g. when -xc-header is provided on the
+  // commandline).
+  if (isInPrimaryFile() && TUKind != TU_Prefix && !getLangOpts().IsHeaderFile) {
     Diag(OnceTok, diag::pp_pragma_once_in_main_file);
     return;
   }
Index: test/Preprocessor/header_is_main_file.c
===================================================================
--- /dev/null
+++ test/Preprocessor/header_is_main_file.c
@@ -0,0 +1,8 @@
+// RUN: %clang_cc1 -x c-header -ffreestanding -Eonly -verify %s
+// expected-no-diagnostics
+
+#pragma once
+#include_next "stdint.h"
+#if !__has_include_next("stdint.h")
+#error "__has_include_next failed"
+#endif